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5. Extensions to the C Language Family

GNU C provides several language features not found in ISO standard C. (The `-pedantic' option directs GCC to print a warning message if any of these features is used.) To test for the availability of these features in conditional compilation, check for a predefined macro __GNUC__, which is always defined under GCC.

These extensions are available in C and Objective-C. Most of them are also available in C++. See section Extensions to the C++ Language, for extensions that apply only to C++.

Some features that are in ISO C99 but not C89 or C++ are also, as extensions, accepted by GCC in C89 mode and in C++.

5.1 Statements and Declarations in Expressions  Putting statements and declarations inside expressions.
5.2 Locally Declared Labels  Labels local to a statement-expression.
5.3 Labels as Values  Getting pointers to labels, and computed gotos.
5.4 Nested Functions  As in Algol and Pascal, lexical scoping of functions.
5.5 Constructing Function Calls  Dispatching a call to another function.
5.6 Referring to a Type with typeof  typeof: referring to the type of an expression.
5.7 Generalized Lvalues  Using `?:', `,' and casts in lvalues.
5.8 Conditionals with Omitted Operands  Omitting the middle operand of a `?:' expression.
5.9 Double-Word Integers  Double-word integers---long long int.
5.10 Complex Numbers  Data types for complex numbers.
5.11 Hex Floats  Hexadecimal floating-point constants.
5.12 Arrays of Length Zero  Zero-length arrays.
5.14 Arrays of Variable Length  Arrays whose length is computed at run time.
5.13 Structures With No Members  Structures with no members.
5.15 Macros with a Variable Number of Arguments.  Macros with a variable number of arguments.
5.16 Slightly Looser Rules for Escaped Newlines  Slightly looser rules for escaped newlines.
5.17 Non-Lvalue Arrays May Have Subscripts  Any array can be subscripted, even if not an lvalue.
5.18 Arithmetic on void- and Function-Pointers  Arithmetic on void-pointers and function pointers.
5.19 Non-Constant Initializers  Non-constant initializers.
5.20 Compound Literals  Compound literals give structures, unions or arrays as values.
5.21 Designated Initializers  Labeling elements of initializers.
5.23 Cast to a Union Type  Casting to union type from any member of the union.
5.22 Case Ranges  `case 1 ... 9' and such.
5.24 Mixed Declarations and Code  Mixing declarations and code.
5.25 Declaring Attributes of Functions  Declaring that functions have no side effects, or that they can never return.
5.26 Attribute Syntax  Formal syntax for attributes.
5.27 Prototypes and Old-Style Function Definitions  Prototype declarations and old-style definitions.
5.28 C++ Style Comments  C++ comments are recognized.
5.29 Dollar Signs in Identifier Names  Dollar sign is allowed in identifiers.
5.30 The Character ESC in Constants  `\e' stands for the character ESC.
5.32 Specifying Attributes of Variables  Specifying attributes of variables.
5.33 Specifying Attributes of Types  Specifying attributes of types.
5.31 Inquiring on Alignment of Types or Variables  Inquiring about the alignment of a type or variable.
5.34 An Inline Function is As Fast As a Macro  Defining inline functions (as fast as macros).
5.35 Assembler Instructions with C Expression Operands  Assembler instructions with C expressions as operands.
                         (With them you can define "built-in" functions.)
5.36 Constraints for asm Operands  Constraints for asm operands
5.37 Controlling Names Used in Assembler Code  Specifying the assembler name to use for a C symbol.
5.38 Variables in Specified Registers  Defining variables residing in specified registers.
5.39 Alternate Keywords  __const__, __asm__, etc., for header files.
5.40 Incomplete enum Types  enum foo;, with details to follow.
5.41 Function Names as Strings  Printable strings which are the name of the current function.
5.42 Getting the Return or Frame Address of a Function  Getting the return or frame address of a function.
5.43 Using vector instructions through built-in functions  
5.44 Other built-in functions provided by GCC  Other built-in functions.
5.45 Built-in Functions Specific to Particular Target Machines  Built-in functions specific to particular targets.
5.46 Pragmas Accepted by GCC  Pragmas accepted by GCC.
5.47 Unnamed struct/union fields within structs/unions.  
5.48 Thread-Local Storage  Per-thread variables.


5.1 Statements and Declarations in Expressions

A compound statement enclosed in parentheses may appear as an expression in GNU C. This allows you to use loops, switches, and local variables within an expression.

Recall that a compound statement is a sequence of statements surrounded by braces; in this construct, parentheses go around the braces. For example:

 
({ int y = foo (); int z;
   if (y > 0) z = y;
   else z = - y;
   z; })

is a valid (though slightly more complex than necessary) expression for the absolute value of foo ().

The last thing in the compound statement should be an expression followed by a semicolon; the value of this subexpression serves as the value of the entire construct. (If you use some other kind of statement last within the braces, the construct has type void, and thus effectively no value.)

This feature is especially useful in making macro definitions "safe" (so that they evaluate each operand exactly once). For example, the "maximum" function is commonly defined as a macro in standard C as follows:

 
#define max(a,b) ((a) > (b) ? (a) : (b))

But this definition computes either a or b twice, with bad results if the operand has side effects. In GNU C, if you know the type of the operands (here let's assume int), you can define the macro safely as follows:

 
#define maxint(a,b) \
  ({int _a = (a), _b = (b); _a > _b ? _a : _b; })

Embedded statements are not allowed in constant expressions, such as the value of an enumeration constant, the width of a bit-field, or the initial value of a static variable.

If you don't know the type of the operand, you can still do this, but you must use typeof (see section 5.6 Referring to a Type with typeof).

Statement expressions are not supported fully in G++, and their fate there is unclear. (It is possible that they will become fully supported at some point, or that they will be deprecated, or that the bugs that are present will continue to exist indefinitely.) Presently, statement expressions do not work well as default arguments.

In addition, there are semantic issues with statement-expressions in C++. If you try to use statement-expressions instead of inline functions in C++, you may be surprised at the way object destruction is handled. For example:

 
#define foo(a)  ({int b = (a); b + 3; })

does not work the same way as:

 
inline int foo(int a) { int b = a; return b + 3; }

In particular, if the expression passed into foo involves the creation of temporaries, the destructors for those temporaries will be run earlier in the case of the macro than in the case of the function.

These considerations mean that it is probably a bad idea to use statement-expressions of this form in header files that are designed to work with C++. (Note that some versions of the GNU C Library contained header files using statement-expression that lead to precisely this bug.)


5.2 Locally Declared Labels

Each statement expression is a scope in which local labels can be declared. A local label is simply an identifier; you can jump to it with an ordinary goto statement, but only from within the statement expression it belongs to.

A local label declaration looks like this:

 
__label__ label;

or

 
__label__ label1, label2, /* ... */;

Local label declarations must come at the beginning of the statement expression, right after the `({', before any ordinary declarations.

The label declaration defines the label name, but does not define the label itself. You must do this in the usual way, with label:, within the statements of the statement expression.

The local label feature is useful because statement expressions are often used in macros. If the macro contains nested loops, a goto can be useful for breaking out of them. However, an ordinary label whose scope is the whole function cannot be used: if the macro can be expanded several times in one function, the label will be multiply defined in that function. A local label avoids this problem. For example:

 
#define SEARCH(array, target)                     \
({                                                \
  __label__ found;                                \
  typeof (target) _SEARCH_target = (target);      \
  typeof (*(array)) *_SEARCH_array = (array);     \
  int i, j;                                       \
  int value;                                      \
  for (i = 0; i < max; i++)                       \
    for (j = 0; j < max; j++)                     \
      if (_SEARCH_array[i][j] == _SEARCH_target)  \
        { value = i; goto found; }                \
  value = -1;                                     \
 found:                                           \
  value;                                          \
})


5.3 Labels as Values

You can get the address of a label defined in the current function (or a containing function) with the unary operator `&&'. The value has type void *. This value is a constant and can be used wherever a constant of that type is valid. For example:

 
void *ptr;
/* ... */
ptr = &&foo;

To use these values, you need to be able to jump to one. This is done with the computed goto statement(3), goto *exp;. For example,

 
goto *ptr;

Any expression of type void * is allowed.

One way of using these constants is in initializing a static array that will serve as a jump table:

 
static void *array[] = { &&foo, &&bar, &&hack };

Then you can select a label with indexing, like this:

 
goto *array[i];

Note that this does not check whether the subscript is in bounds--array indexing in C never does that.

Such an array of label values serves a purpose much like that of the switch statement. The switch statement is cleaner, so use that rather than an array unless the problem does not fit a switch statement very well.

Another use of label values is in an interpreter for threaded code. The labels within the interpreter function can be stored in the threaded code for super-fast dispatching.

You may not use this mechanism to jump to code in a different function. If you do that, totally unpredictable things will happen. The best way to avoid this is to store the label address only in automatic variables and never pass it as an argument.

An alternate way to write the above example is

 
static const int array[] = { &&foo - &&foo, &&bar - &&foo,
                             &&hack - &&foo };
goto *(&&foo + array[i]);

This is more friendly to code living in shared libraries, as it reduces the number of dynamic relocations that are needed, and by consequence, allows the data to be read-only.


5.4 Nested Functions

A nested function is a function defined inside another function. (Nested functions are not supported for GNU C++.) The nested function's name is local to the block where it is defined. For example, here we define a nested function named square, and call it twice:

 
foo (double a, double b)
{
  double square (double z) { return z * z; }

  return square (a) + square (b);
}

The nested function can access all the variables of the containing function that are visible at the point of its definition. This is called lexical scoping. For example, here we show a nested function which uses an inherited variable named offset:

 
bar (int *array, int offset, int size)
{
  int access (int *array, int index)
    { return array[index + offset]; }
  int i;
  /* ... */
  for (i = 0; i < size; i++)
    /* ... */ access (array, i) /* ... */
}

Nested function definitions are permitted within functions in the places where variable definitions are allowed; that is, in any block, before the first statement in the block.

It is possible to call the nested function from outside the scope of its name by storing its address or passing the address to another function:

 
hack (int *array, int size)
{
  void store (int index, int value)
    { array[index] = value; }

  intermediate (store, size);
}

Here, the function intermediate receives the address of store as an argument. If intermediate calls store, the arguments given to store are used to store into array. But this technique works only so long as the containing function (hack, in this example) does not exit.

If you try to call the nested function through its address after the containing function has exited, all hell will break loose. If you try to call it after a containing scope level has exited, and if it refers to some of the variables that are no longer in scope, you may be lucky, but it's not wise to take the risk. If, however, the nested function does not refer to anything that has gone out of scope, you should be safe.

GCC implements taking the address of a nested function using a technique called trampolines. A paper describing them is available as

http://people.debian.org/~aaronl/Usenix88-lexic.pdf.

A nested function can jump to a label inherited from a containing function, provided the label was explicitly declared in the containing function (see section 5.2 Locally Declared Labels). Such a jump returns instantly to the containing function, exiting the nested function which did the goto and any intermediate functions as well. Here is an example:

 
bar (int *array, int offset, int size)
{
  __label__ failure;
  int access (int *array, int index)
    {
      if (index > size)
        goto failure;
      return array[index + offset];
    }
  int i;
  /* ... */
  for (i = 0; i < size; i++)
    /* ... */ access (array, i) /* ... */
  /* ... */
  return 0;

 /* Control comes here from access
    if it detects an error.  */
 failure:
  return -1;
}

A nested function always has internal linkage. Declaring one with extern is erroneous. If you need to declare the nested function before its definition, use auto (which is otherwise meaningless for function declarations).

 
bar (int *array, int offset, int size)
{
  __label__ failure;
  auto int access (int *, int);
  /* ... */
  int access (int *array, int index)
    {
      if (index > size)
        goto failure;
      return array[index + offset];
    }
  /* ... */
}


5.5 Constructing Function Calls

Using the built-in functions described below, you can record the arguments a function received, and call another function with the same arguments, without knowing the number or types of the arguments.

You can also record the return value of that function call, and later return that value, without knowing what data type the function tried to return (as long as your caller expects that data type).

Built-in Function: void * __builtin_apply_args ()
This built-in function returns a pointer to data describing how to perform a call with the same arguments as were passed to the current function.

The function saves the arg pointer register, structure value address, and all registers that might be used to pass arguments to a function into a block of memory allocated on the stack. Then it returns the address of that block.

Built-in Function: void * __builtin_apply (void (*function)(), void *arguments, size_t size)
This built-in function invokes function with a copy of the parameters described by arguments and size.

The value of arguments should be the value returned by __builtin_apply_args. The argument size specifies the size of the stack argument data, in bytes.

This function returns a pointer to data describing how to return whatever value was returned by function. The data is saved in a block of memory allocated on the stack.

It is not always simple to compute the proper value for size. The value is used by __builtin_apply to compute the amount of data that should be pushed on the stack and copied from the incoming argument area.

Built-in Function: void __builtin_return (void *result)
This built-in function returns the value described by result from the containing function. You should specify, for result, a value returned by __builtin_apply.


5.6 Referring to a Type with typeof

Another way to refer to the type of an expression is with typeof. The syntax of using of this keyword looks like sizeof, but the construct acts semantically like a type name defined with typedef.

There are two ways of writing the argument to typeof: with an expression or with a type. Here is an example with an expression:

 
typeof (x[0](1))

This assumes that x is an array of pointers to functions; the type described is that of the values of the functions.

Here is an example with a typename as the argument:

 
typeof (int *)

Here the type described is that of pointers to int.

If you are writing a header file that must work when included in ISO C programs, write __typeof__ instead of typeof. See section 5.39 Alternate Keywords.

A typeof-construct can be used anywhere a typedef name could be used. For example, you can use it in a declaration, in a cast, or inside of sizeof or typeof.

typeof is often useful in conjunction with the statements-within-expressions feature. Here is how the two together can be used to define a safe "maximum" macro that operates on any arithmetic type and evaluates each of its arguments exactly once:

 
#define max(a,b) \
  ({ typeof (a) _a = (a); \
      typeof (b) _b = (b); \
    _a > _b ? _a : _b; })

The reason for using names that start with underscores for the local variables is to avoid conflicts with variable names that occur within the expressions that are substituted for a and b. Eventually we hope to design a new form of declaration syntax that allows you to declare variables whose scopes start only after their initializers; this will be a more reliable way to prevent such conflicts.

Some more examples of the use of typeof:

Compatibility Note: In addition to typeof, GCC 2 supported a more limited extension which permitted one to write

 
typedef T = expr;

with the effect of declaring T to have the type of the expression expr. This extension does not work with GCC 3 (versions between 3.0 and 3.2 will crash; 3.2.1 and later give an error). Code which relies on it should be rewritten to use typeof:

 
typedef typeof(expr) T;

This will work with all versions of GCC.


5.7 Generalized Lvalues

Compound expressions, conditional expressions and casts are allowed as lvalues provided their operands are lvalues. This means that you can take their addresses or store values into them.

All these extensions are deprecated.

For example, a compound expression can be assigned, provided the last expression in the sequence is an lvalue. These two expressions are equivalent:

 
(a, b) += 5
a, (b += 5)

Similarly, the address of the compound expression can be taken. These two expressions are equivalent:

 
&(a, b)
a, &b

A conditional expression is a valid lvalue if its type is not void and the true and false branches are both valid lvalues. For example, these two expressions are equivalent:

 
(a ? b : c) = 5
(a ? b = 5 : (c = 5))

A cast is a valid lvalue if its operand is an lvalue. A simple assignment whose left-hand side is a cast works by converting the right-hand side first to the specified type, then to the type of the inner left-hand side expression. After this is stored, the value is converted back to the specified type to become the value of the assignment. Thus, if a has type char *, the following two expressions are equivalent:

 
(int)a = 5
(int)(a = (char *)(int)5)

An assignment-with-arithmetic operation such as `+=' applied to a cast performs the arithmetic using the type resulting from the cast, and then continues as in the previous case. Therefore, these two expressions are equivalent:

 
(int)a += 5
(int)(a = (char *)(int) ((int)a + 5))

You cannot take the address of an lvalue cast, because the use of its address would not work out coherently. Suppose that &(int)f were permitted, where f has type float. Then the following statement would try to store an integer bit-pattern where a floating point number belongs:

 
*&(int)f = 1;

This is quite different from what (int)f = 1 would do--that would convert 1 to floating point and store it. Rather than cause this inconsistency, we think it is better to prohibit use of `&' on a cast.

If you really do want an int * pointer with the address of f, you can simply write (int *)&f.


5.8 Conditionals with Omitted Operands

The middle operand in a conditional expression may be omitted. Then if the first operand is nonzero, its value is the value of the conditional expression.

Therefore, the expression

 
x ? : y

has the value of x if that is nonzero; otherwise, the value of y.

This example is perfectly equivalent to

 
x ? x : y

In this simple case, the ability to omit the middle operand is not especially useful. When it becomes useful is when the first operand does, or may (if it is a macro argument), contain a side effect. Then repeating the operand in the middle would perform the side effect twice. Omitting the middle operand uses the value already computed without the undesirable effects of recomputing it.


5.9 Double-Word Integers

ISO C99 supports data types for integers that are at least 64 bits wide, and as an extension GCC supports them in C89 mode and in C++. Simply write long long int for a signed integer, or unsigned long long int for an unsigned integer. To make an integer constant of type long long int, add the suffix `LL' to the integer. To make an integer constant of type unsigned long long int, add the suffix `ULL' to the integer.

You can use these types in arithmetic like any other integer types. Addition, subtraction, and bitwise boolean operations on these types are open-coded on all types of machines. Multiplication is open-coded if the machine supports fullword-to-doubleword a widening multiply instruction. Division and shifts are open-coded only on machines that provide special support. The operations that are not open-coded use special library routines that come with GCC.

There may be pitfalls when you use long long types for function arguments, unless you declare function prototypes. If a function expects type int for its argument, and you pass a value of type long long int, confusion will result because the caller and the subroutine will disagree about the number of bytes for the argument. Likewise, if the function expects long long int and you pass int. The best way to avoid such problems is to use prototypes.


5.10 Complex Numbers

ISO C99 supports complex floating data types, and as an extension GCC supports them in C89 mode and in C++, and supports complex integer data types which are not part of ISO C99. You can declare complex types using the keyword _Complex. As an extension, the older GNU keyword __complex__ is also supported.

For example, `_Complex double x;' declares x as a variable whose real part and imaginary part are both of type double. `_Complex short int y;' declares y to have real and imaginary parts of type short int; this is not likely to be useful, but it shows that the set of complex types is complete.

To write a constant with a complex data type, use the suffix `i' or `j' (either one; they are equivalent). For example, 2.5fi has type _Complex float and 3i has type _Complex int. Such a constant always has a pure imaginary value, but you can form any complex value you like by adding one to a real constant. This is a GNU extension; if you have an ISO C99 conforming C library (such as GNU libc), and want to construct complex constants of floating type, you should include <complex.h> and use the macros I or _Complex_I instead.

To extract the real part of a complex-valued expression exp, write __real__ exp. Likewise, use __imag__ to extract the imaginary part. This is a GNU extension; for values of floating type, you should use the ISO C99 functions crealf, creal, creall, cimagf, cimag and cimagl, declared in <complex.h> and also provided as built-in functions by GCC.

The operator `~' performs complex conjugation when used on a value with a complex type. This is a GNU extension; for values of floating type, you should use the ISO C99 functions conjf, conj and conjl, declared in <complex.h> and also provided as built-in functions by GCC.

GCC can allocate complex automatic variables in a noncontiguous fashion; it's even possible for the real part to be in a register while the imaginary part is on the stack (or vice-versa). Only the DWARF2 debug info format can represent this, so use of DWARF2 is recommended. If you are using the stabs debug info format, GCC describes a noncontiguous complex variable as if it were two separate variables of noncomplex type. If the variable's actual name is foo, the two fictitious variables are named foo$real and foo$imag. You can examine and set these two fictitious variables with your debugger.


5.11 Hex Floats

ISO C99 supports floating-point numbers written not only in the usual decimal notation, such as 1.55e1, but also numbers such as 0x1.fp3 written in hexadecimal format. As a GNU extension, GCC supports this in C89 mode (except in some cases when strictly conforming) and in C++. In that format the `0x' hex introducer and the `p' or `P' exponent field are mandatory. The exponent is a decimal number that indicates the power of 2 by which the significant part will be multiplied. Thus `0x1.f' is 1 15/16, `p3' multiplies it by 8, and the value of 0x1.fp3 is the same as 1.55e1.

Unlike for floating-point numbers in the decimal notation the exponent is always required in the hexadecimal notation. Otherwise the compiler would not be able to resolve the ambiguity of, e.g., 0x1.f. This could mean 1.0f or 1.9375 since `f' is also the extension for floating-point constants of type float.


5.12 Arrays of Length Zero

Zero-length arrays are allowed in GNU C. They are very useful as the last element of a structure which is really a header for a variable-length object:

 
struct line {
  int length;
  char contents[0];
};

struct line *thisline = (struct line *)
  malloc (sizeof (struct line) + this_length);
thisline->length = this_length;

In ISO C90, you would have to give contents a length of 1, which means either you waste space or complicate the argument to malloc.

In ISO C99, you would use a flexible array member, which is slightly different in syntax and semantics:

GCC versions before 3.0 allowed zero-length arrays to be statically initialized, as if they were flexible arrays. In addition to those cases that were useful, it also allowed initializations in situations that would corrupt later data. Non-empty initialization of zero-length arrays is now treated like any case where there are more initializer elements than the array holds, in that a suitable warning about "excess elements in array" is given, and the excess elements (all of them, in this case) are ignored.

Instead GCC allows static initialization of flexible array members. This is equivalent to defining a new structure containing the original structure followed by an array of sufficient size to contain the data. I.e. in the following, f1 is constructed as if it were declared like f2.

 
struct f1 {
  int x; int y[];
} f1 = { 1, { 2, 3, 4 } };

struct f2 {
  struct f1 f1; int data[3];
} f2 = { { 1 }, { 2, 3, 4 } };

The convenience of this extension is that f1 has the desired type, eliminating the need to consistently refer to f2.f1.

This has symmetry with normal static arrays, in that an array of unknown size is also written with [].

Of course, this extension only makes sense if the extra data comes at the end of a top-level object, as otherwise we would be overwriting data at subsequent offsets. To avoid undue complication and confusion with initialization of deeply nested arrays, we simply disallow any non-empty initialization except when the structure is the top-level object. For example:

 
struct foo { int x; int y[]; };
struct bar { struct foo z; };

struct foo a = { 1, { 2, 3, 4 } };        // Valid.
struct bar b = { { 1, { 2, 3, 4 } } };    // Invalid.
struct bar c = { { 1, { } } };            // Valid.
struct foo d[1] = { { 1 { 2, 3, 4 } } };  // Invalid.


5.13 Structures With No Members

GCC permits a C structure to have no members:

 
struct empty {
};

The structure will have size zero. In C++, empty structures are part of the language. G++ treats empty structures as if they had a single member of type char.


5.14 Arrays of Variable Length

Variable-length automatic arrays are allowed in ISO C99, and as an extension GCC accepts them in C89 mode and in C++. (However, GCC's implementation of variable-length arrays does not yet conform in detail to the ISO C99 standard.) These arrays are declared like any other automatic arrays, but with a length that is not a constant expression. The storage is allocated at the point of declaration and deallocated when the brace-level is exited. For example:

 
FILE *
concat_fopen (char *s1, char *s2, char *mode)
{
  char str[strlen (s1) + strlen (s2) + 1];
  strcpy (str, s1);
  strcat (str, s2);
  return fopen (str, mode);
}

Jumping or breaking out of the scope of the array name deallocates the storage. Jumping into the scope is not allowed; you get an error message for it.

You can use the function alloca to get an effect much like variable-length arrays. The function alloca is available in many other C implementations (but not in all). On the other hand, variable-length arrays are more elegant.

There are other differences between these two methods. Space allocated with alloca exists until the containing function returns. The space for a variable-length array is deallocated as soon as the array name's scope ends. (If you use both variable-length arrays and alloca in the same function, deallocation of a variable-length array will also deallocate anything more recently allocated with alloca.)

You can also use variable-length arrays as arguments to functions:

 
struct entry
tester (int len, char data[len][len])
{
  /* ... */
}

The length of an array is computed once when the storage is allocated and is remembered for the scope of the array in case you access it with sizeof.

If you want to pass the array first and the length afterward, you can use a forward declaration in the parameter list--another GNU extension.

 
struct entry
tester (int len; char data[len][len], int len)
{
  /* ... */
}

The `int len' before the semicolon is a parameter forward declaration, and it serves the purpose of making the name len known when the declaration of data is parsed.

You can write any number of such parameter forward declarations in the parameter list. They can be separated by commas or semicolons, but the last one must end with a semicolon, which is followed by the "real" parameter declarations. Each forward declaration must match a "real" declaration in parameter name and data type. ISO C99 does not support parameter forward declarations.


5.15 Macros with a Variable Number of Arguments.

In the ISO C standard of 1999, a macro can be declared to accept a variable number of arguments much as a function can. The syntax for defining the macro is similar to that of a function. Here is an example:

 
#define debug(format, ...) fprintf (stderr, format, __VA_ARGS__)

Here `...' is a variable argument. In the invocation of such a macro, it represents the zero or more tokens until the closing parenthesis that ends the invocation, including any commas. This set of tokens replaces the identifier __VA_ARGS__ in the macro body wherever it appears. See the CPP manual for more information.

GCC has long supported variadic macros, and used a different syntax that allowed you to give a name to the variable arguments just like any other argument. Here is an example:

 
#define debug(format, args...) fprintf (stderr, format, args)

This is in all ways equivalent to the ISO C example above, but arguably more readable and descriptive.

GNU CPP has two further variadic macro extensions, and permits them to be used with either of the above forms of macro definition.

In standard C, you are not allowed to leave the variable argument out entirely; but you are allowed to pass an empty argument. For example, this invocation is invalid in ISO C, because there is no comma after the string:

 
debug ("A message")

GNU CPP permits you to completely omit the variable arguments in this way. In the above examples, the compiler would complain, though since the expansion of the macro still has the extra comma after the format string.

To help solve this problem, CPP behaves specially for variable arguments used with the token paste operator, `##'. If instead you write

 
#define debug(format, ...) fprintf (stderr, format, ## __VA_ARGS__)

and if the variable arguments are omitted or empty, the `##' operator causes the preprocessor to remove the comma before it. If you do provide some variable arguments in your macro invocation, GNU CPP does not complain about the paste operation and instead places the variable arguments after the comma. Just like any other pasted macro argument, these arguments are not macro expanded.


5.16 Slightly Looser Rules for Escaped Newlines

Recently, the preprocessor has relaxed its treatment of escaped newlines. Previously, the newline had to immediately follow a backslash. The current implementation allows whitespace in the form of spaces, horizontal and vertical tabs, and form feeds between the backslash and the subsequent newline. The preprocessor issues a warning, but treats it as a valid escaped newline and combines the two lines to form a single logical line. This works within comments and tokens, as well as between tokens. Comments are not treated as whitespace for the purposes of this relaxation, since they have not yet been replaced with spaces.


5.17 Non-Lvalue Arrays May Have Subscripts

In ISO C99, arrays that are not lvalues still decay to pointers, and may be subscripted, although they may not be modified or used after the next sequence point and the unary `&' operator may not be applied to them. As an extension, GCC allows such arrays to be subscripted in C89 mode, though otherwise they do not decay to pointers outside C99 mode. For example, this is valid in GNU C though not valid in C89:

 
struct foo {int a[4];};

struct foo f();

bar (int index)
{
  return f().a[index];
}


5.18 Arithmetic on void- and Function-Pointers

In GNU C, addition and subtraction operations are supported on pointers to void and on pointers to functions. This is done by treating the size of a void or of a function as 1.

A consequence of this is that sizeof is also allowed on void and on function types, and returns 1.

The option `-Wpointer-arith' requests a warning if these extensions are used.


5.19 Non-Constant Initializers

As in standard C++ and ISO C99, the elements of an aggregate initializer for an automatic variable are not required to be constant expressions in GNU C. Here is an example of an initializer with run-time varying elements:

 
foo (float f, float g)
{
  float beat_freqs[2] = { f-g, f+g };
  /* ... */
}


5.20 Compound Literals

ISO C99 supports compound literals. A compound literal looks like a cast containing an initializer. Its value is an object of the type specified in the cast, containing the elements specified in the initializer; it is an lvalue. As an extension, GCC supports compound literals in C89 mode and in C++.

Usually, the specified type is a structure. Assume that struct foo and structure are declared as shown:

 
struct foo {int a; char b[2];} structure;

Here is an example of constructing a struct foo with a compound literal:

 
structure = ((struct foo) {x + y, 'a', 0});

This is equivalent to writing the following:

 
{
  struct foo temp = {x + y, 'a', 0};
  structure = temp;
}

You can also construct an array. If all the elements of the compound literal are (made up of) simple constant expressions, suitable for use in initializers of objects of static storage duration, then the compound literal can be coerced to a pointer to its first element and used in such an initializer, as shown here:

 
char **foo = (char *[]) { "x", "y", "z" };

Compound literals for scalar types and union types are is also allowed, but then the compound literal is equivalent to a cast.

As a GNU extension, GCC allows initialization of objects with static storage duration by compound literals (which is not possible in ISO C99, because the initializer is not a constant). It is handled as if the object was initialized only with the bracket enclosed list if compound literal's and object types match. The initializer list of the compound literal must be constant. If the object being initialized has array type of unknown size, the size is determined by compound literal size.

 
static struct foo x = (struct foo) {1, 'a', 'b'};
static int y[] = (int []) {1, 2, 3};
static int z[] = (int [3]) {1};

The above lines are equivalent to the following:
 
static struct foo x = {1, 'a', 'b'};
static int y[] = {1, 2, 3};
static int z[] = {1, 0, 0};


5.21 Designated Initializers

Standard C89 requires the elements of an initializer to appear in a fixed order, the same as the order of the elements in the array or structure being initialized.

In ISO C99 you can give the elements in any order, specifying the array indices or structure field names they apply to, and GNU C allows this as an extension in C89 mode as well. This extension is not implemented in GNU C++.

To specify an array index, write `[index] =' before the element value. For example,

 
int a[6] = { [4] = 29, [2] = 15 };

is equivalent to

 
int a[6] = { 0, 0, 15, 0, 29, 0 };

The index values must be constant expressions, even if the array being initialized is automatic.

An alternative syntax for this which has been obsolete since GCC 2.5 but GCC still accepts is to write `[index]' before the element value, with no `='.

To initialize a range of elements to the same value, write `[first ... last] = value'. This is a GNU extension. For example,

 
int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 };

If the value in it has side-effects, the side-effects will happen only once, not for each initialized field by the range initializer.

Note that the length of the array is the highest value specified plus one.

In a structure initializer, specify the name of a field to initialize with `.fieldname =' before the element value. For example, given the following structure,

 
struct point { int x, y; };

the following initialization

 
struct point p = { .y = yvalue, .x = xvalue };

is equivalent to

 
struct point p = { xvalue, yvalue };

Another syntax which has the same meaning, obsolete since GCC 2.5, is `fieldname:', as shown here:

 
struct point p = { y: yvalue, x: xvalue };

The `[index]' or `.fieldname' is known as a designator. You can also use a designator (or the obsolete colon syntax) when initializing a union, to specify which element of the union should be used. For example,

 
union foo { int i; double d; };

union foo f = { .d = 4 };

will convert 4 to a double to store it in the union using the second element. By contrast, casting 4 to type union foo would store it into the union as the integer i, since it is an integer. (See section 5.23 Cast to a Union Type.)

You can combine this technique of naming elements with ordinary C initialization of successive elements. Each initializer element that does not have a designator applies to the next consecutive element of the array or structure. For example,

 
int a[6] = { [1] = v1, v2, [4] = v4 };

is equivalent to

 
int a[6] = { 0, v1, v2, 0, v4, 0 };

Labeling the elements of an array initializer is especially useful when the indices are characters or belong to an enum type. For example:

 
int whitespace[256]
  = { [' '] = 1, ['\t'] = 1, ['\h'] = 1,
      ['\f'] = 1, ['\n'] = 1, ['\r'] = 1 };

You can also write a series of `.fieldname' and `[index]' designators before an `=' to specify a nested subobject to initialize; the list is taken relative to the subobject corresponding to the closest surrounding brace pair. For example, with the `struct point' declaration above:

 
struct point ptarray[10] = { [2].y = yv2, [2].x = xv2, [0].x = xv0 };

If the same field is initialized multiple times, it will have value from the last initialization. If any such overridden initialization has side-effect, it is unspecified whether the side-effect happens or not. Currently, gcc will discard them and issue a warning.


5.22 Case Ranges

You can specify a range of consecutive values in a single case label, like this:

 
case low ... high:

This has the same effect as the proper number of individual case labels, one for each integer value from low to high, inclusive.

This feature is especially useful for ranges of ASCII character codes:

 
case 'A' ... 'Z':

Be careful: Write spaces around the ..., for otherwise it may be parsed wrong when you use it with integer values. For example, write this:

 
case 1 ... 5:

rather than this:

 
case 1...5:


5.23 Cast to a Union Type

A cast to union type is similar to other casts, except that the type specified is a union type. You can specify the type either with union tag or with a typedef name. A cast to union is actually a constructor though, not a cast, and hence does not yield an lvalue like normal casts. (See section 5.20 Compound Literals.)

The types that may be cast to the union type are those of the members of the union. Thus, given the following union and variables:

 
union foo { int i; double d; };
int x;
double y;

both x and y can be cast to type union foo.

Using the cast as the right-hand side of an assignment to a variable of union type is equivalent to storing in a member of the union:

 
union foo u;
/* ... */
u = (union foo) x  ==  u.i = x
u = (union foo) y  ==  u.d = y

You can also use the union cast as a function argument:

 
void hack (union foo);
/* ... */
hack ((union foo) x);


5.24 Mixed Declarations and Code

ISO C99 and ISO C++ allow declarations and code to be freely mixed within compound statements. As an extension, GCC also allows this in C89 mode. For example, you could do:

 
int i;
/* ... */
i++;
int j = i + 2;

Each identifier is visible from where it is declared until the end of the enclosing block.


5.25 Declaring Attributes of Functions

In GNU C, you declare certain things about functions called in your program which help the compiler optimize function calls and check your code more carefully.

The keyword __attribute__ allows you to specify special attributes when making a declaration. This keyword is followed by an attribute specification inside double parentheses. The following attributes are currently defined for functions on all targets: noreturn, noinline, always_inline, pure, const, nothrow, format, format_arg, no_instrument_function, section, constructor, destructor, used, unused, deprecated, weak, malloc, alias, and nonnull. Several other attributes are defined for functions on particular target systems. Other attributes, including section are supported for variables declarations (see section 5.32 Specifying Attributes of Variables) and for types (see section 5.33 Specifying Attributes of Types).

You may also specify attributes with `__' preceding and following each keyword. This allows you to use them in header files without being concerned about a possible macro of the same name. For example, you may use __noreturn__ instead of noreturn.

See section 5.26 Attribute Syntax, for details of the exact syntax for using attributes.

noreturn
A few standard library functions, such as abort and exit, cannot return. GCC knows this automatically. Some programs define their own functions that never return. You can declare them noreturn to tell the compiler this fact. For example,

 
void fatal () __attribute__ ((noreturn));

void
fatal (/* ... */)
{
  /* ... */ /* Print error message. */ /* ... */
  exit (1);
}

The noreturn keyword tells the compiler to assume that fatal cannot return. It can then optimize without regard to what would happen if fatal ever did return. This makes slightly better code. More importantly, it helps avoid spurious warnings of uninitialized variables.

Do not assume that registers saved by the calling function are restored before calling the noreturn function.

It does not make sense for a noreturn function to have a return type other than void.

The attribute noreturn is not implemented in GCC versions earlier than 2.5. An alternative way to declare that a function does not return, which works in the current version and in some older versions, is as follows:

 
typedef void voidfn ();

volatile voidfn fatal;

noinline
This function attribute prevents a function from being considered for inlining.

always_inline
Generally, functions are not inlined unless optimization is specified. For functions declared inline, this attribute inlines the function even if no optimization level was specified.

pure
Many functions have no effects except the return value and their return value depends only on the parameters and/or global variables. Such a function can be subject to common subexpression elimination and loop optimization just as an arithmetic operator would be. These functions should be declared with the attribute pure. For example,

 
int square (int) __attribute__ ((pure));

says that the hypothetical function square is safe to call fewer times than the program says.

Some of common examples of pure functions are strlen or memcmp. Interesting non-pure functions are functions with infinite loops or those depending on volatile memory or other system resource, that may change between two consecutive calls (such as feof in a multithreading environment).

The attribute pure is not implemented in GCC versions earlier than 2.96.

const
Many functions do not examine any values except their arguments, and have no effects except the return value. Basically this is just slightly more strict class than the pure attribute above, since function is not allowed to read global memory.

Note that a function that has pointer arguments and examines the data pointed to must not be declared const. Likewise, a function that calls a non-const function usually must not be const. It does not make sense for a const function to return void.

The attribute const is not implemented in GCC versions earlier than 2.5. An alternative way to declare that a function has no side effects, which works in the current version and in some older versions, is as follows:

 
typedef int intfn ();

extern const intfn square;

This approach does not work in GNU C++ from 2.6.0 on, since the language specifies that the `const' must be attached to the return value.

nothrow
The nothrow attribute is used to inform the compiler that a function cannot throw an exception. For example, most functions in the standard C library can be guaranteed not to throw an exception with the notable exceptions of qsort and bsearch that take function pointer arguments. The nothrow attribute is not implemented in GCC versions earlier than 3.2.

format (archetype, string-index, first-to-check)
The format attribute specifies that a function takes printf, scanf, strftime or strfmon style arguments which should be type-checked against a format string. For example, the declaration:

 
extern int
my_printf (void *my_object, const char *my_format, ...)
      __attribute__ ((format (printf, 2, 3)));

causes the compiler to check the arguments in calls to my_printf for consistency with the printf style format string argument my_format.

The parameter archetype determines how the format string is interpreted, and should be printf, scanf, strftime or strfmon. (You can also use __printf__, __scanf__, __strftime__ or __strfmon__.) The parameter string-index specifies which argument is the format string argument (starting from 1), while first-to-check is the number of the first argument to check against the format string. For functions where the arguments are not available to be checked (such as vprintf), specify the third parameter as zero. In this case the compiler only checks the format string for consistency. For strftime formats, the third parameter is required to be zero. Since non-static C++ methods have an implicit this argument, the arguments of such methods should be counted from two, not one, when giving values for string-index and first-to-check.

In the example above, the format string (my_format) is the second argument of the function my_print, and the arguments to check start with the third argument, so the correct parameters for the format attribute are 2 and 3.

The format attribute allows you to identify your own functions which take format strings as arguments, so that GCC can check the calls to these functions for errors. The compiler always (unless `-ffreestanding' is used) checks formats for the standard library functions printf, fprintf, sprintf, scanf, fscanf, sscanf, strftime, vprintf, vfprintf and vsprintf whenever such warnings are requested (using `-Wformat'), so there is no need to modify the header file `stdio.h'. In C99 mode, the functions snprintf, vsnprintf, vscanf, vfscanf and vsscanf are also checked. Except in strictly conforming C standard modes, the X/Open function strfmon is also checked as are printf_unlocked and fprintf_unlocked. See section Options Controlling C Dialect.

format_arg (string-index)
The format_arg attribute specifies that a function takes a format string for a printf, scanf, strftime or strfmon style function and modifies it (for example, to translate it into another language), so the result can be passed to a printf, scanf, strftime or strfmon style function (with the remaining arguments to the format function the same as they would have been for the unmodified string). For example, the declaration:

 
extern char *
my_dgettext (char *my_domain, const char *my_format)
      __attribute__ ((format_arg (2)));

causes the compiler to check the arguments in calls to a printf, scanf, strftime or strfmon type function, whose format string argument is a call to the my_dgettext function, for consistency with the format string argument my_format. If the format_arg attribute had not been specified, all the compiler could tell in such calls to format functions would be that the format string argument is not constant; this would generate a warning when `-Wformat-nonliteral' is used, but the calls could not be checked without the attribute.

The parameter string-index specifies which argument is the format string argument (starting from one). Since non-static C++ methods have an implicit this argument, the arguments of such methods should be counted from two.

The format-arg attribute allows you to identify your own functions which modify format strings, so that GCC can check the calls to printf, scanf, strftime or strfmon type function whose operands are a call to one of your own function. The compiler always treats gettext, dgettext, and dcgettext in this manner except when strict ISO C support is requested by `-ansi' or an appropriate `-std' option, or `-ffreestanding' is used. See section Options Controlling C Dialect.

nonnull (arg-index, ...)
The nonnull attribute specifies that some function parameters should be non-null pointers. For instance, the declaration:

 
extern void *
my_memcpy (void *dest, const void *src, size_t len)
	__attribute__((nonnull (1, 2)));

causes the compiler to check that, in calls to my_memcpy, arguments dest and src are non-null. If the compiler determines that a null pointer is passed in an argument slot marked as non-null, and the `-Wnonnull' option is enabled, a warning is issued. The compiler may also choose to make optimizations based on the knowledge that certain function arguments will not be null.

If no argument index list is given to the nonnull attribute, all pointer arguments are marked as non-null. To illustrate, the following declaration is equivalent to the previous example:

 
extern void *
my_memcpy (void *dest, const void *src, size_t len)
	__attribute__((nonnull));

no_instrument_function
If `-finstrument-functions' is given, profiling function calls will be generated at entry and exit of most user-compiled functions. Functions with this attribute will not be so instrumented.

section ("section-name")
Normally, the compiler places the code it generates in the text section. Sometimes, however, you need additional sections, or you need certain particular functions to appear in special sections. The section attribute specifies that a function lives in a particular section. For example, the declaration:

 
extern void foobar (void) __attribute__ ((section ("bar")));

puts the function foobar in the bar section.

Some file formats do not support arbitrary sections so the section attribute is not available on all platforms. If you need to map the entire contents of a module to a particular section, consider using the facilities of the linker instead.

constructor
destructor
The constructor attribute causes the function to be called automatically before execution enters main (). Similarly, the destructor attribute causes the function to be called automatically after main () has completed or exit () has been called. Functions with these attributes are useful for initializing data that will be used implicitly during the execution of the program.

These attributes are not currently implemented for Objective-C.

unused
This attribute, attached to a function, means that the function is meant to be possibly unused. GCC will not produce a warning for this function. GNU C++ does not currently support this attribute as definitions without parameters are valid in C++.

used
This attribute, attached to a function, means that code must be emitted for the function even if it appears that the function is not referenced. This is useful, for example, when the function is referenced only in inline assembly.

deprecated
The deprecated attribute results in a warning if the function is used anywhere in the source file. This is useful when identifying functions that are expected to be removed in a future version of a program. The warning also includes the location of the declaration of the deprecated function, to enable users to easily find further information about why the function is deprecated, or what they should do instead. Note that the warnings only occurs for uses:

 
int old_fn () __attribute__ ((deprecated));
int old_fn ();
int (*fn_ptr)() = old_fn;

results in a warning on line 3 but not line 2.

The deprecated attribute can also be used for variables and types (see section 5.32 Specifying Attributes of Variables, see section 5.33 Specifying Attributes of Types.)

weak
The weak attribute causes the declaration to be emitted as a weak symbol rather than a global. This is primarily useful in defining library functions which can be overridden in user code, though it can also be used with non-function declarations. Weak symbols are supported for ELF targets, and also for a.out targets when using the GNU assembler and linker.

malloc
The malloc attribute is used to tell the compiler that a function may be treated as if it were the malloc function. The compiler assumes that calls to malloc result in pointers that cannot alias anything. This will often improve optimization.

alias ("target")
The alias attribute causes the declaration to be emitted as an alias for another symbol, which must be specified. For instance,

 
void __f () { /* Do something. */; }
void f () __attribute__ ((weak, alias ("__f")));

declares `f' to be a weak alias for `__f'. In C++, the mangled name for the target must be used.

Not all target machines support this attribute.

visibility ("visibility_type")
The visibility attribute on ELF targets causes the declaration to be emitted with default, hidden, protected or internal visibility.

 
void __attribute__ ((visibility ("protected")))
f () { /* Do something. */; }
int i __attribute__ ((visibility ("hidden")));

See the ELF gABI for complete details, but the short story is:

default
Default visibility is the normal case for ELF. This value is available for the visibility attribute to override other options that may change the assumed visibility of symbols.

hidden
Hidden visibility indicates that the symbol will not be placed into the dynamic symbol table, so no other module (executable or shared library) can reference it directly.

protected
Protected visibility indicates that the symbol will be placed in the dynamic symbol table, but that references within the defining module will bind to the local symbol. That is, the symbol cannot be overridden by another module.

internal
Internal visibility is like hidden visibility, but with additional processor specific semantics. Unless otherwise specified by the psABI, gcc defines internal visibility to mean that the function is never called from another module. Note that hidden symbols, while they cannot be referenced directly by other modules, can be referenced indirectly via function pointers. By indicating that a symbol cannot be called from outside the module, gcc may for instance omit the load of a PIC register since it is known that the calling function loaded the correct value.

Not all ELF targets support this attribute.

regparm (number)
On the Intel 386, the regparm attribute causes the compiler to pass up to number integer arguments in registers EAX, EDX, and ECX instead of on the stack. Functions that take a variable number of arguments will continue to be passed all of their arguments on the stack.

Beware that on some ELF systems this attribute is unsuitable for global functions in shared libraries with lazy binding (which is the default). Lazy binding will send the first call via resolving code in the loader, which might assume EAX, EDX and ECX can be clobbered, as per the standard calling conventions. Solaris 8 is affected by this. GNU systems with GLIBC 2.1 or higher, and FreeBSD, are believed to be safe since the loaders there save all registers. (Lazy binding can be disabled with the linker or the loader if desired, to avoid the problem.)

stdcall
On the Intel 386, the stdcall attribute causes the compiler to assume that the called function will pop off the stack space used to pass arguments, unless it takes a variable number of arguments.

cdecl
On the Intel 386, the cdecl attribute causes the compiler to assume that the calling function will pop off the stack space used to pass arguments. This is useful to override the effects of the `-mrtd' switch.

longcall/shortcall
On the RS/6000 and PowerPC, the longcall attribute causes the compiler to always call this function via a pointer, just as it would if the `-mlongcall' option had been specified. The shortcall attribute causes the compiler not to do this. These attributes override both the `-mlongcall' switch and the #pragma longcall setting.

See section 3.17.10 IBM RS/6000 and PowerPC Options, for more information on whether long calls are necessary.

long_call/short_call
This attribute specifies how a particular function is called on ARM. Both attributes override the `-mlong-calls' (see section 3.17.5 ARM Options) command line switch and #pragma long_calls settings. The long_call attribute causes the compiler to always call the function by first loading its address into a register and then using the contents of that register. The short_call attribute always places the offset to the function from the call site into the `BL' instruction directly.

function_vector
Use this attribute on the H8/300 and H8/300H to indicate that the specified function should be called through the function vector. Calling a function through the function vector will reduce code size, however; the function vector has a limited size (maximum 128 entries on the H8/300 and 64 entries on the H8/300H) and shares space with the interrupt vector.

You must use GAS and GLD from GNU binutils version 2.7 or later for this attribute to work correctly.

interrupt
Use this attribute on the ARM, AVR, M32R/D and Xstormy16 ports to indicate that the specified function is an interrupt handler. The compiler will generate function entry and exit sequences suitable for use in an interrupt handler when this attribute is present.

Note, interrupt handlers for the H8/300, H8/300H and SH processors can be specified via the interrupt_handler attribute.

Note, on the AVR, interrupts will be enabled inside the function.

Note, for the ARM, you can specify the kind of interrupt to be handled by adding an optional parameter to the interrupt attribute like this:

 
void f () __attribute__ ((interrupt ("IRQ")));

Permissible values for this parameter are: IRQ, FIQ, SWI, ABORT and UNDEF.

interrupt_handler
Use this attribute on the H8/300, H8/300H and SH to indicate that the specified function is an interrupt handler. The compiler will generate function entry and exit sequences suitable for use in an interrupt handler when this attribute is present.

sp_switch
Use this attribute on the SH to indicate an interrupt_handler function should switch to an alternate stack. It expects a string argument that names a global variable holding the address of the alternate stack.

 
void *alt_stack;
void f () __attribute__ ((interrupt_handler,
                          sp_switch ("alt_stack")));

trap_exit
Use this attribute on the SH for an interrupt_handle to return using trapa instead of rte. This attribute expects an integer argument specifying the trap number to be used.

eightbit_data
Use this attribute on the H8/300 and H8/300H to indicate that the specified variable should be placed into the eight bit data section. The compiler will generate more efficient code for certain operations on data in the eight bit data area. Note the eight bit data area is limited to 256 bytes of data.

You must use GAS and GLD from GNU binutils version 2.7 or later for this attribute to work correctly.

tiny_data
Use this attribute on the H8/300H to indicate that the specified variable should be placed into the tiny data section. The compiler will generate more efficient code for loads and stores on data in the tiny data section. Note the tiny data area is limited to slightly under 32kbytes of data.

signal
Use this attribute on the AVR to indicate that the specified function is a signal handler. The compiler will generate function entry and exit sequences suitable for use in a signal handler when this attribute is present. Interrupts will be disabled inside the function.

naked
Use this attribute on the ARM, AVR and IP2K ports to indicate that the specified function does not need prologue/epilogue sequences generated by the compiler. It is up to the programmer to provide these sequences.

model (model-name)
Use this attribute on the M32R/D to set the addressability of an object, and of the code generated for a function. The identifier model-name is one of small, medium, or large, representing each of the code models.

Small model objects live in the lower 16MB of memory (so that their addresses can be loaded with the ld24 instruction), and are callable with the bl instruction.

Medium model objects may live anywhere in the 32-bit address space (the compiler will generate seth/add3 instructions to load their addresses), and are callable with the bl instruction.

Large model objects may live anywhere in the 32-bit address space (the compiler will generate seth/add3 instructions to load their addresses), and may not be reachable with the bl instruction (the compiler will generate the much slower seth/add3/jl instruction sequence).

far
On 68HC11 and 68HC12 the far attribute causes the compiler to use a calling convention that takes care of switching memory banks when entering and leaving a function. This calling convention is also the default when using the `-mlong-calls' option.

On 68HC12 the compiler will use the call and rtc instructions to call and return from a function.

On 68HC11 the compiler will generate a sequence of instructions to invoke a board-specific routine to switch the memory bank and call the real function. The board-specific routine simulates a call. At the end of a function, it will jump to a board-specific routine instead of using rts. The board-specific return routine simulates the rtc.

The far attribute must not be used when the interrupt or trap attributes are used.

near
On 68HC11 and 68HC12 the near attribute causes the compiler to use the normal calling convention based on jsr and rts. This attribute can be used to cancel the effect of the `-mlong-calls' option.

page0
On 68HC11 and 68HC12, the page0 attribute indicates that a global or static variable is put in the page0 section and the compiler can use the direct addressing mode. On 68HC11 the compiler will be able to use bset and bclr on these variables. Note that the page0 is limited to the absolute address range 0..0x0ff.

trap
On 68HC11 and 68HC12, the trap attribute marks the function as being a trap handler. It will use rti instead of rts to return from the function. Offset of function parameters are also adjusted to take into account the trap frame.

dllimport
On Windows targets, the dllimport attribute causes the compiler to reference a function or variable via a global pointer to a pointer that is set up by the Windows dll library. The pointer name is formed by combining _imp__ and the function or variable name. The attribute implies extern storage.

Currently, the attribute is ignored for inlined functions. If the attribute is applied to a symbol definition, an error is reported. If a symbol previously declared dllimport is later defined, the attribute is ignored in subsequent references, and a warning is emitted. The attribute is also overridden by a subsequent declaration as dllexport.

When applied to C++ classes, the attribute marks non-inlined member functions and static data members as imports. However, the attribute is ignored for virtual methods to allow creation of vtables using thunks.

On cygwin, mingw and arm-pe targets, __declspec(dllimport) is recognized as a synonym for __attribute__ ((dllimport)) for compatibility with other Windows compilers.

The use of the dllimport attribute on functions is not necessary, but provides a small performance benefit by eliminating a thunk in the dll. The use of the dllimport attribute on imported variables was required on older versions of GNU ld, but can now be avoided by passing the `--enable-auto-import' switch to ld. As with functions, using the attribute for a variable eliminates a thunk in the dll.

One drawback to using this attribute is that a pointer to a function or variable marked as dllimport cannot be used as a constant address. The attribute can be disabled for functions by setting the `-mnop-fun-dllimport' flag.

dllexport
On Windows targets the dllexport attribute causes the compiler to provide a global pointer to a pointer in a dll, so that it can be referenced with the dllimport attribute. The pointer name is formed by combining _imp__ and the function or variable name.

Currently, the dllexportattribute is ignored for inlined functions, but export can be forced by using the `-fkeep-inline-functions' flag. The attribute is also ignored for undefined symbols.

When applied to C++ classes. the attribute marks defined non-inlined member functions and static data members as exports. Static consts initialized in-class are not marked unless they are also defined out-of-class.

On cygwin, mingw and arm-pe targets, __declspec(dllexport) is recognized as a synonym for __attribute__ ((dllexport)) for compatibility with other Windows compilers.

Alternative methods for including the symbol in the dll's export table are to use a .def file with an EXPORTS section or, with GNU ld, using the `--export-all' linker flag.

You can specify multiple attributes in a declaration by separating them by commas within the double parentheses or by immediately following an attribute declaration with another attribute declaration.

Some people object to the __attribute__ feature, suggesting that ISO C's #pragma should be used instead. At the time __attribute__ was designed, there were two reasons for not doing this.

  1. It is impossible to generate #pragma commands from a macro.

  2. There is no telling what the same #pragma might mean in another compiler.

These two reasons applied to almost any application that might have been proposed for #pragma. It was basically a mistake to use #pragma for anything.

The ISO C99 standard includes _Pragma, which now allows pragmas to be generated from macros. In addition, a #pragma GCC namespace is now in use for GCC-specific pragmas. However, it has been found convenient to use __attribute__ to achieve a natural attachment of attributes to their corresponding declarations, whereas #pragma GCC is of use for constructs that do not naturally form part of the grammar. See section `Miscellaneous Preprocessing Directives' in The GNU C Preprocessor.


5.26 Attribute Syntax

This section describes the syntax with which __attribute__ may be used, and the constructs to which attribute specifiers bind, for the C language. Some details may vary for C++ and Objective-C. Because of infelicities in the grammar for attributes, some forms described here may not be successfully parsed in all cases.

There are some problems with the semantics of attributes in C++. For example, there are no manglings for attributes, although they may affect code generation, so problems may arise when attributed types are used in conjunction with templates or overloading. Similarly, typeid does not distinguish between types with different attributes. Support for attributes in C++ may be restricted in future to attributes on declarations only, but not on nested declarators.

See section 5.25 Declaring Attributes of Functions, for details of the semantics of attributes applying to functions. See section 5.32 Specifying Attributes of Variables, for details of the semantics of attributes applying to variables. See section 5.33 Specifying Attributes of Types, for details of the semantics of attributes applying to structure, union and enumerated types.

An attribute specifier is of the form __attribute__ ((attribute-list)). An attribute list is a possibly empty comma-separated sequence of attributes, where each attribute is one of the following:

An attribute specifier list is a sequence of one or more attribute specifiers, not separated by any other tokens.

An attribute specifier list may appear after the colon following a label, other than a case or default label. The only attribute it makes sense to use after a label is unused. This feature is intended for code generated by programs which contains labels that may be unused but which is compiled with `-Wall'. It would not normally be appropriate to use in it human-written code, though it could be useful in cases where the code that jumps to the label is contained within an #ifdef conditional.

An attribute specifier list may appear as part of a struct, union or enum specifier. It may go either immediately after the struct, union or enum keyword, or after the closing brace. It is ignored if the content of the structure, union or enumerated type is not defined in the specifier in which the attribute specifier list is used--that is, in usages such as struct __attribute__((foo)) bar with no following opening brace. Where attribute specifiers follow the closing brace, they are considered to relate to the structure, union or enumerated type defined, not to any enclosing declaration the type specifier appears in, and the type defined is not complete until after the attribute specifiers.

Otherwise, an attribute specifier appears as part of a declaration, counting declarations of unnamed parameters and type names, and relates to that declaration (which may be nested in another declaration, for example in the case of a parameter declaration), or to a particular declarator within a declaration. Where an attribute specifier is applied to a parameter declared as a function or an array, it should apply to the function or array rather than the pointer to which the parameter is implicitly converted, but this is not yet correctly implemented.

Any list of specifiers and qualifiers at the start of a declaration may contain attribute specifiers, whether or not such a list may in that context contain storage class specifiers. (Some attributes, however, are essentially in the nature of storage class specifiers, and only make sense where storage class specifiers may be used; for example, section.) There is one necessary limitation to this syntax: the first old-style parameter declaration in a function definition cannot begin with an attribute specifier, because such an attribute applies to the function instead by syntax described below (which, however, is not yet implemented in this case). In some other cases, attribute specifiers are permitted by this grammar but not yet supported by the compiler. All attribute specifiers in this place relate to the declaration as a whole. In the obsolescent usage where a type of int is implied by the absence of type specifiers, such a list of specifiers and qualifiers may be an attribute specifier list with no other specifiers or qualifiers.

An attribute specifier list may appear immediately before a declarator (other than the first) in a comma-separated list of declarators in a declaration of more than one identifier using a single list of specifiers and qualifiers. Such attribute specifiers apply only to the identifier before whose declarator they appear. For example, in

 
__attribute__((noreturn)) void d0 (void),
    __attribute__((format(printf, 1, 2))) d1 (const char *, ...),
     d2 (void)

the noreturn attribute applies to all the functions declared; the format attribute only applies to d1.

An attribute specifier list may appear immediately before the comma, = or semicolon terminating the declaration of an identifier other than a function definition. At present, such attribute specifiers apply to the declared object or function, but in future they may attach to the outermost adjacent declarator. In simple cases there is no difference, but, for example, in

 
void (****f)(void) __attribute__((noreturn));

at present the noreturn attribute applies to f, which causes a warning since f is not a function, but in future it may apply to the function ****f. The precise semantics of what attributes in such cases will apply to are not yet specified. Where an assembler name for an object or function is specified (see section 5.37 Controlling Names Used in Assembler Code), at present the attribute must follow the asm specification; in future, attributes before the asm specification may apply to the adjacent declarator, and those after it to the declared object or function.

An attribute specifier list may, in future, be permitted to appear after the declarator in a function definition (before any old-style parameter declarations or the function body).

Attribute specifiers may be mixed with type qualifiers appearing inside the [] of a parameter array declarator, in the C99 construct by which such qualifiers are applied to the pointer to which the array is implicitly converted. Such attribute specifiers apply to the pointer, not to the array, but at present this is not implemented and they are ignored.

An attribute specifier list may appear at the start of a nested declarator. At present, there are some limitations in this usage: the attributes correctly apply to the declarator, but for most individual attributes the semantics this implies are not implemented. When attribute specifiers follow the * of a pointer declarator, they may be mixed with any type qualifiers present. The following describes the formal semantics of this syntax. It will make the most sense if you are familiar with the formal specification of declarators in the ISO C standard.

Consider (as in C99 subclause 6.7.5 paragraph 4) a declaration T D1, where T contains declaration specifiers that specify a type Type (such as int) and D1 is a declarator that contains an identifier ident. The type specified for ident for derived declarators whose type does not include an attribute specifier is as in the ISO C standard.

If D1 has the form ( attribute-specifier-list D ), and the declaration T D specifies the type "derived-declarator-type-list Type" for ident, then T D1 specifies the type "derived-declarator-type-list attribute-specifier-list Type" for ident.

If D1 has the form * type-qualifier-and-attribute-specifier-list D, and the declaration T D specifies the type "derived-declarator-type-list Type" for ident, then T D1 specifies the type "derived-declarator-type-list type-qualifier-and-attribute-specifier-list Type" for ident.

For example,

 
void (__attribute__((noreturn)) ****f) (void);

specifies the type "pointer to pointer to pointer to pointer to non-returning function returning void". As another example,

 
char *__attribute__((aligned(8))) *f;

specifies the type "pointer to 8-byte-aligned pointer to char". Note again that this does not work with most attributes; for example, the usage of `aligned' and `noreturn' attributes given above is not yet supported.

For compatibility with existing code written for compiler versions that did not implement attributes on nested declarators, some laxity is allowed in the placing of attributes. If an attribute that only applies to types is applied to a declaration, it will be treated as applying to the type of that declaration. If an attribute that only applies to declarations is applied to the type of a declaration, it will be treated as applying to that declaration; and, for compatibility with code placing the attributes immediately before the identifier declared, such an attribute applied to a function return type will be treated as applying to the function type, and such an attribute applied to an array element type will be treated as applying to the array type. If an attribute that only applies to function types is applied to a pointer-to-function type, it will be treated as applying to the pointer target type; if such an attribute is applied to a function return type that is not a pointer-to-function type, it will be treated as applying to the function type.


5.27 Prototypes and Old-Style Function Definitions

GNU C extends ISO C to allow a function prototype to override a later old-style non-prototype definition. Consider the following example:

 
/* Use prototypes unless the compiler is old-fashioned.  */
#ifdef __STDC__
#define P(x) x
#else
#define P(x) ()
#endif

/* Prototype function declaration.  */
int isroot P((uid_t));

/* Old-style function definition.  */
int
isroot (x)   /* ??? lossage here ??? */
     uid_t x;
{
  return x == 0;
}

Suppose the type uid_t happens to be short. ISO C does not allow this example, because subword arguments in old-style non-prototype definitions are promoted. Therefore in this example the function definition's argument is really an int, which does not match the prototype argument type of short.

This restriction of ISO C makes it hard to write code that is portable to traditional C compilers, because the programmer does not know whether the uid_t type is short, int, or long. Therefore, in cases like these GNU C allows a prototype to override a later old-style definition. More precisely, in GNU C, a function prototype argument type overrides the argument type specified by a later old-style definition if the former type is the same as the latter type before promotion. Thus in GNU C the above example is equivalent to the following:

 
int isroot (uid_t);

int
isroot (uid_t x)
{
  return x == 0;
}

GNU C++ does not support old-style function definitions, so this extension is irrelevant.


5.28 C++ Style Comments

In GNU C, you may use C++ style comments, which start with `//' and continue until the end of the line. Many other C implementations allow such comments, and they are included in the 1999 C standard. However, C++ style comments are not recognized if you specify an `-std' option specifying a version of ISO C before C99, or `-ansi' (equivalent to `-std=c89').


5.29 Dollar Signs in Identifier Names

In GNU C, you may normally use dollar signs in identifier names. This is because many traditional C implementations allow such identifiers. However, dollar signs in identifiers are not supported on a few target machines, typically because the target assembler does not allow them.


5.30 The Character ESC in Constants

You can use the sequence `\e' in a string or character constant to stand for the ASCII character ESC.


5.31 Inquiring on Alignment of Types or Variables

The keyword __alignof__ allows you to inquire about how an object is aligned, or the minimum alignment usually required by a type. Its syntax is just like sizeof.

For example, if the target machine requires a double value to be aligned on an 8-byte boundary, then __alignof__ (double) is 8. This is true on many RISC machines. On more traditional machine designs, __alignof__ (double) is 4 or even 2.

Some machines never actually require alignment; they allow reference to any data type even at an odd address. For these machines, __alignof__ reports the recommended alignment of a type.

If the operand of __alignof__ is an lvalue rather than a type, its value is the required alignment for its type, taking into account any minimum alignment specified with GCC's __attribute__ extension (see section 5.32 Specifying Attributes of Variables). For example, after this declaration:

 
struct foo { int x; char y; } foo1;

the value of __alignof__ (foo1.y) is 1, even though its actual alignment is probably 2 or 4, the same as __alignof__ (int).

It is an error to ask for the alignment of an incomplete type.


5.32 Specifying Attributes of Variables

The keyword __attribute__ allows you to specify special attributes of variables or structure fields. This keyword is followed by an attribute specification inside double parentheses. Some attributes are currently defined generically for variables. Other attributes are defined for variables on particular target systems. Other attributes are available for functions (see section 5.25 Declaring Attributes of Functions) and for types (see section 5.33 Specifying Attributes of Types). Other front ends might define more attributes (see section Extensions to the C++ Language).

You may also specify attributes with `__' preceding and following each keyword. This allows you to use them in header files without being concerned about a possible macro of the same name. For example, you may use __aligned__ instead of aligned.

See section 5.26 Attribute Syntax, for details of the exact syntax for using attributes.

aligned (alignment)
This attribute specifies a minimum alignment for the variable or structure field, measured in bytes. For example, the declaration:

 
int x __attribute__ ((aligned (16))) = 0;

causes the compiler to allocate the global variable x on a 16-byte boundary. On a 68040, this could be used in conjunction with an asm expression to access the move16 instruction which requires 16-byte aligned operands.

You can also specify the alignment of structure fields. For example, to create a double-word aligned int pair, you could write:

 
struct foo { int x[2] __attribute__ ((aligned (8))); };

This is an alternative to creating a union with a double member that forces the union to be double-word aligned.

As in the preceding examples, you can explicitly specify the alignment (in bytes) that you wish the compiler to use for a given variable or structure field. Alternatively, you can leave out the alignment factor and just ask the compiler to align a variable or field to the maximum useful alignment for the target machine you are compiling for. For example, you could write:

 
short array[3] __attribute__ ((aligned));

Whenever you leave out the alignment factor in an aligned attribute specification, the compiler automatically sets the alignment for the declared variable or field to the largest alignment which is ever used for any data type on the target machine you are compiling for. Doing this can often make copy operations more efficient, because the compiler can use whatever instructions copy the biggest chunks of memory when performing copies to or from the variables or fields that you have aligned this way.

The aligned attribute can only increase the alignment; but you can decrease it by specifying packed as well. See below.

Note that the effectiveness of aligned attributes may be limited by inherent limitations in your linker. On many systems, the linker is only able to arrange for variables to be aligned up to a certain maximum alignment. (For some linkers, the maximum supported alignment may be very very small.) If your linker is only able to align variables up to a maximum of 8 byte alignment, then specifying aligned(16) in an __attribute__ will still only provide you with 8 byte alignment. See your linker documentation for further information.

cleanup (cleanup_function)
The cleanup attribute runs a function when the variable goes out of scope. This attribute can only be applied to auto function scope variables; it may not be applied to parameters or variables with static storage duration. The function must take one parameter, a pointer to a type compatible with the variable. The return value of the function (if any) is ignored.

If `-fexceptions' is enabled, then cleanup_function will be run during the stack unwinding that happens during the processing of the exception. Note that the cleanup attribute does not allow the exception to be caught, only to perform an action. It is undefined what happens if cleanup_function does not return normally.

common
nocommon
The common attribute requests GCC to place a variable in "common" storage. The nocommon attribute requests the opposite -- to allocate space for it directly.

These attributes override the default chosen by the `-fno-common' and `-fcommon' flags respectively.

deprecated
The deprecated attribute results in a warning if the variable is used anywhere in the source file. This is useful when identifying variables that are expected to be removed in a future version of a program. The warning also includes the location of the declaration of the deprecated variable, to enable users to easily find further information about why the variable is deprecated, or what they should do instead. Note that the warning only occurs for uses:

 
extern int old_var __attribute__ ((deprecated));
extern int old_var;
int new_fn () { return old_var; }

results in a warning on line 3 but not line 2.

The deprecated attribute can also be used for functions and types (see section 5.25 Declaring Attributes of Functions, see section 5.33 Specifying Attributes of Types.)

mode (mode)
This attribute specifies the data type for the declaration--whichever type corresponds to the mode mode. This in effect lets you request an integer or floating point type according to its width.

You may also specify a mode of `byte' or `__byte__' to indicate the mode corresponding to a one-byte integer, `word' or `__word__' for the mode of a one-word integer, and `pointer' or `__pointer__' for the mode used to represent pointers.

packed
The packed attribute specifies that a variable or structure field should have the smallest possible alignment--one byte for a variable, and one bit for a field, unless you specify a larger value with the aligned attribute.

Here is a structure in which the field x is packed, so that it immediately follows a:

 
struct foo
{
  char a;
  int x[2] __attribute__ ((packed));
};

section ("section-name")
Normally, the compiler places the objects it generates in sections like data and bss. Sometimes, however, you need additional sections, or you need certain particular variables to appear in special sections, for example to map to special hardware. The section attribute specifies that a variable (or function) lives in a particular section. For example, this small program uses several specific section names:

 
struct duart a __attribute__ ((section ("DUART_A"))) = { 0 };
struct duart b __attribute__ ((section ("DUART_B"))) = { 0 };
char stack[10000] __attribute__ ((section ("STACK"))) = { 0 };
int init_data __attribute__ ((section ("INITDATA"))) = 0;

main()
{
  /* Initialize stack pointer */
  init_sp (stack + sizeof (stack));

  /* Initialize initialized data */
  memcpy (&init_data, &data, &edata - &data);

  /* Turn on the serial ports */
  init_duart (&a);
  init_duart (&b);
}

Use the section attribute with an initialized definition of a global variable, as shown in the example. GCC issues a warning and otherwise ignores the section attribute in uninitialized variable declarations.

You may only use the section attribute with a fully initialized global definition because of the way linkers work. The linker requires each object be defined once, with the exception that uninitialized variables tentatively go in the common (or bss) section and can be multiply "defined". You can force a variable to be initialized with the `-fno-common' flag or the nocommon attribute.

Some file formats do not support arbitrary sections so the section attribute is not available on all platforms. If you need to map the entire contents of a module to a particular section, consider using the facilities of the linker instead.

shared
On Windows, in addition to putting variable definitions in a named section, the section can also be shared among all running copies of an executable or DLL. For example, this small program defines shared data by putting it in a named section shared and marking the section shareable:

 
int foo __attribute__((section ("shared"), shared)) = 0;

int
main()
{
  /* Read and write foo.  All running
     copies see the same value.  */
  return 0;
}

You may only use the shared attribute along with section attribute with a fully initialized global definition because of the way linkers work. See section attribute for more information.

The shared attribute is only available on Windows.

tls_model ("tls_model")
The tls_model attribute sets thread-local storage model (see section 5.48 Thread-Local Storage) of a particular __thread variable, overriding -ftls-model= command line switch on a per-variable basis. The tls_model argument should be one of global-dynamic, local-dynamic, initial-exec or local-exec.

Not all targets support this attribute.

transparent_union
This attribute, attached to a function parameter which is a union, means that the corresponding argument may have the type of any union member, but the argument is passed as if its type were that of the first union member. For more details see See section 5.33 Specifying Attributes of Types. You can also use this attribute on a typedef for a union data type; then it applies to all function parameters with that type.

unused
This attribute, attached to a variable, means that the variable is meant to be possibly unused. GCC will not produce a warning for this variable.

vector_size (bytes)
This attribute specifies the vector size for the variable, measured in bytes. For example, the declaration:

 
int foo __attribute__ ((vector_size (16)));

causes the compiler to set the mode for foo, to be 16 bytes, divided into int sized units. Assuming a 32-bit int (a vector of 4 units of 4 bytes), the corresponding mode of foo will be V4SI.

This attribute is only applicable to integral and float scalars, although arrays, pointers, and function return values are allowed in conjunction with this construct.

Aggregates with this attribute are invalid, even if they are of the same size as a corresponding scalar. For example, the declaration:

 
struct S { int a; };
struct S  __attribute__ ((vector_size (16))) foo;

is invalid even if the size of the structure is the same as the size of the int.

weak
The weak attribute is described in See section 5.25 Declaring Attributes of Functions.

model (model-name)
Use this attribute on the M32R/D to set the addressability of an object. The identifier model-name is one of small, medium, or large, representing each of the code models.

Small model objects live in the lower 16MB of memory (so that their addresses can be loaded with the ld24 instruction).

Medium and large model objects may live anywhere in the 32-bit address space (the compiler will generate seth/add3 instructions to load their addresses).

dllimport
The dllimport attribute is described in See section 5.25 Declaring Attributes of Functions.

dlexport
The dllexport attribute is described in See section 5.25 Declaring Attributes of Functions.

To specify multiple attributes, separate them by commas within the double parentheses: for example, `__attribute__ ((aligned (16), packed))'.


5.33 Specifying Attributes of Types

The keyword __attribute__ allows you to specify special attributes of struct and union types when you define such types. This keyword is followed by an attribute specification inside double parentheses. Six attributes are currently defined for types: aligned, packed, transparent_union, unused, deprecated and may_alias. Other attributes are defined for functions (see section 5.25 Declaring Attributes of Functions) and for variables (see section 5.32 Specifying Attributes of Variables).

You may also specify any one of these attributes with `__' preceding and following its keyword. This allows you to use these attributes in header files without being concerned about a possible macro of the same name. For example, you may use __aligned__ instead of aligned.

You may specify the aligned and transparent_union attributes either in a typedef declaration or just past the closing curly brace of a complete enum, struct or union type definition and the packed attribute only past the closing brace of a definition.

You may also specify attributes between the enum, struct or union tag and the name of the type rather than after the closing brace.

See section 5.26 Attribute Syntax, for details of the exact syntax for using attributes.

aligned (alignment)
This attribute specifies a minimum alignment (in bytes) for variables of the specified type. For example, the declarations:

 
struct S { short f[3]; } __attribute__ ((aligned (8)));
typedef int more_aligned_int __attribute__ ((aligned (8)));

force the compiler to insure (as far as it can) that each variable whose type is struct S or more_aligned_int will be allocated and aligned at least on a 8-byte boundary. On a SPARC, having all variables of type struct S aligned to 8-byte boundaries allows the compiler to use the ldd and std (doubleword load and store) instructions when copying one variable of type struct S to another, thus improving run-time efficiency.

Note that the alignment of any given struct or union type is required by the ISO C standard to be at least a perfect multiple of the lowest common multiple of the alignments of all of the members of the struct or union in question. This means that you can effectively adjust the alignment of a struct or union type by attaching an aligned attribute to any one of the members of such a type, but the notation illustrated in the example above is a more obvious, intuitive, and readable way to request the compiler to adjust the alignment of an entire struct or union type.

As in the preceding example, you can explicitly specify the alignment (in bytes) that you wish the compiler to use for a given struct or union type. Alternatively, you can leave out the alignment factor and just ask the compiler to align a type to the maximum useful alignment for the target machine you are compiling for. For example, you could write:

 
struct S { short f[3]; } __attribute__ ((aligned));

Whenever you leave out the alignment factor in an aligned attribute specification, the compiler automatically sets the alignment for the type to the largest alignment which is ever used for any data type on the target machine you are compiling for. Doing this can often make copy operations more efficient, because the compiler can use whatever instructions copy the biggest chunks of memory when performing copies to or from the variables which have types that you have aligned this way.

In the example above, if the size of each short is 2 bytes, then the size of the entire struct S type is 6 bytes. The smallest power of two which is greater than or equal to that is 8, so the compiler sets the alignment for the entire struct S type to 8 bytes.

Note that although you can ask the compiler to select a time-efficient alignment for a given type and then declare only individual stand-alone objects of that type, the compiler's ability to select a time-efficient alignment is primarily useful only when you plan to create arrays of variables having the relevant (efficiently aligned) type. If you declare or use arrays of variables of an efficiently-aligned type, then it is likely that your program will also be doing pointer arithmetic (or subscripting, which amounts to the same thing) on pointers to the relevant type, and the code that the compiler generates for these pointer arithmetic operations will often be more efficient for efficiently-aligned types than for other types.

The aligned attribute can only increase the alignment; but you can decrease it by specifying packed as well. See below.

Note that the effectiveness of aligned attributes may be limited by inherent limitations in your linker. On many systems, the linker is only able to arrange for variables to be aligned up to a certain maximum alignment. (For some linkers, the maximum supported alignment may be very very small.) If your linker is only able to align variables up to a maximum of 8 byte alignment, then specifying aligned(16) in an __attribute__ will still only provide you with 8 byte alignment. See your linker documentation for further information.

packed
This attribute, attached to an enum, struct, or union type definition, specifies that the minimum required memory be used to represent the type.

Specifying this attribute for struct and union types is equivalent to specifying the packed attribute on each of the structure or union members. Specifying the `-fshort-enums' flag on the line is equivalent to specifying the packed attribute on all enum definitions.

You may only specify this attribute after a closing curly brace on an enum definition, not in a typedef declaration, unless that declaration also contains the definition of the enum.

transparent_union
This attribute, attached to a union type definition, indicates that any function parameter having that union type causes calls to that function to be treated in a special way.

First, the argument corresponding to a transparent union type can be of any type in the union; no cast is required. Also, if the union contains a pointer type, the corresponding argument can be a null pointer constant or a void pointer expression; and if the union contains a void pointer type, the corresponding argument can be any pointer expression. If the union member type is a pointer, qualifiers like const on the referenced type must be respected, just as with normal pointer conversions.

Second, the argument is passed to the function using the calling conventions of the first member of the transparent union, not the calling conventions of the union itself. All members of the union must have the same machine representation; this is necessary for this argument passing to work properly.

Transparent unions are designed for library functions that have multiple interfaces for compatibility reasons. For example, suppose the wait function must accept either a value of type int * to comply with Posix, or a value of type union wait * to comply with the 4.1BSD interface. If wait's parameter were void *, wait would accept both kinds of arguments, but it would also accept any other pointer type and this would make argument type checking less useful. Instead, <sys/wait.h> might define the interface as follows:

 
typedef union
  {
    int *__ip;
    union wait *__up;
  } wait_status_ptr_t __attribute__ ((__transparent_union__));

pid_t wait (wait_status_ptr_t);

This interface allows either int * or union wait * arguments to be passed, using the int * calling convention. The program can call wait with arguments of either type:

 
int w1 () { int w; return wait (&w); }
int w2 () { union wait w; return wait (&w); }

With this interface, wait's implementation might look like this:

 
pid_t wait (wait_status_ptr_t p)
{
  return waitpid (-1, p.__ip, 0);
}

unused
When attached to a type (including a union or a struct), this attribute means that variables of that type are meant to appear possibly unused. GCC will not produce a warning for any variables of that type, even if the variable appears to do nothing. This is often the case with lock or thread classes, which are usually defined and then not referenced, but contain constructors and destructors that have nontrivial bookkeeping functions.

deprecated
The deprecated attribute results in a warning if the type is used anywhere in the source file. This is useful when identifying types that are expected to be removed in a future version of a program. If possible, the warning also includes the location of the declaration of the deprecated type, to enable users to easily find further information about why the type is deprecated, or what they should do instead. Note that the warnings only occur for uses and then only if the type is being applied to an identifier that itself is not being declared as deprecated.

 
typedef int T1 __attribute__ ((deprecated));
T1 x;
typedef T1 T2;
T2 y;
typedef T1 T3 __attribute__ ((deprecated));
T3 z __attribute__ ((deprecated));

results in a warning on line 2 and 3 but not lines 4, 5, or 6. No warning is issued for line 4 because T2 is not explicitly deprecated. Line 5 has no warning because T3 is explicitly deprecated. Similarly for line 6.

The deprecated attribute can also be used for functions and variables (see section 5.25 Declaring Attributes of Functions, see section 5.32 Specifying Attributes of Variables.)

may_alias
Accesses to objects with types with this attribute are not subjected to type-based alias analysis, but are instead assumed to be able to alias any other type of objects, just like the char type. See `-fstrict-aliasing' for more information on aliasing issues.

Example of use:

 
typedef short __attribute__((__may_alias__)) short_a;

int
main (void)
{
  int a = 0x12345678;
  short_a *b = (short_a *) &a;

  b[1] = 0;

  if (a == 0x12345678)
    abort();

  exit(0);
}

If you replaced short_a with short in the variable declaration, the above program would abort when compiled with `-fstrict-aliasing', which is on by default at `-O2' or above in recent GCC versions.

To specify multiple attributes, separate them by commas within the double parentheses: for example, `__attribute__ ((aligned (16), packed))'.


5.34 An Inline Function is As Fast As a Macro

By declaring a function inline, you can direct GCC to integrate that function's code into the code for its callers. This makes execution faster by eliminating the function-call overhead; in addition, if any of the actual argument values are constant, their known values may permit simplifications at compile time so that not all of the inline function's code needs to be included. The effect on code size is less predictable; object code may be larger or smaller with function inlining, depending on the particular case. Inlining of functions is an optimization and it really "works" only in optimizing compilation. If you don't use `-O', no function is really inline.

Inline functions are included in the ISO C99 standard, but there are currently substantial differences between what GCC implements and what the ISO C99 standard requires.

To declare a function inline, use the inline keyword in its declaration, like this:

 
inline int
inc (int *a)
{
  (*a)++;
}

(If you are writing a header file to be included in ISO C programs, write __inline__ instead of inline. See section 5.39 Alternate Keywords.) You can also make all "simple enough" functions inline with the option `-finline-functions'.

Note that certain usages in a function definition can make it unsuitable for inline substitution. Among these usages are: use of varargs, use of alloca, use of variable sized data types (see section 5.14 Arrays of Variable Length), use of computed goto (see section 5.3 Labels as Values), use of nonlocal goto, and nested functions (see section 5.4 Nested Functions). Using `-Winline' will warn when a function marked inline could not be substituted, and will give the reason for the failure.

Note that in C and Objective-C, unlike C++, the inline keyword does not affect the linkage of the function.

GCC automatically inlines member functions defined within the class body of C++ programs even if they are not explicitly declared inline. (You can override this with `-fno-default-inline'; see section Options Controlling C++ Dialect.)

When a function is both inline and static, if all calls to the function are integrated into the caller, and the function's address is never used, then the function's own assembler code is never referenced. In this case, GCC does not actually output assembler code for the function, unless you specify the option `-fkeep-inline-functions'. Some calls cannot be integrated for various reasons (in particular, calls that precede the function's definition cannot be integrated, and neither can recursive calls within the definition). If there is a nonintegrated call, then the function is compiled to assembler code as usual. The function must also be compiled as usual if the program refers to its address, because that can't be inlined.

When an inline function is not static, then the compiler must assume that there may be calls from other source files; since a global symbol can be defined only once in any program, the function must not be defined in the other source files, so the calls therein cannot be integrated. Therefore, a non-static inline function is always compiled on its own in the usual fashion.

If you specify both inline and extern in the function definition, then the definition is used only for inlining. In no case is the function compiled on its own, not even if you refer to its address explicitly. Such an address becomes an external reference, as if you had only declared the function, and had not defined it.

This combination of inline and extern has almost the effect of a macro. The way to use it is to put a function definition in a header file with these keywords, and put another copy of the definition (lacking inline and extern) in a library file. The definition in the header file will cause most calls to the function to be inlined. If any uses of the function remain, they will refer to the single copy in the library.

Since GCC eventually will implement ISO C99 semantics for inline functions, it is best to use static inline only to guarantee compatibility. (The existing semantics will remain available when `-std=gnu89' is specified, but eventually the default will be `-std=gnu99' and that will implement the C99 semantics, though it does not do so yet.)

GCC does not inline any functions when not optimizing unless you specify the `always_inline' attribute for the function, like this:

 
/* Prototype.  */
inline void foo (const char) __attribute__((always_inline));


5.35 Assembler Instructions with C Expression Operands

In an assembler instruction using asm, you can specify the operands of the instruction using C expressions. This means you need not guess which registers or memory locations will contain the data you want to use.

You must specify an assembler instruction template much like what appears in a machine description, plus an operand constraint string for each operand.

For example, here is how to use the 68881's fsinx instruction:

 
asm ("fsinx %1,%0" : "=f" (result) : "f" (angle));

Here angle is the C expression for the input operand while result is that of the output operand. Each has `"f"' as its operand constraint, saying that a floating point register is required. The `=' in `=f' indicates that the operand is an output; all output operands' constraints must use `='. The constraints use the same language used in the machine description (see section 5.36 Constraints for asm Operands).

Each operand is described by an operand-constraint string followed by the C expression in parentheses. A colon separates the assembler template from the first output operand and another separates the last output operand from the first input, if any. Commas separate the operands within each group. The total number of operands is currently limited to 30; this limitation may be lifted in some future version of GCC.

If there are no output operands but there are input operands, you must place two consecutive colons surrounding the place where the output operands would go.

As of GCC version 3.1, it is also possible to specify input and output operands using symbolic names which can be referenced within the assembler code. These names are specified inside square brackets preceding the constraint string, and can be referenced inside the assembler code using %[name] instead of a percentage sign followed by the operand number. Using named operands the above example could look like:

 
asm ("fsinx %[angle],%[output]"
     : [output] "=f" (result)
     : [angle] "f" (angle));

Note that the symbolic operand names have no relation whatsoever to other C identifiers. You may use any name you like, even those of existing C symbols, but you must ensure that no two operands within the same assembler construct use the same symbolic name.

Output operand expressions must be lvalues; the compiler can check this. The input operands need not be lvalues. The compiler cannot check whether the operands have data types that are reasonable for the instruction being executed. It does not parse the assembler instruction template and does not know what it means or even whether it is valid assembler input. The extended asm feature is most often used for machine instructions the compiler itself does not know exist. If the output expression cannot be directly addressed (for example, it is a bit-field), your constraint must allow a register. In that case, GCC will use the register as the output of the asm, and then store that register into the output.

The ordinary output operands must be write-only; GCC will assume that the values in these operands before the instruction are dead and need not be generated. Extended asm supports input-output or read-write operands. Use the constraint character `+' to indicate such an operand and list it with the output operands.

When the constraints for the read-write operand (or the operand in which only some of the bits are to be changed) allows a register, you may, as an alternative, logically split its function into two separate operands, one input operand and one write-only output operand. The connection between them is expressed by constraints which say they need to be in the same location when the instruction executes. You can use the same C expression for both operands, or different expressions. For example, here we write the (fictitious) `combine' instruction with bar as its read-only source operand and foo as its read-write destination:

 
asm ("combine %2,%0" : "=r" (foo) : "0" (foo), "g" (bar));

The constraint `"0"' for operand 1 says that it must occupy the same location as operand 0. A number in constraint is allowed only in an input operand and it must refer to an output operand.

Only a number in the constraint can guarantee that one operand will be in the same place as another. The mere fact that foo is the value of both operands is not enough to guarantee that they will be in the same place in the generated assembler code. The following would not work reliably:

 
asm ("combine %2,%0" : "=r" (foo) : "r" (foo), "g" (bar));

Various optimizations or reloading could cause operands 0 and 1 to be in different registers; GCC knows no reason not to do so. For example, the compiler might find a copy of the value of foo in one register and use it for operand 1, but generate the output operand 0 in a different register (copying it afterward to foo's own address). Of course, since the register for operand 1 is not even mentioned in the assembler code, the result will not work, but GCC can't tell that.

As of GCC version 3.1, one may write [name] instead of the operand number for a matching constraint. For example:

 
asm ("cmoveq %1,%2,%[result]"
     : [result] "=r"(result)
     : "r" (test), "r"(new), "[result]"(old));

Some instructions clobber specific hard registers. To describe this, write a third colon after the input operands, followed by the names of the clobbered hard registers (given as strings). Here is a realistic example for the VAX:

 
asm volatile ("movc3 %0,%1,%2"
              : /* no outputs */
              : "g" (from), "g" (to), "g" (count)
              : "r0", "r1", "r2", "r3", "r4", "r5");

You may not write a clobber description in a way that overlaps with an input or output operand. For example, you may not have an operand describing a register class with one member if you mention that register in the clobber list. Variables declared to live in specific registers (see section 5.38 Variables in Specified Registers), and used as asm input or output operands must have no part mentioned in the clobber description. There is no way for you to specify that an input operand is modified without also specifying it as an output operand. Note that if all the output operands you specify are for this purpose (and hence unused), you will then also need to specify volatile for the asm construct, as described below, to prevent GCC from deleting the asm statement as unused.

If you refer to a particular hardware register from the assembler code, you will probably have to list the register after the third colon to tell the compiler the register's value is modified. In some assemblers, the register names begin with `%'; to produce one `%' in the assembler code, you must write `%%' in the input.

If your assembler instruction can alter the condition code register, add `cc' to the list of clobbered registers. GCC on some machines represents the condition codes as a specific hardware register; `cc' serves to name this register. On other machines, the condition code is handled differently, and specifying `cc' has no effect. But it is valid no matter what the machine.

If your assembler instructions access memory in an unpredictable fashion, add `memory' to the list of clobbered registers. This will cause GCC to not keep memory values cached in registers across the assembler instruction and not optimize stores or loads to that memory. You will also want to add the volatile keyword if the memory affected is not listed in the inputs or outputs of the asm, as the `memory' clobber does not count as a side-effect of the asm. If you know how large the accessed memory is, you can add it as input or output but if this is not known, you should add `memory'. As an example, if you access ten bytes of a string, you can use a memory input like:

 
{"m"( ({ struct { char x[10]; } *p = (void *)ptr ; *p; }) )}.

Note that in the following example the memory input is necessary, otherwise GCC might optimize the store to x away:
 
int foo ()
{
  int x = 42;
  int *y = &x;
  int result;
  asm ("magic stuff accessing an 'int' pointed to by '%1'"
        "=&d" (r) : "a" (y), "m" (*y));
  return result;     
}

You can put multiple assembler instructions together in a single asm template, separated by the characters normally used in assembly code for the system. A combination that works in most places is a newline to break the line, plus a tab character to move to the instruction field (written as `\n\t'). Sometimes semicolons can be used, if the assembler allows semicolons as a line-breaking character. Note that some assembler dialects use semicolons to start a comment. The input operands are guaranteed not to use any of the clobbered registers, and neither will the output operands' addresses, so you can read and write the clobbered registers as many times as you like. Here is an example of multiple instructions in a template; it assumes the subroutine _foo accepts arguments in registers 9 and 10:

 
asm ("movl %0,r9\n\tmovl %1,r10\n\tcall _foo"
     : /* no outputs */
     : "g" (from), "g" (to)
     : "r9", "r10");

Unless an output operand has the `&' constraint modifier, GCC may allocate it in the same register as an unrelated input operand, on the assumption the inputs are consumed before the outputs are produced. This assumption may be false if the assembler code actually consists of more than one instruction. In such a case, use `&' for each output operand that may not overlap an input. See section 5.36.3 Constraint Modifier Characters.

If you want to test the condition code produced by an assembler instruction, you must include a branch and a label in the asm construct, as follows:

 
asm ("clr %0\n\tfrob %1\n\tbeq 0f\n\tmov #1,%0\n0:"
     : "g" (result)
     : "g" (input));

This assumes your assembler supports local labels, as the GNU assembler and most Unix assemblers do.

Speaking of labels, jumps from one asm to another are not supported. The compiler's optimizers do not know about these jumps, and therefore they cannot take account of them when deciding how to optimize.

Usually the most convenient way to use these asm instructions is to encapsulate them in macros that look like functions. For example,

 
#define sin(x)       \
({ double __value, __arg = (x);   \
   asm ("fsinx %1,%0": "=f" (__value): "f" (__arg));  \
   __value; })

Here the variable __arg is used to make sure that the instruction operates on a proper double value, and to accept only those arguments x which can convert automatically to a double.

Another way to make sure the instruction operates on the correct data type is to use a cast in the asm. This is different from using a variable __arg in that it converts more different types. For example, if the desired type were int, casting the argument to int would accept a pointer with no complaint, while assigning the argument to an int variable named __arg would warn about using a pointer unless the caller explicitly casts it.

If an asm has output operands, GCC assumes for optimization purposes the instruction has no side effects except to change the output operands. This does not mean instructions with a side effect cannot be used, but you must be careful, because the compiler may eliminate them if the output operands aren't used, or move them out of loops, or replace two with one if they constitute a common subexpression. Also, if your instruction does have a side effect on a variable that otherwise appears not to change, the old value of the variable may be reused later if it happens to be found in a register.

You can prevent an asm instruction from being deleted, moved significantly, or combined, by writing the keyword volatile after the asm. For example:

 
#define get_and_set_priority(new)              \
({ int __old;                                  \
   asm volatile ("get_and_set_priority %0, %1" \
                 : "=g" (__old) : "g" (new));  \
   __old; })

If you write an asm instruction with no outputs, GCC will know the instruction has side-effects and will not delete the instruction or move it outside of loops.

The volatile keyword indicates that the instruction has important side-effects. GCC will not delete a volatile asm if it is reachable. (The instruction can still be deleted if GCC can prove that control-flow will never reach the location of the instruction.) In addition, GCC will not reschedule instructions across a volatile asm instruction. For example:

 
*(volatile int *)addr = foo;
asm volatile ("eieio" : : );

Assume addr contains the address of a memory mapped device register. The PowerPC eieio instruction (Enforce In-order Execution of I/O) tells the CPU to make sure that the store to that device register happens before it issues any other I/O.

Note that even a volatile asm instruction can be moved in ways that appear insignificant to the compiler, such as across jump instructions. You can't expect a sequence of volatile asm instructions to remain perfectly consecutive. If you want consecutive output, use a single asm. Also, GCC will perform some optimizations across a volatile asm instruction; GCC does not "forget everything" when it encounters a volatile asm instruction the way some other compilers do.

An asm instruction without any operands or clobbers (an "old style" asm) will be treated identically to a volatile asm instruction.

It is a natural idea to look for a way to give access to the condition code left by the assembler instruction. However, when we attempted to implement this, we found no way to make it work reliably. The problem is that output operands might need reloading, which would result in additional following "store" instructions. On most machines, these instructions would alter the condition code before there was time to test it. This problem doesn't arise for ordinary "test" and "compare" instructions because they don't have any output operands.

For reasons similar to those described above, it is not possible to give an assembler instruction access to the condition code left by previous instructions.

If you are writing a header file that should be includable in ISO C programs, write __asm__ instead of asm. See section 5.39 Alternate Keywords.


5.35.1 i386 floating point asm operands

There are several rules on the usage of stack-like regs in asm_operands insns. These rules apply only to the operands that are stack-like regs:

  1. Given a set of input regs that die in an asm_operands, it is necessary to know which are implicitly popped by the asm, and which must be explicitly popped by gcc.

    An input reg that is implicitly popped by the asm must be explicitly clobbered, unless it is constrained to match an output operand.

  2. For any input reg that is implicitly popped by an asm, it is necessary to know how to adjust the stack to compensate for the pop. If any non-popped input is closer to the top of the reg-stack than the implicitly popped reg, it would not be possible to know what the stack looked like--it's not clear how the rest of the stack "slides up".

    All implicitly popped input regs must be closer to the top of the reg-stack than any input that is not implicitly popped.

    It is possible that if an input dies in an insn, reload might use the input reg for an output reload. Consider this example:

     
    asm ("foo" : "=t" (a) : "f" (b));
    

    This asm says that input B is not popped by the asm, and that the asm pushes a result onto the reg-stack, i.e., the stack is one deeper after the asm than it was before. But, it is possible that reload will think that it can use the same reg for both the input and the output, if input B dies in this insn.

    If any input operand uses the f constraint, all output reg constraints must use the & earlyclobber.

    The asm above would be written as

     
    asm ("foo" : "=&t" (a) : "f" (b));
    

  3. Some operands need to be in particular places on the stack. All output operands fall in this category--there is no other way to know which regs the outputs appear in unless the user indicates this in the constraints.

    Output operands must specifically indicate which reg an output appears in after an asm. =f is not allowed: the operand constraints must select a class with a single reg.

  4. Output operands may not be "inserted" between existing stack regs. Since no 387 opcode uses a read/write operand, all output operands are dead before the asm_operands, and are pushed by the asm_operands. It makes no sense to push anywhere but the top of the reg-stack.

    Output operands must start at the top of the reg-stack: output operands may not "skip" a reg.

  5. Some asm statements may need extra stack space for internal calculations. This can be guaranteed by clobbering stack registers unrelated to the inputs and outputs.

Here are a couple of reasonable asms to want to write. This asm takes one input, which is internally popped, and produces two outputs.

 
asm ("fsincos" : "=t" (cos), "=u" (sin) : "0" (inp));

This asm takes two inputs, which are popped by the fyl2xp1 opcode, and replaces them with one output. The user must code the st(1) clobber for reg-stack.c to know that fyl2xp1 pops both inputs.

 
asm ("fyl2xp1" : "=t" (result) : "0" (x), "u" (y) : "st(1)");


5.36 Constraints for asm Operands

Here are specific details on what constraint letters you can use with asm operands. Constraints can say whether an operand may be in a register, and which kinds of register; whether the operand can be a memory reference, and which kinds of address; whether the operand may be an immediate constant, and which possible values it may have. Constraints can also require two operands to match.

5.36.1 Simple Constraints  Basic use of constraints.
5.36.2 Multiple Alternative Constraints  When an insn has two alternative constraint-patterns.
5.36.3 Constraint Modifier Characters  More precise control over effects of constraints.
5.36.4 Constraints for Particular Machines  Special constraints for some particular machines.


5.36.1 Simple Constraints

The simplest kind of constraint is a string full of letters, each of which describes one kind of operand that is permitted. Here are the letters that are allowed:

whitespace
Whitespace characters are ignored and can be inserted at any position except the first. This enables each alternative for different operands to be visually aligned in the machine description even if they have different number of constraints and modifiers.

`m'
A memory operand is allowed, with any kind of address that the machine supports in general.

`o'
A memory operand is allowed, but only if the address is offsettable. This means that adding a small integer (actually, the width in bytes of the operand, as determined by its machine mode) may be added to the address and the result is also a valid memory address.

For example, an address which is constant is offsettable; so is an address that is the sum of a register and a constant (as long as a slightly larger constant is also within the range of address-offsets supported by the machine); but an autoincrement or autodecrement address is not offsettable. More complicated indirect/indexed addresses may or may not be offsettable depending on the other addressing modes that the machine supports.

Note that in an output operand which can be matched by another operand, the constraint letter `o' is valid only when accompanied by both `<' (if the target machine has predecrement addressing) and `>' (if the target machine has preincrement addressing).

`V'
A memory operand that is not offsettable. In other words, anything that would fit the `m' constraint but not the `o' constraint.

`<'
A memory operand with autodecrement addressing (either predecrement or postdecrement) is allowed.

`>'
A memory operand with autoincrement addressing (either preincrement or postincrement) is allowed.

`r'
A register operand is allowed provided that it is in a general register.

`i'
An immediate integer operand (one with constant value) is allowed. This includes symbolic constants whose values will be known only at assembly time.

`n'
An immediate integer operand with a known numeric value is allowed. Many systems cannot support assembly-time constants for operands less than a word wide. Constraints for these operands should use `n' rather than `i'.

`I', `J', `K', ... `P'
Other letters in the range `I' through `P' may be defined in a machine-dependent fashion to permit immediate integer operands with explicit integer values in specified ranges. For example, on the 68000, `I' is defined to stand for the range of values 1 to 8. This is the range permitted as a shift count in the shift instructions.

`E'
An immediate floating operand (expression code const_double) is allowed, but only if the target floating point format is the same as that of the host machine (on which the compiler is running).

`F'
An immediate floating operand (expression code const_double or const_vector) is allowed.

`G', `H'
`G' and `H' may be defined in a machine-dependent fashion to permit immediate floating operands in particular ranges of values.

`s'
An immediate integer operand whose value is not an explicit integer is allowed.

This might appear strange; if an insn allows a constant operand with a value not known at compile time, it certainly must allow any known value. So why use `s' instead of `i'? Sometimes it allows better code to be generated.

For example, on the 68000 in a fullword instruction it is possible to use an immediate operand; but if the immediate value is between -128 and 127, better code results from loading the value into a register and using the register. This is because the load into the register can be done with a `moveq' instruction. We arrange for this to happen by defining the letter `K' to mean "any integer outside the range -128 to 127", and then specifying `Ks' in the operand constraints.

`g'
Any register, memory or immediate integer operand is allowed, except for registers that are not general registers.

`X'
Any operand whatsoever is allowed.

`0', `1', `2', ... `9'
An operand that matches the specified operand number is allowed. If a digit is used together with letters within the same alternative, the digit should come last.

This number is allowed to be more than a single digit. If multiple digits are encountered consecutively, they are interpreted as a single decimal integer. There is scant chance for ambiguity, since to-date it has never been desirable that `10' be interpreted as matching either operand 1 or operand 0. Should this be desired, one can use multiple alternatives instead.

This is called a matching constraint and what it really means is that the assembler has only a single operand that fills two roles which asm distinguishes. For example, an add instruction uses two input operands and an output operand, but on most CISC machines an add instruction really has only two operands, one of them an input-output operand:

 
addl #35,r12

Matching constraints are used in these circumstances. More precisely, the two operands that match must include one input-only operand and one output-only operand. Moreover, the digit must be a smaller number than the number of the operand that uses it in the constraint.

`p'
An operand that is a valid memory address is allowed. This is for "load address" and "push address" instructions.

`p' in the constraint must be accompanied by address_operand as the predicate in the match_operand. This predicate interprets the mode specified in the match_operand as the mode of the memory reference for which the address would be valid.

other-letters
Other letters can be defined in machine-dependent fashion to stand for particular classes of registers or other arbitrary operand types. `d', `a' and `f' are defined on the 68000/68020 to stand for data, address and floating point registers.


5.36.2 Multiple Alternative Constraints

Sometimes a single instruction has multiple alternative sets of possible operands. For example, on the 68000, a logical-or instruction can combine register or an immediate value into memory, or it can combine any kind of operand into a register; but it cannot combine one memory location into another.

These constraints are represented as multiple alternatives. An alternative can be described by a series of letters for each operand. The overall constraint for an operand is made from the letters for this operand from the first alternative, a comma, the letters for this operand from the second alternative, a comma, and so on until the last alternative.

If all the operands fit any one alternative, the instruction is valid. Otherwise, for each alternative, the compiler counts how many instructions must be added to copy the operands so that that alternative applies. The alternative requiring the least copying is chosen. If two alternatives need the same amount of copying, the one that comes first is chosen. These choices can be altered with the `?' and `!' characters:

?
Disparage slightly the alternative that the `?' appears in, as a choice when no alternative applies exactly. The compiler regards this alternative as one unit more costly for each `?' that appears in it.

!
Disparage severely the alternative that the `!' appears in. This alternative can still be used if it fits without reloading, but if reloading is needed, some other alternative will be used.


5.36.3 Constraint Modifier Characters

Here are constraint modifier characters.

`='
Means that this operand is write-only for this instruction: the previous value is discarded and replaced by output data.

`+'
Means that this operand is both read and written by the instruction.

When the compiler fixes up the operands to satisfy the constraints, it needs to know which operands are inputs to the instruction and which are outputs from it. `=' identifies an output; `+' identifies an operand that is both input and output; all other operands are assumed to be input only.

If you specify `=' or `+' in a constraint, you put it in the first character of the constraint string.

`&'
Means (in a particular alternative) that this operand is an earlyclobber operand, which is modified before the instruction is finished using the input operands. Therefore, this operand may not lie in a register that is used as an input operand or as part of any memory address.

`&' applies only to the alternative in which it is written. In constraints with multiple alternatives, sometimes one alternative requires `&' while others do not. See, for example, the `movdf' insn of the 68000.

An input operand can be tied to an earlyclobber operand if its only use as an input occurs before the early result is written. Adding alternatives of this form often allows GCC to produce better code when only some of the inputs can be affected by the earlyclobber. See, for example, the `mulsi3' insn of the ARM.

`&' does not obviate the need to write `='.

`%'
Declares the instruction to be commutative for this operand and the following operand. This means that the compiler may interchange the two operands if that is the cheapest way to make all operands fit the constraints. GCC can only handle one commutative pair in an asm; if you use more, the compiler may fail.

`#'
Says that all following characters, up to the next comma, are to be ignored as a constraint. They are significant only for choosing register preferences.

`*'
Says that the following character should be ignored when choosing register preferences. `*' has no effect on the meaning of the constraint as a constraint, and no effect on reloading.


5.36.4 Constraints for Particular Machines

Whenever possible, you should use the general-purpose constraint letters in asm arguments, since they will convey meaning more readily to people reading your code. Failing that, use the constraint letters that usually have very similar meanings across architectures. The most commonly used constraints are `m' and `r' (for memory and general-purpose registers respectively; see section 5.36.1 Simple Constraints), and `I', usually the letter indicating the most common immediate-constant format.

For each machine architecture, the `config/machine/machine.h' file defines additional constraints. These constraints are used by the compiler itself for instruction generation, as well as for asm statements; therefore, some of the constraints are not particularly interesting for asm. The constraints are defined through these macros:

REG_CLASS_FROM_LETTER
Register class constraints (usually lower case).

CONST_OK_FOR_LETTER_P
Immediate constant constraints, for non-floating point constants of word size or smaller precision (usually upper case).

CONST_DOUBLE_OK_FOR_LETTER_P
Immediate constant constraints, for all floating point constants and for constants of greater than word size precision (usually upper case).

EXTRA_CONSTRAINT
Special cases of registers or memory. This macro is not required, and is only defined for some machines.

Inspecting these macro definitions in the compiler source for your machine is the best way to be certain you have the right constraints. However, here is a summary of the machine-dependent constraints available on some particular machines.

ARM family---`arm.h'
f
Floating-point register

F
One of the floating-point constants 0.0, 0.5, 1.0, 2.0, 3.0, 4.0, 5.0 or 10.0

G
Floating-point constant that would satisfy the constraint `F' if it were negated

I
Integer that is valid as an immediate operand in a data processing instruction. That is, an integer in the range 0 to 255 rotated by a multiple of 2

J
Integer in the range -4095 to 4095

K
Integer that satisfies constraint `I' when inverted (ones complement)

L
Integer that satisfies constraint `I' when negated (twos complement)

M
Integer in the range 0 to 32

Q
A memory reference where the exact address is in a single register (``m'' is preferable for asm statements)

R
An item in the constant pool

S
A symbol in the text segment of the current file

AVR family---`avr.h'
l
Registers from r0 to r15

a
Registers from r16 to r23

d
Registers from r16 to r31

w
Registers from r24 to r31. These registers can be used in `adiw' command

e
Pointer register (r26--r31)

b
Base pointer register (r28--r31)

q
Stack pointer register (SPH:SPL)

t
Temporary register r0

x
Register pair X (r27:r26)

y
Register pair Y (r29:r28)

z
Register pair Z (r31:r30)

I
Constant greater than -1, less than 64

J
Constant greater than -64, less than 1

K
Constant integer 2

L
Constant integer 0

M
Constant that fits in 8 bits

N
Constant integer -1

O
Constant integer 8, 16, or 24

P
Constant integer 1

G
A floating point constant 0.0

IBM RS6000---`rs6000.h'
b
Address base register

f
Floating point register

h
`MQ', `CTR', or `LINK' register

q
`MQ' register

c
`CTR' register

l
`LINK' register

x
`CR' register (condition register) number 0

y
`CR' register (condition register)

z
`FPMEM' stack memory for FPR-GPR transfers

I
Signed 16-bit constant

J
Unsigned 16-bit constant shifted left 16 bits (use `L' instead for SImode constants)

K
Unsigned 16-bit constant

L
Signed 16-bit constant shifted left 16 bits

M
Constant larger than 31

N
Exact power of 2

O
Zero

P
Constant whose negation is a signed 16-bit constant

G
Floating point constant that can be loaded into a register with one instruction per word

Q
Memory operand that is an offset from a register (`m' is preferable for asm statements)

R
AIX TOC entry

S
Constant suitable as a 64-bit mask operand

T
Constant suitable as a 32-bit mask operand

U
System V Release 4 small data area reference

Intel 386---`i386.h'
q
`a', b, c, or d register for the i386. For x86-64 it is equivalent to `r' class. (for 8-bit instructions that do not use upper halves)

Q
`a', b, c, or d register. (for 8-bit instructions, that do use upper halves)

R
Legacy register--equivalent to r class in i386 mode. (for non-8-bit registers used together with 8-bit upper halves in a single instruction)

A
Specifies the `a' or `d' registers. This is primarily useful for 64-bit integer values (when in 32-bit mode) intended to be returned with the `d' register holding the most significant bits and the `a' register holding the least significant bits.

f
Floating point register

t
First (top of stack) floating point register

u
Second floating point register

a
`a' register

b
`b' register

c
`c' register

C
Specifies constant that can be easily constructed in SSE register without loading it from memory.

d
`d' register

D
`di' register

S
`si' register

x
`xmm' SSE register

y
MMX register

I
Constant in range 0 to 31 (for 32-bit shifts)

J
Constant in range 0 to 63 (for 64-bit shifts)

K
`0xff'

L
`0xffff'

M
0, 1, 2, or 3 (shifts for lea instruction)

N
Constant in range 0 to 255 (for out instruction)

Z
Constant in range 0 to 0xffffffff or symbolic reference known to fit specified range. (for using immediates in zero extending 32-bit to 64-bit x86-64 instructions)

e
Constant in range -2147483648 to 2147483647 or symbolic reference known to fit specified range. (for using immediates in 64-bit x86-64 instructions)

G
Standard 80387 floating point constant

Intel 960---`i960.h'
f
Floating point register (fp0 to fp3)

l
Local register (r0 to r15)

b
Global register (g0 to g15)

d
Any local or global register

I
Integers from 0 to 31

J
0

K
Integers from -31 to 0

G
Floating point 0

H
Floating point 1

Intel IA-64---`ia64.h'
a
General register r0 to r3 for addl instruction

b
Branch register

c
Predicate register (`c' as in "conditional")

d
Application register residing in M-unit

e
Application register residing in I-unit

f
Floating-point register

m
Memory operand. Remember that `m' allows postincrement and postdecrement which require printing with `%Pn' on IA-64. Use `S' to disallow postincrement and postdecrement.

G
Floating-point constant 0.0 or 1.0

I
14-bit signed integer constant

J
22-bit signed integer constant

K
8-bit signed integer constant for logical instructions

L
8-bit adjusted signed integer constant for compare pseudo-ops

M
6-bit unsigned integer constant for shift counts

N
9-bit signed integer constant for load and store postincrements

O
The constant zero

P
0 or -1 for dep instruction

Q
Non-volatile memory for floating-point loads and stores

R
Integer constant in the range 1 to 4 for shladd instruction

S
Memory operand except postincrement and postdecrement

FRV---`frv.h'
a
Register in the class ACC_REGS (acc0 to acc7).

b
Register in the class EVEN_ACC_REGS (acc0 to acc7).

c
Register in the class CC_REGS (fcc0 to fcc3 and icc0 to icc3).

d
Register in the class GPR_REGS (gr0 to gr63).

e
Register in the class EVEN_REGS (gr0 to gr63). Odd registers are excluded not in the class but through the use of a machine mode larger than 4 bytes.

f
Register in the class FPR_REGS (fr0 to fr63).

h
Register in the class FEVEN_REGS (fr0 to fr63). Odd registers are excluded not in the class but through the use of a machine mode larger than 4 bytes.

l
Register in the class LR_REG (the lr register).

q
Register in the class QUAD_REGS (gr2 to gr63). Register numbers not divisible by 4 are excluded not in the class but through the use of a machine mode larger than 8 bytes.

t
Register in the class ICC_REGS (icc0 to icc3).

u
Register in the class FCC_REGS (fcc0 to fcc3).

v
Register in the class ICR_REGS (cc4 to cc7).

w
Register in the class FCR_REGS (cc0 to cc3).

x
Register in the class QUAD_FPR_REGS (fr0 to fr63). Register numbers not divisible by 4 are excluded not in the class but through the use of a machine mode larger than 8 bytes.

z
Register in the class SPR_REGS (lcr and lr).

A
Register in the class QUAD_ACC_REGS (acc0 to acc7).

B
Register in the class ACCG_REGS (accg0 to accg7).

C
Register in the class CR_REGS (cc0 to cc7).

G
Floating point constant zero

I
6-bit signed integer constant

J
10-bit signed integer constant

L
16-bit signed integer constant

M
16-bit unsigned integer constant

N
12-bit signed integer constant that is negative--i.e. in the range of -2048 to -1

O
Constant zero

P
12-bit signed integer constant that is greater than zero--i.e. in the range of 1 to 2047.

IP2K---`ip2k.h'
a
`DP' or `IP' registers (general address)

f
`IP' register

j
`IPL' register

k
`IPH' register

b
`DP' register

y
`DPH' register

z
`DPL' register

q
`SP' register

c
`DP' or `SP' registers (offsettable address)

d
Non-pointer registers (not `SP', `DP', `IP')

u
Non-SP registers (everything except `SP')

R
Indirect thru `IP' - Avoid this except for QImode, since we can't access extra bytes

S
Indirect thru `SP' or `DP' with short displacement (0..127)

T
Data-section immediate value

I
Integers from -255 to -1

J
Integers from 0 to 7--valid bit number in a register

K
Integers from 0 to 127--valid displacement for addressing mode

L
Integers from 1 to 127

M
Integer -1

N
Integer 1

O
Zero

P
Integers from 0 to 255

MIPS---`mips.h'
d
General-purpose integer register

f
Floating-point register (if available)

h
`Hi' register

l
`Lo' register

x
`Hi' or `Lo' register

y
General-purpose integer register

z
Floating-point status register

I
Signed 16-bit constant (for arithmetic instructions)

J
Zero

K
Zero-extended 16-bit constant (for logic instructions)

L
Constant with low 16 bits zero (can be loaded with lui)

M
32-bit constant which requires two instructions to load (a constant which is not `I', `K', or `L')

N
Negative 16-bit constant

O
Exact power of two

P
Positive 16-bit constant

G
Floating point zero

Q
Memory reference that can be loaded with more than one instruction (`m' is preferable for asm statements)

R
Memory reference that can be loaded with one instruction (`m' is preferable for asm statements)

S
Memory reference in external OSF/rose PIC format (`m' is preferable for asm statements)

Motorola 680x0---`m68k.h'
a
Address register

d
Data register

f
68881 floating-point register, if available

x
Sun FPA (floating-point) register, if available

y
First 16 Sun FPA registers, if available

I
Integer in the range 1 to 8

J
16-bit signed number

K
Signed number whose magnitude is greater than 0x80

L
Integer in the range -8 to -1

M
Signed number whose magnitude is greater than 0x100

G
Floating point constant that is not a 68881 constant

H
Floating point constant that can be used by Sun FPA

Motorola 68HC11 & 68HC12 families---`m68hc11.h'
a
Register 'a'

b
Register 'b'

d
Register 'd'

q
An 8-bit register

t
Temporary soft register _.tmp

u
A soft register _.d1 to _.d31

w
Stack pointer register

x
Register 'x'

y
Register 'y'

z
Pseudo register 'z' (replaced by 'x' or 'y' at the end)

A
An address register: x, y or z

B
An address register: x or y

D
Register pair (x:d) to form a 32-bit value

L
Constants in the range -65536 to 65535

M
Constants whose 16-bit low part is zero

N
Constant integer 1 or -1

O
Constant integer 16

P
Constants in the range -8 to 2

SPARC---`sparc.h'
f
Floating-point register on the SPARC-V8 architecture and lower floating-point register on the SPARC-V9 architecture.

e
Floating-point register. It is equivalent to `f' on the SPARC-V8 architecture and contains both lower and upper floating-point registers on the SPARC-V9 architecture.

c
Floating-point condition code register.

d
Lower floating-point register. It is only valid on the SPARC-V9 architecture when the Visual Instruction Set is available.

b
Floating-point register. It is only valid on the SPARC-V9 architecture when the Visual Instruction Set is available.

h
64-bit global or out register for the SPARC-V8+ architecture.

I
Signed 13-bit constant

J
Zero

K
32-bit constant with the low 12 bits clear (a constant that can be loaded with the sethi instruction)

L
A constant in the range supported by movcc instructions

M
A constant in the range supported by movrcc instructions

N
Same as `K', except that it verifies that bits that are not in the lower 32-bit range are all zero. Must be used instead of `K' for modes wider than SImode

O
The constant 4096

G
Floating-point zero

H
Signed 13-bit constant, sign-extended to 32 or 64 bits

Q
Floating-point constant whose integral representation can be moved into an integer register using a single sethi instruction

R
Floating-point constant whose integral representation can be moved into an integer register using a single mov instruction

S
Floating-point constant whose integral representation can be moved into an integer register using a high/lo_sum instruction sequence

T
Memory address aligned to an 8-byte boundary

U
Even register

W
Memory address for `e' constraint registers.

TMS320C3x/C4x---`c4x.h'
a
Auxiliary (address) register (ar0-ar7)

b
Stack pointer register (sp)

c
Standard (32-bit) precision integer register

f
Extended (40-bit) precision register (r0-r11)

k
Block count register (bk)

q
Extended (40-bit) precision low register (r0-r7)

t
Extended (40-bit) precision register (r0-r1)

u
Extended (40-bit) precision register (r2-r3)

v
Repeat count register (rc)

x
Index register (ir0-ir1)

y
Status (condition code) register (st)

z
Data page register (dp)

G
Floating-point zero

H
Immediate 16-bit floating-point constant

I
Signed 16-bit constant

J
Signed 8-bit constant

K
Signed 5-bit constant

L
Unsigned 16-bit constant

M
Unsigned 8-bit constant

N
Ones complement of unsigned 16-bit constant

O
High 16-bit constant (32-bit constant with 16 LSBs zero)

Q
Indirect memory reference with signed 8-bit or index register displacement

R
Indirect memory reference with unsigned 5-bit displacement

S
Indirect memory reference with 1 bit or index register displacement

T
Direct memory reference

U
Symbolic address

S/390 and zSeries---`s390.h'
a
Address register (general purpose register except r0)

d
Data register (arbitrary general purpose register)

f
Floating-point register

I
Unsigned 8-bit constant (0--255)

J
Unsigned 12-bit constant (0--4095)

K
Signed 16-bit constant (-32768--32767)

L
Unsigned 16-bit constant (0--65535)

Q
Memory reference without index register

S
Symbolic constant suitable for use with the larl instruction

Xstormy16---`stormy16.h'
a
Register r0.

b
Register r1.

c
Register r2.

d
Register r8.

e
Registers r0 through r7.

t
Registers r0 and r1.

y
The carry register.

z
Registers r8 and r9.

I
A constant between 0 and 3 inclusive.

J
A constant that has exactly one bit set.

K
A constant that has exactly one bit clear.

L
A constant between 0 and 255 inclusive.

M
A constant between -255 and 0 inclusive.

N
A constant between -3 and 0 inclusive.

O
A constant between 1 and 4 inclusive.

P
A constant between -4 and -1 inclusive.

Q
A memory reference that is a stack push.

R
A memory reference that is a stack pop.

S
A memory reference that refers to a constant address of known value.

T
The register indicated by Rx (not implemented yet).

U
A constant that is not between 2 and 15 inclusive.

Xtensa---`xtensa.h'
a
General-purpose 32-bit register

b
One-bit boolean register

A
MAC16 40-bit accumulator register

I
Signed 12-bit integer constant, for use in MOVI instructions

J
Signed 8-bit integer constant, for use in ADDI instructions

K
Integer constant valid for BccI instructions

L
Unsigned constant valid for BccUI instructions


5.37 Controlling Names Used in Assembler Code

You can specify the name to be used in the assembler code for a C function or variable by writing the asm (or __asm__) keyword after the declarator as follows:

 
int foo asm ("myfoo") = 2;

This specifies that the name to be used for the variable foo in the assembler code should be `myfoo' rather than the usual `_foo'.

On systems where an underscore is normally prepended to the name of a C function or variable, this feature allows you to define names for the linker that do not start with an underscore.

It does not make sense to use this feature with a non-static local variable since such variables do not have assembler names. If you are trying to put the variable in a particular register, see 5.38 Variables in Specified Registers. GCC presently accepts such code with a warning, but will probably be changed to issue an error, rather than a warning, in the future.

You cannot use asm in this way in a function definition; but you can get the same effect by writing a declaration for the function before its definition and putting asm there, like this:

 
extern func () asm ("FUNC");

func (x, y)
     int x, y;
/* ... */

It is up to you to make sure that the assembler names you choose do not conflict with any other assembler symbols. Also, you must not use a register name; that would produce completely invalid assembler code. GCC does not as yet have the ability to store static variables in registers. Perhaps that will be added.


5.38 Variables in Specified Registers

GNU C allows you to put a few global variables into specified hardware registers. You can also specify the register in which an ordinary register variable should be allocated.

5.38.1 Defining Global Register Variables  
5.38.2 Specifying Registers for Local Variables  


5.38.1 Defining Global Register Variables

You can define a global register variable in GNU C like this:

 
register int *foo asm ("a5");

Here a5 is the name of the register which should be used. Choose a register which is normally saved and restored by function calls on your machine, so that library routines will not clobber it.

Naturally the register name is cpu-dependent, so you would need to conditionalize your program according to cpu type. The register a5 would be a good choice on a 68000 for a variable of pointer type. On machines with register windows, be sure to choose a "global" register that is not affected magically by the function call mechanism.

In addition, operating systems on one type of cpu may differ in how they name the registers; then you would need additional conditionals. For example, some 68000 operating systems call this register %a5.

Eventually there may be a way of asking the compiler to choose a register automatically, but first we need to figure out how it should choose and how to enable you to guide the choice. No solution is evident.

Defining a global register variable in a certain register reserves that register entirely for this use, at least within the current compilation. The register will not be allocated for any other purpose in the functions in the current compilation. The register will not be saved and restored by these functions. Stores into this register are never deleted even if they would appear to be dead, but references may be deleted or moved or simplified.

It is not safe to access the global register variables from signal handlers, or from more than one thread of control, because the system library routines may temporarily use the register for other things (unless you recompile them specially for the task at hand).

It is not safe for one function that uses a global register variable to call another such function foo by way of a third function lose that was compiled without knowledge of this variable (i.e. in a different source file in which the variable wasn't declared). This is because lose might save the register and put some other value there. For example, you can't expect a global register variable to be available in the comparison-function that you pass to qsort, since qsort might have put something else in that register. (If you are prepared to recompile qsort with the same global register variable, you can solve this problem.)

If you want to recompile qsort or other source files which do not actually use your global register variable, so that they will not use that register for any other purpose, then it suffices to specify the compiler option `-ffixed-reg'. You need not actually add a global register declaration to their source code.

A function which can alter the value of a global register variable cannot safely be called from a function compiled without this variable, because it could clobber the value the caller expects to find there on return. Therefore, the function which is the entry point into the part of the program that uses the global register variable must explicitly save and restore the value which belongs to its caller.

On most machines, longjmp will restore to each global register variable the value it had at the time of the setjmp. On some machines, however, longjmp will not change the value of global register variables. To be portable, the function that called setjmp should make other arrangements to save the values of the global register variables, and to restore them in a longjmp. This way, the same thing will happen regardless of what longjmp does.

All global register variable declarations must precede all function definitions. If such a declaration could appear after function definitions, the declaration would be too late to prevent the register from being used for other purposes in the preceding functions.

Global register variables may not have initial values, because an executable file has no means to supply initial contents for a register.

On the SPARC, there are reports that g3 ... g7 are suitable registers, but certain library functions, such as getwd, as well as the subroutines for division and remainder, modify g3 and g4. g1 and g2 are local temporaries.

On the 68000, a2 ... a5 should be suitable, as should d2 ... d7. Of course, it will not do to use more than a few of those.


5.38.2 Specifying Registers for Local Variables

You can define a local register variable with a specified register like this:

 
register int *foo asm ("a5");

Here a5 is the name of the register which should be used. Note that this is the same syntax used for defining global register variables, but for a local variable it would appear within a function.

Naturally the register name is cpu-dependent, but this is not a problem, since specific registers are most often useful with explicit assembler instructions (see section 5.35 Assembler Instructions with C Expression Operands). Both of these things generally require that you conditionalize your program according to cpu type.

In addition, operating systems on one type of cpu may differ in how they name the registers; then you would need additional conditionals. For example, some 68000 operating systems call this register %a5.

Defining such a register variable does not reserve the register; it remains available for other uses in places where flow control determines the variable's value is not live. However, these registers are made unavailable for use in the reload pass; excessive use of this feature leaves the compiler too few available registers to compile certain functions.

This option does not guarantee that GCC will generate code that has this variable in the register you specify at all times. You may not code an explicit reference to this register in an asm statement and assume it will always refer to this variable.

Stores into local register variables may be deleted when they appear to be dead according to dataflow analysis. References to local register variables may be deleted or moved or simplified.


5.39 Alternate Keywords

`-ansi' and the various `-std' options disable certain keywords. This causes trouble when you want to use GNU C extensions, or a general-purpose header file that should be usable by all programs, including ISO C programs. The keywords asm, typeof and inline are not available in programs compiled with `-ansi' or `-std' (although inline can be used in a program compiled with `-std=c99'). The ISO C99 keyword restrict is only available when `-std=gnu99' (which will eventually be the default) or `-std=c99' (or the equivalent `-std=iso9899:1999') is used.

The way to solve these problems is to put `__' at the beginning and end of each problematical keyword. For example, use __asm__ instead of asm, and __inline__ instead of inline.

Other C compilers won't accept these alternative keywords; if you want to compile with another compiler, you can define the alternate keywords as macros to replace them with the customary keywords. It looks like this:

 
#ifndef __GNUC__
#define __asm__ asm
#endif

`-pedantic' and other options cause warnings for many GNU C extensions. You can prevent such warnings within one expression by writing __extension__ before the expression. __extension__ has no effect aside from this.


5.40 Incomplete enum Types

You can define an enum tag without specifying its possible values. This results in an incomplete type, much like what you get if you write struct foo without describing the elements. A later declaration which does specify the possible values completes the type.

You can't allocate variables or storage using the type while it is incomplete. However, you can work with pointers to that type.

This extension may not be very useful, but it makes the handling of enum more consistent with the way struct and union are handled.

This extension is not supported by GNU C++.


5.41 Function Names as Strings

GCC predefines two magic identifiers to hold the name of the current function. The identifier __FUNCTION__ holds the name of the function as it appears in the source. The identifier __PRETTY_FUNCTION__ holds the name of the function pretty printed in a language specific fashion.

These names are always the same in a C function, but in a C++ function they may be different. For example, this program:

 
extern "C" {
extern int printf (char *, ...);
}

class a {
 public:
  sub (int i)
    {
      printf ("__FUNCTION__ = %s\n", __FUNCTION__);
      printf ("__PRETTY_FUNCTION__ = %s\n", __PRETTY_FUNCTION__);
    }
};

int
main (void)
{
  a ax;
  ax.sub (0);
  return 0;
}

gives this output:

 
__FUNCTION__ = sub
__PRETTY_FUNCTION__ = int  a::sub (int)

The compiler automagically replaces the identifiers with a string literal containing the appropriate name. Thus, they are neither preprocessor macros, like __FILE__ and __LINE__, nor variables. This means that they catenate with other string literals, and that they can be used to initialize char arrays. For example

 
char here[] = "Function " __FUNCTION__ " in " __FILE__;

On the other hand, `#ifdef __FUNCTION__' does not have any special meaning inside a function, since the preprocessor does not do anything special with the identifier __FUNCTION__.

Note that these semantics are deprecated, and that GCC 3.2 will handle __FUNCTION__ and __PRETTY_FUNCTION__ the same way as __func__. __func__ is defined by the ISO standard C99:

 
The identifier __func__ is implicitly declared by the translator
as if, immediately following the opening brace of each function
definition, the declaration

 
static const char __func__[] = "function-name";

appeared, where function-name is the name of the lexically-enclosing function. This name is the unadorned name of the function.

By this definition, __func__ is a variable, not a string literal. In particular, __func__ does not catenate with other string literals.

In C++, __FUNCTION__ and __PRETTY_FUNCTION__ are variables, declared in the same way as __func__.


5.42 Getting the Return or Frame Address of a Function

These functions may be used to get information about the callers of a function.

Built-in Function: void * __builtin_return_address (unsigned int level)
This function returns the return address of the current function, or of one of its callers. The level argument is number of frames to scan up the call stack. A value of 0 yields the return address of the current function, a value of 1 yields the return address of the caller of the current function, and so forth. When inlining the expected behavior is that the function will return the address of the function that will be returned to. To work around this behavior use the noinline function attribute.

The level argument must be a constant integer.

On some machines it may be impossible to determine the return address of any function other than the current one; in such cases, or when the top of the stack has been reached, this function will return 0 or a random value. In addition, __builtin_frame_address may be used to determine if the top of the stack has been reached.

This function should only be used with a nonzero argument for debugging purposes.

Built-in Function: void * __builtin_frame_address (unsigned int level)
This function is similar to __builtin_return_address, but it returns the address of the function frame rather than the return address of the function. Calling __builtin_frame_address with a value of 0 yields the frame address of the current function, a value of 1 yields the frame address of the caller of the current function, and so forth.

The frame is the area on the stack which holds local variables and saved registers. The frame address is normally the address of the first word pushed on to the stack by the function. However, the exact definition depends upon the processor and the calling convention. If the processor has a dedicated frame pointer register, and the function has a frame, then __builtin_frame_address will return the value of the frame pointer register.

On some machines it may be impossible to determine the frame address of any function other than the current one; in such cases, or when the top of the stack has been reached, this function will return 0 if the first frame pointer is properly initialized by the startup code.

This function should only be used with a nonzero argument for debugging purposes.


5.43 Using vector instructions through built-in functions

On some targets, the instruction set contains SIMD vector instructions that operate on multiple values contained in one large register at the same time. For example, on the i386 the MMX, 3Dnow! and SSE extensions can be used this way.

The first step in using these extensions is to provide the necessary data types. This should be done using an appropriate typedef:

 
typedef int v4si __attribute__ ((mode(V4SI)));

The base type int is effectively ignored by the compiler, the actual properties of the new type v4si are defined by the __attribute__. It defines the machine mode to be used; for vector types these have the form VnB; n should be the number of elements in the vector, and B should be the base mode of the individual elements. The following can be used as base modes:

QI
An integer that is as wide as the smallest addressable unit, usually 8 bits.
HI
An integer, twice as wide as a QI mode integer, usually 16 bits.
SI
An integer, four times as wide as a QI mode integer, usually 32 bits.
DI
An integer, eight times as wide as a QI mode integer, usually 64 bits.
SF
A floating point value, as wide as a SI mode integer, usually 32 bits.
DF
A floating point value, as wide as a DI mode integer, usually 64 bits.

Specifying a combination that is not valid for the current architecture will cause gcc to synthesize the instructions using a narrower mode. For example, if you specify a variable of type V4SI and your architecture does not allow for this specific SIMD type, gcc will produce code that uses 4 SIs.

The types defined in this manner can be used with a subset of normal C operations. Currently, gcc will allow using the following operators on these types: +, -, *, /, unary minus.

The operations behave like C++ valarrays. Addition is defined as the addition of the corresponding elements of the operands. For example, in the code below, each of the 4 elements in a will be added to the corresponding 4 elements in b and the resulting vector will be stored in c.

 
typedef int v4si __attribute__ ((mode(V4SI)));

v4si a, b, c;

c = a + b;

Subtraction, multiplication, and division operate in a similar manner. Likewise, the result of using the unary minus operator on a vector type is a vector whose elements are the negative value of the corresponding elements in the operand.

You can declare variables and use them in function calls and returns, as well as in assignments and some casts. You can specify a vector type as a return type for a function. Vector types can also be used as function arguments. It is possible to cast from one vector type to another, provided they are of the same size (in fact, you can also cast vectors to and from other datatypes of the same size).

You cannot operate between vectors of different lengths or different signedness without a cast.

A port that supports hardware vector operations, usually provides a set of built-in functions that can be used to operate on vectors. For example, a function to add two vectors and multiply the result by a third could look like this:

 
v4si f (v4si a, v4si b, v4si c)
{
  v4si tmp = __builtin_addv4si (a, b);
  return __builtin_mulv4si (tmp, c);
}


5.44 Other built-in functions provided by GCC

GCC provides a large number of built-in functions other than the ones mentioned above. Some of these are for internal use in the processing of exceptions or variable-length argument lists and will not be documented here because they may change from time to time; we do not recommend general use of these functions.

The remaining functions are provided for optimization purposes.

GCC includes built-in versions of many of the functions in the standard C library. The versions prefixed with __builtin_ will always be treated as having the same meaning as the C library function even if you specify the `-fno-builtin' option. (see section 3.4 Options Controlling C Dialect) Many of these functions are only optimized in certain cases; if they are not optimized in a particular case, a call to the library function will be emitted.

The functions abort, exit, _Exit and _exit are recognized and presumed not to return, but otherwise are not built in. _exit is not recognized in strict ISO C mode (`-ansi', `-std=c89' or `-std=c99'). _Exit is not recognized in strict C89 mode (`-ansi' or `-std=c89'). All these functions have corresponding versions prefixed with __builtin_, which may be used even in strict C89 mode.

Outside strict ISO C mode, the functions alloca, bcmp, bzero, index, rindex, ffs, fputs_unlocked, printf_unlocked and fprintf_unlocked may be handled as built-in functions. All these functions have corresponding versions prefixed with __builtin_, which may be used even in strict C89 mode.

The ISO C99 functions conj, conjf, conjl, creal, crealf, creall, cimag, cimagf, cimagl, imaxabs, llabs, snprintf, vscanf, vsnprintf and vsscanf are handled as built-in functions except in strict ISO C90 mode. There are also built-in versions of the ISO C99 functions cosf, cosl, expf, expl, fabsf, fabsl, logf, logl, sinf, sinl, sqrtf, and sqrtl, that are recognized in any mode since ISO C90 reserves these names for the purpose to which ISO C99 puts them. All these functions have corresponding versions prefixed with __builtin_.

The ISO C90 functions abs, cos, exp, fabs, fprintf, fputs, labs, log, memcmp, memcpy, memset, printf, putchar, puts, scanf, sin, snprintf, sprintf, sqrt, sscanf, strcat, strchr, strcmp, strcpy, strcspn, strlen, strncat, strncmp, strncpy, strpbrk, strrchr, strspn, strstr, vprintf and vsprintf are all recognized as built-in functions unless `-fno-builtin' is specified (or `-fno-builtin-function' is specified for an individual function). All of these functions have corresponding versions prefixed with __builtin_.

GCC provides built-in versions of the ISO C99 floating point comparison macros that avoid raising exceptions for unordered operands. They have the same names as the standard macros ( isgreater, isgreaterequal, isless, islessequal, islessgreater, and isunordered) , with __builtin_ prefixed. We intend for a library implementor to be able to simply #define each standard macro to its built-in equivalent.

Built-in Function: int __builtin_types_compatible_p (type1, type2)

You can use the built-in function __builtin_types_compatible_p to determine whether two types are the same.

This built-in function returns 1 if the unqualified versions of the types type1 and type2 (which are types, not expressions) are compatible, 0 otherwise. The result of this built-in function can be used in integer constant expressions.

This built-in function ignores top level qualifiers (e.g., const, volatile). For example, int is equivalent to const int.

The type int[] and int[5] are compatible. On the other hand, int and char * are not compatible, even if the size of their types, on the particular architecture are the same. Also, the amount of pointer indirection is taken into account when determining similarity. Consequently, short * is not similar to short **. Furthermore, two types that are typedefed are considered compatible if their underlying types are compatible.

An enum type is considered to be compatible with another enum type. For example, enum {foo, bar} is similar to enum {hot, dog}.

You would typically use this function in code whose execution varies depending on the arguments' types. For example:

 
#define foo(x)                                                  \
  ({                                                           \
    typeof (x) tmp;                                             \
    if (__builtin_types_compatible_p (typeof (x), long double)) \
      tmp = foo_long_double (tmp);                              \
    else if (__builtin_types_compatible_p (typeof (x), double)) \
      tmp = foo_double (tmp);                                   \
    else if (__builtin_types_compatible_p (typeof (x), float))  \
      tmp = foo_float (tmp);                                    \
    else                                                        \
      abort ();                                                 \
    tmp;                                                        \
  })

Note: This construct is only available for C.

Built-in Function: type __builtin_choose_expr (const_exp, exp1, exp2)

You can use the built-in function __builtin_choose_expr to evaluate code depending on the value of a constant expression. This built-in function returns exp1 if const_exp, which is a constant expression that must be able to be determined at compile time, is nonzero. Otherwise it returns 0.

This built-in function is analogous to the `? :' operator in C, except that the expression returned has its type unaltered by promotion rules. Also, the built-in function does not evaluate the expression that was not chosen. For example, if const_exp evaluates to true, exp2 is not evaluated even if it has side-effects.

This built-in function can return an lvalue if the chosen argument is an lvalue.

If exp1 is returned, the return type is the same as exp1's type. Similarly, if exp2 is returned, its return type is the same as exp2.

Example:

 
#define foo(x)                                                    \
  __builtin_choose_expr (                                         \
    __builtin_types_compatible_p (typeof (x), double),            \
    foo_double (x),                                               \
    __builtin_choose_expr (                                       \
      __builtin_types_compatible_p (typeof (x), float),           \
      foo_float (x),                                              \
      /* The void expression results in a compile-time error  \
         when assigning the result to something.  */          \
      (void)0))

Note: This construct is only available for C. Furthermore, the unused expression (exp1 or exp2 depending on the value of const_exp) may still generate syntax errors. This may change in future revisions.

Built-in Function: int __builtin_constant_p (exp)
You can use the built-in function __builtin_constant_p to determine if a value is known to be constant at compile-time and hence that GCC can perform constant-folding on expressions involving that value. The argument of the function is the value to test. The function returns the integer 1 if the argument is known to be a compile-time constant and 0 if it is not known to be a compile-time constant. A return of 0 does not indicate that the value is not a constant, but merely that GCC cannot prove it is a constant with the specified value of the `-O' option.

You would typically use this function in an embedded application where memory was a critical resource. If you have some complex calculation, you may want it to be folded if it involves constants, but need to call a function if it does not. For example:

 
#define Scale_Value(X)      \
  (__builtin_constant_p (X) \
  ? ((X) * SCALE + OFFSET) : Scale (X))

You may use this built-in function in either a macro or an inline function. However, if you use it in an inlined function and pass an argument of the function as the argument to the built-in, GCC will never return 1 when you call the inline function with a string constant or compound literal (see section 5.20 Compound Literals) and will not return 1 when you pass a constant numeric value to the inline function unless you specify the `-O' option.

You may also use __builtin_constant_p in initializers for static data. For instance, you can write

 
static const int table[] = {
   __builtin_constant_p (EXPRESSION) ? (EXPRESSION) : -1,
   /* ... */
};

This is an acceptable initializer even if EXPRESSION is not a constant expression. GCC must be more conservative about evaluating the built-in in this case, because it has no opportunity to perform optimization.

Previous versions of GCC did not accept this built-in in data initializers. The earliest version where it is completely safe is 3.0.1.

Built-in Function: long __builtin_expect (long exp, long c)
You may use __builtin_expect to provide the compiler with branch prediction information. In general, you should prefer to use actual profile feedback for this (`-fprofile-arcs'), as programmers are notoriously bad at predicting how their programs actually perform. However, there are applications in which this data is hard to collect.

The return value is the value of exp, which should be an integral expression. The value of c must be a compile-time constant. The semantics of the built-in are that it is expected that exp == c. For example:

 
if (__builtin_expect (x, 0))
  foo ();

would indicate that we do not expect to call foo, since we expect x to be zero. Since you are limited to integral expressions for exp, you should use constructions such as

 
if (__builtin_expect (ptr != NULL, 1))
  error ();

when testing pointer or floating-point values.

Built-in Function: void __builtin_prefetch (const void *addr, ...)
This function is used to minimize cache-miss latency by moving data into a cache before it is accessed. You can insert calls to __builtin_prefetch into code for which you know addresses of data in memory that is likely to be accessed soon. If the target supports them, data prefetch instructions will be generated. If the prefetch is done early enough before the access then the data will be in the cache by the time it is accessed.

The value of addr is the address of the memory to prefetch. There are two optional arguments, rw and locality. The value of rw is a compile-time constant one or zero; one means that the prefetch is preparing for a write to the memory address and zero, the default, means that the prefetch is preparing for a read. The value locality must be a compile-time constant integer between zero and three. A value of zero means that the data has no temporal locality, so it need not be left in the cache after the access. A value of three means that the data has a high degree of temporal locality and should be left in all levels of cache possible. Values of one and two mean, respectively, a low or moderate degree of temporal locality. The default is three.

 
for (i = 0; i < n; i++)
  {
    a[i] = a[i] + b[i];
    __builtin_prefetch (&a[i+j], 1, 1);
    __builtin_prefetch (&b[i+j], 0, 1);
    /* ... */
  }

Data prefetch does not generate faults if addr is invalid, but the address expression itself must be valid. For example, a prefetch of p->next will not fault if p->next is not a valid address, but evaluation will fault if p is not a valid address.

If the target does not support data prefetch, the address expression is evaluated if it includes side effects but no other code is generated and GCC does not issue a warning.

Built-in Function: double __builtin_huge_val (void)
Returns a positive infinity, if supported by the floating-point format, else DBL_MAX. This function is suitable for implementing the ISO C macro HUGE_VAL.

Built-in Function: float __builtin_huge_valf (void)
Similar to __builtin_huge_val, except the return type is float.

Built-in Function: long double __builtin_huge_vall (void)
Similar to __builtin_huge_val, except the return type is long double.

Built-in Function: double __builtin_inf (void)
Similar to __builtin_huge_val, except a warning is generated if the target floating-point format does not support infinities. This function is suitable for implementing the ISO C99 macro INFINITY.

Built-in Function: float __builtin_inff (void)
Similar to __builtin_inf, except the return type is float.

Built-in Function: long double __builtin_infl (void)
Similar to __builtin_inf, except the return type is long double.

Built-in Function: double __builtin_nan (const char *str)
This is an implementation of the ISO C99 function nan.

Since ISO C99 defines this function in terms of strtod, which we do not implement, a description of the parsing is in order. The string is parsed as by strtol; that is, the base is recognized by leading `0' or `0x' prefixes. The number parsed is placed in the significand such that the least significant bit of the number is at the least significant bit of the significand. The number is truncated to fit the significand field provided. The significand is forced to be a quiet NaN.

This function, if given a string literal, is evaluated early enough that it is considered a compile-time constant.

Built-in Function: float __builtin_nanf (const char *str)
Similar to __builtin_nan, except the return type is float.

Built-in Function: long double __builtin_nanl (const char *str)
Similar to __builtin_nan, except the return type is long double.

Built-in Function: double __builtin_nans (const char *str)
Similar to __builtin_nan, except the significand is forced to be a signaling NaN. The nans function is proposed by WG14 N965.

Built-in Function: float __builtin_nansf (const char *str)
Similar to __builtin_nans, except the return type is float.

Built-in Function: long double __builtin_nansl (const char *str)
Similar to __builtin_nans, except the return type is long double.


5.45 Built-in Functions Specific to Particular Target Machines

On some target machines, GCC supports many built-in functions specific to those machines. Generally these generate calls to specific machine instructions, but allow the compiler to schedule those calls.

5.45.1 Alpha Built-in Functions  
5.45.2 X86 Built-in Functions  
5.45.3 PowerPC AltiVec Built-in Functions  


5.45.1 Alpha Built-in Functions

These built-in functions are available for the Alpha family of processors, depending on the command-line switches used.

The following built-in functions are always available. They all generate the machine instruction that is part of the name.

 
long __builtin_alpha_implver (void)
long __builtin_alpha_rpcc (void)
long __builtin_alpha_amask (long)
long __builtin_alpha_cmpbge (long, long)
long __builtin_alpha_extbl (long, long)
long __builtin_alpha_extwl (long, long)
long __builtin_alpha_extll (long, long)
long __builtin_alpha_extql (long, long)
long __builtin_alpha_extwh (long, long)
long __builtin_alpha_extlh (long, long)
long __builtin_alpha_extqh (long, long)
long __builtin_alpha_insbl (long, long)
long __builtin_alpha_inswl (long, long)
long __builtin_alpha_insll (long, long)
long __builtin_alpha_insql (long, long)
long __builtin_alpha_inswh (long, long)
long __builtin_alpha_inslh (long, long)
long __builtin_alpha_insqh (long, long)
long __builtin_alpha_mskbl (long, long)
long __builtin_alpha_mskwl (long, long)
long __builtin_alpha_mskll (long, long)
long __builtin_alpha_mskql (long, long)
long __builtin_alpha_mskwh (long, long)
long __builtin_alpha_msklh (long, long)
long __builtin_alpha_mskqh (long, long)
long __builtin_alpha_umulh (long, long)
long __builtin_alpha_zap (long, long)
long __builtin_alpha_zapnot (long, long)

The following built-in functions are always with `-mmax' or `-mcpu=cpu' where cpu is pca56 or later. They all generate the machine instruction that is part of the name.

 
long __builtin_alpha_pklb (long)
long __builtin_alpha_pkwb (long)
long __builtin_alpha_unpkbl (long)
long __builtin_alpha_unpkbw (long)
long __builtin_alpha_minub8 (long, long)
long __builtin_alpha_minsb8 (long, long)
long __builtin_alpha_minuw4 (long, long)
long __builtin_alpha_minsw4 (long, long)
long __builtin_alpha_maxub8 (long, long)
long __builtin_alpha_maxsb8 (long, long)
long __builtin_alpha_maxuw4 (long, long)
long __builtin_alpha_maxsw4 (long, long)
long __builtin_alpha_perr (long, long)

The following built-in functions are always with `-mcix' or `-mcpu=cpu' where cpu is ev67 or later. They all generate the machine instruction that is part of the name.

 
long __builtin_alpha_cttz (long)
long __builtin_alpha_ctlz (long)
long __builtin_alpha_ctpop (long)

The following builtins are available on systems that use the OSF/1 PALcode. Normally they invoke the rduniq and wruniq PAL calls, but when invoked with `-mtls-kernel', they invoke rdval and wrval.

 
void *__builtin_thread_pointer (void)
void __builtin_set_thread_pointer (void *)


5.45.2 X86 Built-in Functions

These built-in functions are available for the i386 and x86-64 family of computers, depending on the command-line switches used.

The following machine modes are available for use with MMX built-in functions (see section 5.43 Using vector instructions through built-in functions): V2SI for a vector of two 32-bit integers, V4HI for a vector of four 16-bit integers, and V8QI for a vector of eight 8-bit integers. Some of the built-in functions operate on MMX registers as a whole 64-bit entity, these use DI as their mode.

If 3Dnow extensions are enabled, V2SF is used as a mode for a vector of two 32-bit floating point values.

If SSE extensions are enabled, V4SF is used for a vector of four 32-bit floating point values. Some instructions use a vector of four 32-bit integers, these use V4SI. Finally, some instructions operate on an entire vector register, interpreting it as a 128-bit integer, these use mode TI.

The following built-in functions are made available by `-mmmx'. All of them generate the machine instruction that is part of the name.

 
v8qi __builtin_ia32_paddb (v8qi, v8qi)
v4hi __builtin_ia32_paddw (v4hi, v4hi)
v2si __builtin_ia32_paddd (v2si, v2si)
v8qi __builtin_ia32_psubb (v8qi, v8qi)
v4hi __builtin_ia32_psubw (v4hi, v4hi)
v2si __builtin_ia32_psubd (v2si, v2si)
v8qi __builtin_ia32_paddsb (v8qi, v8qi)
v4hi __builtin_ia32_paddsw (v4hi, v4hi)
v8qi __builtin_ia32_psubsb (v8qi, v8qi)
v4hi __builtin_ia32_psubsw (v4hi, v4hi)
v8qi __builtin_ia32_paddusb (v8qi, v8qi)
v4hi __builtin_ia32_paddusw (v4hi, v4hi)
v8qi __builtin_ia32_psubusb (v8qi, v8qi)
v4hi __builtin_ia32_psubusw (v4hi, v4hi)
v4hi __builtin_ia32_pmullw (v4hi, v4hi)
v4hi __builtin_ia32_pmulhw (v4hi, v4hi)
di __builtin_ia32_pand (di, di)
di __builtin_ia32_pandn (di,di)
di __builtin_ia32_por (di, di)
di __builtin_ia32_pxor (di, di)
v8qi __builtin_ia32_pcmpeqb (v8qi, v8qi)
v4hi __builtin_ia32_pcmpeqw (v4hi, v4hi)
v2si __builtin_ia32_pcmpeqd (v2si, v2si)
v8qi __builtin_ia32_pcmpgtb (v8qi, v8qi)
v4hi __builtin_ia32_pcmpgtw (v4hi, v4hi)
v2si __builtin_ia32_pcmpgtd (v2si, v2si)
v8qi __builtin_ia32_punpckhbw (v8qi, v8qi)
v4hi __builtin_ia32_punpckhwd (v4hi, v4hi)
v2si __builtin_ia32_punpckhdq (v2si, v2si)
v8qi __builtin_ia32_punpcklbw (v8qi, v8qi)
v4hi __builtin_ia32_punpcklwd (v4hi, v4hi)
v2si __builtin_ia32_punpckldq (v2si, v2si)
v8qi __builtin_ia32_packsswb (v4hi, v4hi)
v4hi __builtin_ia32_packssdw (v2si, v2si)
v8qi __builtin_ia32_packuswb (v4hi, v4hi)

The following built-in functions are made available either with `-msse', or with a combination of `-m3dnow' and `-march=athlon'. All of them generate the machine instruction that is part of the name.

 
v4hi __builtin_ia32_pmulhuw (v4hi, v4hi)
v8qi __builtin_ia32_pavgb (v8qi, v8qi)
v4hi __builtin_ia32_pavgw (v4hi, v4hi)
v4hi __builtin_ia32_psadbw (v8qi, v8qi)
v8qi __builtin_ia32_pmaxub (v8qi, v8qi)
v4hi __builtin_ia32_pmaxsw (v4hi, v4hi)
v8qi __builtin_ia32_pminub (v8qi, v8qi)
v4hi __builtin_ia32_pminsw (v4hi, v4hi)
int __builtin_ia32_pextrw (v4hi, int)
v4hi __builtin_ia32_pinsrw (v4hi, int, int)
int __builtin_ia32_pmovmskb (v8qi)
void __builtin_ia32_maskmovq (v8qi, v8qi, char *)
void __builtin_ia32_movntq (di *, di)
void __builtin_ia32_sfence (void)

The following built-in functions are available when `-msse' is used. All of them generate the machine instruction that is part of the name.

 
int __builtin_ia32_comieq (v4sf, v4sf)
int __builtin_ia32_comineq (v4sf, v4sf)
int __builtin_ia32_comilt (v4sf, v4sf)
int __builtin_ia32_comile (v4sf, v4sf)
int __builtin_ia32_comigt (v4sf, v4sf)
int __builtin_ia32_comige (v4sf, v4sf)
int __builtin_ia32_ucomieq (v4sf, v4sf)
int __builtin_ia32_ucomineq (v4sf, v4sf)
int __builtin_ia32_ucomilt (v4sf, v4sf)
int __builtin_ia32_ucomile (v4sf, v4sf)
int __builtin_ia32_ucomigt (v4sf, v4sf)
int __builtin_ia32_ucomige (v4sf, v4sf)
v4sf __builtin_ia32_addps (v4sf, v4sf)
v4sf __builtin_ia32_subps (v4sf, v4sf)
v4sf __builtin_ia32_mulps (v4sf, v4sf)
v4sf __builtin_ia32_divps (v4sf, v4sf)
v4sf __builtin_ia32_addss (v4sf, v4sf)
v4sf __builtin_ia32_subss (v4sf, v4sf)
v4sf __builtin_ia32_mulss (v4sf, v4sf)
v4sf __builtin_ia32_divss (v4sf, v4sf)
v4si __builtin_ia32_cmpeqps (v4sf, v4sf)
v4si __builtin_ia32_cmpltps (v4sf, v4sf)
v4si __builtin_ia32_cmpleps (v4sf, v4sf)
v4si __builtin_ia32_cmpgtps (v4sf, v4sf)
v4si __builtin_ia32_cmpgeps (v4sf, v4sf)
v4si __builtin_ia32_cmpunordps (v4sf, v4sf)
v4si __builtin_ia32_cmpneqps (v4sf, v4sf)
v4si __builtin_ia32_cmpnltps (v4sf, v4sf)
v4si __builtin_ia32_cmpnleps (v4sf, v4sf)
v4si __builtin_ia32_cmpngtps (v4sf, v4sf)
v4si __builtin_ia32_cmpngeps (v4sf, v4sf)
v4si __builtin_ia32_cmpordps (v4sf, v4sf)
v4si __builtin_ia32_cmpeqss (v4sf, v4sf)
v4si __builtin_ia32_cmpltss (v4sf, v4sf)
v4si __builtin_ia32_cmpless (v4sf, v4sf)
v4si __builtin_ia32_cmpunordss (v4sf, v4sf)
v4si __builtin_ia32_cmpneqss (v4sf, v4sf)
v4si __builtin_ia32_cmpnlts (v4sf, v4sf)
v4si __builtin_ia32_cmpnless (v4sf, v4sf)
v4si __builtin_ia32_cmpordss (v4sf, v4sf)
v4sf __builtin_ia32_maxps (v4sf, v4sf)
v4sf __builtin_ia32_maxss (v4sf, v4sf)
v4sf __builtin_ia32_minps (v4sf, v4sf)
v4sf __builtin_ia32_minss (v4sf, v4sf)
v4sf __builtin_ia32_andps (v4sf, v4sf)
v4sf __builtin_ia32_andnps (v4sf, v4sf)
v4sf __builtin_ia32_orps (v4sf, v4sf)
v4sf __builtin_ia32_xorps (v4sf, v4sf)
v4sf __builtin_ia32_movss (v4sf, v4sf)
v4sf __builtin_ia32_movhlps (v4sf, v4sf)
v4sf __builtin_ia32_movlhps (v4sf, v4sf)
v4sf __builtin_ia32_unpckhps (v4sf, v4sf)
v4sf __builtin_ia32_unpcklps (v4sf, v4sf)
v4sf __builtin_ia32_cvtpi2ps (v4sf, v2si)
v4sf __builtin_ia32_cvtsi2ss (v4sf, int)
v2si __builtin_ia32_cvtps2pi (v4sf)
int __builtin_ia32_cvtss2si (v4sf)
v2si __builtin_ia32_cvttps2pi (v4sf)
int __builtin_ia32_cvttss2si (v4sf)
v4sf __builtin_ia32_rcpps (v4sf)
v4sf __builtin_ia32_rsqrtps (v4sf)
v4sf __builtin_ia32_sqrtps (v4sf)
v4sf __builtin_ia32_rcpss (v4sf)
v4sf __builtin_ia32_rsqrtss (v4sf)
v4sf __builtin_ia32_sqrtss (v4sf)
v4sf __builtin_ia32_shufps (v4sf, v4sf, int)
void __builtin_ia32_movntps (float *, v4sf)
int __builtin_ia32_movmskps (v4sf)

The following built-in functions are available when `-msse' is used.

v4sf __builtin_ia32_loadaps (float *)
Generates the movaps machine instruction as a load from memory.
void __builtin_ia32_storeaps (float *, v4sf)
Generates the movaps machine instruction as a store to memory.
v4sf __builtin_ia32_loadups (float *)
Generates the movups machine instruction as a load from memory.
void __builtin_ia32_storeups (float *, v4sf)
Generates the movups machine instruction as a store to memory.
v4sf __builtin_ia32_loadsss (float *)
Generates the movss machine instruction as a load from memory.
void __builtin_ia32_storess (float *, v4sf)
Generates the movss machine instruction as a store to memory.
v4sf __builtin_ia32_loadhps (v4sf, v2si *)
Generates the movhps machine instruction as a load from memory.
v4sf __builtin_ia32_loadlps (v4sf, v2si *)
Generates the movlps machine instruction as a load from memory
void __builtin_ia32_storehps (v4sf, v2si *)
Generates the movhps machine instruction as a store to memory.
void __builtin_ia32_storelps (v4sf, v2si *)
Generates the movlps machine instruction as a store to memory.

The following built-in functions are available when `-msse3' is used. All of them generate the machine instruction that is part of the name.

 
v2df __builtin_ia32_addsubpd (v2df, v2df)
v2df __builtin_ia32_addsubps (v2df, v2df)
v2df __builtin_ia32_haddpd (v2df, v2df)
v2df __builtin_ia32_haddps (v2df, v2df)
v2df __builtin_ia32_hsubpd (v2df, v2df)
v2df __builtin_ia32_hsubps (v2df, v2df)
v16qi __builtin_ia32_lddqu (char const *)
void __builtin_ia32_monitor (void *, unsigned int, unsigned int)
v2df __builtin_ia32_movddup (v2df)
v4sf __builtin_ia32_movshdup (v4sf)
v4sf __builtin_ia32_movsldup (v4sf)
void __builtin_ia32_mwait (unsigned int, unsigned int)

The following built-in functions are available when `-msse3' is used.

v2df __builtin_ia32_loadddup (double const *)
Generates the movddup machine instruction as a load from memory.

The following built-in functions are available when `-m3dnow' is used. All of them generate the machine instruction that is part of the name.

 
void __builtin_ia32_femms (void)
v8qi __builtin_ia32_pavgusb (v8qi, v8qi)
v2si __builtin_ia32_pf2id (v2sf)
v2sf __builtin_ia32_pfacc (v2sf, v2sf)
v2sf __builtin_ia32_pfadd (v2sf, v2sf)
v2si __builtin_ia32_pfcmpeq (v2sf, v2sf)
v2si __builtin_ia32_pfcmpge (v2sf, v2sf)
v2si __builtin_ia32_pfcmpgt (v2sf, v2sf)
v2sf __builtin_ia32_pfmax (v2sf, v2sf)
v2sf __builtin_ia32_pfmin (v2sf, v2sf)
v2sf __builtin_ia32_pfmul (v2sf, v2sf)
v2sf __builtin_ia32_pfrcp (v2sf)
v2sf __builtin_ia32_pfrcpit1 (v2sf, v2sf)
v2sf __builtin_ia32_pfrcpit2 (v2sf, v2sf)
v2sf __builtin_ia32_pfrsqrt (v2sf)
v2sf __builtin_ia32_pfrsqrtit1 (v2sf, v2sf)
v2sf __builtin_ia32_pfsub (v2sf, v2sf)
v2sf __builtin_ia32_pfsubr (v2sf, v2sf)
v2sf __builtin_ia32_pi2fd (v2si)
v4hi __builtin_ia32_pmulhrw (v4hi, v4hi)

The following built-in functions are available when both `-m3dnow' and `-march=athlon' are used. All of them generate the machine instruction that is part of the name.

 
v2si __builtin_ia32_pf2iw (v2sf)
v2sf __builtin_ia32_pfnacc (v2sf, v2sf)
v2sf __builtin_ia32_pfpnacc (v2sf, v2sf)
v2sf __builtin_ia32_pi2fw (v2si)
v2sf __builtin_ia32_pswapdsf (v2sf)
v2si __builtin_ia32_pswapdsi (v2si)


5.45.3 PowerPC AltiVec Built-in Functions

These built-in functions are available for the PowerPC family of computers, depending on the command-line switches used.

The following machine modes are available for use with AltiVec built-in functions (see section 5.43 Using vector instructions through built-in functions): V4SI for a vector of four 32-bit integers, V4SF for a vector of four 32-bit floating point numbers, V8HI for a vector of eight 16-bit integers, and V16QI for a vector of sixteen 8-bit integers.

The following functions are made available by including <altivec.h> and using `-maltivec' and `-mabi=altivec'. The functions implement the functionality described in Motorola's AltiVec Programming Interface Manual.

There are a few differences from Motorola's documentation and GCC's implementation. Vector constants are done with curly braces (not parentheses). Vector initializers require no casts if the vector constant is of the same type as the variable it is initializing. The vector bool type is deprecated and will be discontinued in further revisions. Use vector signed instead. If signed or unsigned is omitted, the vector type will default to signed. Lastly, all overloaded functions are implemented with macros for the C implementation. So code the following example will not work:

 
  vec_add ((vector signed int){1, 2, 3, 4}, foo);

Since vec_add is a macro, the vector constant in the above example will be treated as four different arguments. Wrap the entire argument in parentheses for this to work. The C++ implementation does not use macros.

Note: Only the <altivec.h> interface is supported. Internally, GCC uses built-in functions to achieve the functionality in the aforementioned header file, but they are not supported and are subject to change without notice.

 
vector signed char vec_abs (vector signed char, vector signed char);
vector signed short vec_abs (vector signed short, vector signed short);
vector signed int vec_abs (vector signed int, vector signed int);
vector signed float vec_abs (vector signed float, vector signed float);

vector signed char vec_abss (vector signed char, vector signed char);
vector signed short vec_abss (vector signed short, vector signed short);

vector signed char vec_add (vector signed char, vector signed char);
vector unsigned char vec_add (vector signed char, vector unsigned char);

vector unsigned char vec_add (vector unsigned char, vector signed char);

vector unsigned char vec_add (vector unsigned char,
                              vector unsigned char);
vector signed short vec_add (vector signed short, vector signed short);
vector unsigned short vec_add (vector signed short,
                               vector unsigned short);
vector unsigned short vec_add (vector unsigned short,
                               vector signed short);
vector unsigned short vec_add (vector unsigned short,
                               vector unsigned short);
vector signed int vec_add (vector signed int, vector signed int);
vector unsigned int vec_add (vector signed int, vector unsigned int);
vector unsigned int vec_add (vector unsigned int, vector signed int);
vector unsigned int vec_add (vector unsigned int, vector unsigned int);
vector float vec_add (vector float, vector float);

vector unsigned int vec_addc (vector unsigned int, vector unsigned int);

vector unsigned char vec_adds (vector signed char,
                               vector unsigned char);
vector unsigned char vec_adds (vector unsigned char,
                               vector signed char);
vector unsigned char vec_adds (vector unsigned char,
                               vector unsigned char);
vector signed char vec_adds (vector signed char, vector signed char);
vector unsigned short vec_adds (vector signed short,
                                vector unsigned short);
vector unsigned short vec_adds (vector unsigned short,
                                vector signed short);
vector unsigned short vec_adds (vector unsigned short,
                                vector unsigned short);
vector signed short vec_adds (vector signed short, vector signed short);

vector unsigned int vec_adds (vector signed int, vector unsigned int);
vector unsigned int vec_adds (vector unsigned int, vector signed int);
vector unsigned int vec_adds (vector unsigned int, vector unsigned int);

vector signed int vec_adds (vector signed int, vector signed int);

vector float vec_and (vector float, vector float);
vector float vec_and (vector float, vector signed int);
vector float vec_and (vector signed int, vector float);
vector signed int vec_and (vector signed int, vector signed int);
vector unsigned int vec_and (vector signed int, vector unsigned int);
vector unsigned int vec_and (vector unsigned int, vector signed int);
vector unsigned int vec_and (vector unsigned int, vector unsigned int);
vector signed short vec_and (vector signed short, vector signed short);
vector unsigned short vec_and (vector signed short,
                               vector unsigned short);
vector unsigned short vec_and (vector unsigned short,
                               vector signed short);
vector unsigned short vec_and (vector unsigned short,
                               vector unsigned short);
vector signed char vec_and (vector signed char, vector signed char);
vector unsigned char vec_and (vector signed char, vector unsigned char);

vector unsigned char vec_and (vector unsigned char, vector signed char);

vector unsigned char vec_and (vector unsigned char,
                              vector unsigned char);

vector float vec_andc (vector float, vector float);
vector float vec_andc (vector float, vector signed int);
vector float vec_andc (vector signed int, vector float);
vector signed int vec_andc (vector signed int, vector signed int);
vector unsigned int vec_andc (vector signed int, vector unsigned int);
vector unsigned int vec_andc (vector unsigned int, vector signed int);
vector unsigned int vec_andc (vector unsigned int, vector unsigned int);

vector signed short vec_andc (vector signed short, vector signed short);

vector unsigned short vec_andc (vector signed short,
                                vector unsigned short);
vector unsigned short vec_andc (vector unsigned short,
                                vector signed short);
vector unsigned short vec_andc (vector unsigned short,
                                vector unsigned short);
vector signed char vec_andc (vector signed char, vector signed char);
vector unsigned char vec_andc (vector signed char,
                               vector unsigned char);
vector unsigned char vec_andc (vector unsigned char,
                               vector signed char);
vector unsigned char vec_andc (vector unsigned char,
                               vector unsigned char);

vector unsigned char vec_avg (vector unsigned char,
                              vector unsigned char);
vector signed char vec_avg (vector signed char, vector signed char);
vector unsigned short vec_avg (vector unsigned short,
                               vector unsigned short);
vector signed short vec_avg (vector signed short, vector signed short);
vector unsigned int vec_avg (vector unsigned int, vector unsigned int);
vector signed int vec_avg (vector signed int, vector signed int);

vector float vec_ceil (vector float);

vector signed int vec_cmpb (vector float, vector float);

vector signed char vec_cmpeq (vector signed char, vector signed char);
vector signed char vec_cmpeq (vector unsigned char,
                              vector unsigned char);
vector signed short vec_cmpeq (vector signed short,
                               vector signed short);
vector signed short vec_cmpeq (vector unsigned short,
                               vector unsigned short);
vector signed int vec_cmpeq (vector signed int, vector signed int);
vector signed int vec_cmpeq (vector unsigned int, vector unsigned int);
vector signed int vec_cmpeq (vector float, vector float);

vector signed int vec_cmpge (vector float, vector float);

vector signed char vec_cmpgt (vector unsigned char,
                              vector unsigned char);
vector signed char vec_cmpgt (vector signed char, vector signed char);
vector signed short vec_cmpgt (vector unsigned short,
                               vector unsigned short);
vector signed short vec_cmpgt (vector signed short,
                               vector signed short);
vector signed int vec_cmpgt (vector unsigned int, vector unsigned int);
vector signed int vec_cmpgt (vector signed int, vector signed int);
vector signed int vec_cmpgt (vector float, vector float);

vector signed int vec_cmple (vector float, vector float);

vector signed char vec_cmplt (vector unsigned char,
                              vector unsigned char);
vector signed char vec_cmplt (vector signed char, vector signed char);
vector signed short vec_cmplt (vector unsigned short,
                               vector unsigned short);
vector signed short vec_cmplt (vector signed short,
                               vector signed short);
vector signed int vec_cmplt (vector unsigned int, vector unsigned int);
vector signed int vec_cmplt (vector signed int, vector signed int);
vector signed int vec_cmplt (vector float, vector float);

vector float vec_ctf (vector unsigned int, const char);
vector float vec_ctf (vector signed int, const char);

vector signed int vec_cts (vector float, const char);

vector unsigned int vec_ctu (vector float, const char);

void vec_dss (const char);

void vec_dssall (void);

void vec_dst (void *, int, const char);

void vec_dstst (void *, int, const char);

void vec_dststt (void *, int, const char);

void vec_dstt (void *, int, const char);

vector float vec_expte (vector float, vector float);

vector float vec_floor (vector float, vector float);

vector float vec_ld (int, vector float *);
vector float vec_ld (int, float *):
vector signed int vec_ld (int, int *);
vector signed int vec_ld (int, vector signed int *);
vector unsigned int vec_ld (int, vector unsigned int *);
vector unsigned int vec_ld (int, unsigned int *);
vector signed short vec_ld (int, short *, vector signed short *);
vector unsigned short vec_ld (int, unsigned short *,
                              vector unsigned short *);
vector signed char vec_ld (int, signed char *);
vector signed char vec_ld (int, vector signed char *);
vector unsigned char vec_ld (int, unsigned char *);
vector unsigned char vec_ld (int, vector unsigned char *);

vector signed char vec_lde (int, signed char *);
vector unsigned char vec_lde (int, unsigned char *);
vector signed short vec_lde (int, short *);
vector unsigned short vec_lde (int, unsigned short *);
vector float vec_lde (int, float *);
vector signed int vec_lde (int, int *);
vector unsigned int vec_lde (int, unsigned int *);

void float vec_ldl (int, float *);
void float vec_ldl (int, vector float *);
void signed int vec_ldl (int, vector signed int *);
void signed int vec_ldl (int, int *);
void unsigned int vec_ldl (int, unsigned int *);
void unsigned int vec_ldl (int, vector unsigned int *);
void signed short vec_ldl (int, vector signed short *);
void signed short vec_ldl (int, short *);
void unsigned short vec_ldl (int, vector unsigned short *);
void unsigned short vec_ldl (int, unsigned short *);
void signed char vec_ldl (int, vector signed char *);
void signed char vec_ldl (int, signed char *);
void unsigned char vec_ldl (int, vector unsigned char *);
void unsigned char vec_ldl (int, unsigned char *);

vector float vec_loge (vector float);

vector unsigned char vec_lvsl (int, void *, int *);

vector unsigned char vec_lvsr (int, void *, int *);

vector float vec_madd (vector float, vector float, vector float);

vector signed short vec_madds (vector signed short, vector signed short,
                               vector signed short);

vector unsigned char vec_max (vector signed char, vector unsigned char);

vector unsigned char vec_max (vector unsigned char, vector signed char);

vector unsigned char vec_max (vector unsigned char,
                              vector unsigned char);
vector signed char vec_max (vector signed char, vector signed char);
vector unsigned short vec_max (vector signed short,
                               vector unsigned short);
vector unsigned short vec_max (vector unsigned short,
                               vector signed short);
vector unsigned short vec_max (vector unsigned short,
                               vector unsigned short);
vector signed short vec_max (vector signed short, vector signed short);
vector unsigned int vec_max (vector signed int, vector unsigned int);
vector unsigned int vec_max (vector unsigned int, vector signed int);
vector unsigned int vec_max (vector unsigned int, vector unsigned int);
vector signed int vec_max (vector signed int, vector signed int);
vector float vec_max (vector float, vector float);

vector signed char vec_mergeh (vector signed char, vector signed char);
vector unsigned char vec_mergeh (vector unsigned char,
                                 vector unsigned char);
vector signed short vec_mergeh (vector signed short,
                                vector signed short);
vector unsigned short vec_mergeh (vector unsigned short,
                                  vector unsigned short);
vector float vec_mergeh (vector float, vector float);
vector signed int vec_mergeh (vector signed int, vector signed int);
vector unsigned int vec_mergeh (vector unsigned int,
                                vector unsigned int);

vector signed char vec_mergel (vector signed char, vector signed char);
vector unsigned char vec_mergel (vector unsigned char,
                                 vector unsigned char);
vector signed short vec_mergel (vector signed short,
                                vector signed short);
vector unsigned short vec_mergel (vector unsigned short,
                                  vector unsigned short);
vector float vec_mergel (vector float, vector float);
vector signed int vec_mergel (vector signed int, vector signed int);
vector unsigned int vec_mergel (vector unsigned int,
                                vector unsigned int);

vector unsigned short vec_mfvscr (void);

vector unsigned char vec_min (vector signed char, vector unsigned char);

vector unsigned char vec_min (vector unsigned char, vector signed char);

vector unsigned char vec_min (vector unsigned char,
                              vector unsigned char);
vector signed char vec_min (vector signed char, vector signed char);
vector unsigned short vec_min (vector signed short,
                               vector unsigned short);
vector unsigned short vec_min (vector unsigned short,
                               vector signed short);
vector unsigned short vec_min (vector unsigned short,
                               vector unsigned short);
vector signed short vec_min (vector signed short, vector signed short);
vector unsigned int vec_min (vector signed int, vector unsigned int);
vector unsigned int vec_min (vector unsigned int, vector signed int);
vector unsigned int vec_min (vector unsigned int, vector unsigned int);
vector signed int vec_min (vector signed int, vector signed int);
vector float vec_min (vector float, vector float);

vector signed short vec_mladd (vector signed short, vector signed short,
                               vector signed short);
vector signed short vec_mladd (vector signed short,
                               vector unsigned short,
                               vector unsigned short);
vector signed short vec_mladd (vector unsigned short,
                               vector signed short,
                               vector signed short);
vector unsigned short vec_mladd (vector unsigned short,
                                 vector unsigned short,
                                 vector unsigned short);

vector signed short vec_mradds (vector signed short,
                                vector signed short,
                                vector signed short);

vector unsigned int vec_msum (vector unsigned char,
                              vector unsigned char,
                              vector unsigned int);
vector signed int vec_msum (vector signed char, vector unsigned char,
                            vector signed int);
vector unsigned int vec_msum (vector unsigned short,
                              vector unsigned short,
                              vector unsigned int);
vector signed int vec_msum (vector signed short, vector signed short,
                            vector signed int);

vector unsigned int vec_msums (vector unsigned short,
                               vector unsigned short,
                               vector unsigned int);
vector signed int vec_msums (vector signed short, vector signed short,
                             vector signed int);

void vec_mtvscr (vector signed int);
void vec_mtvscr (vector unsigned int);
void vec_mtvscr (vector signed short);
void vec_mtvscr (vector unsigned short);
void vec_mtvscr (vector signed char);
void vec_mtvscr (vector unsigned char);

vector unsigned short vec_mule (vector unsigned char,
                                vector unsigned char);
vector signed short vec_mule (vector signed char, vector signed char);
vector unsigned int vec_mule (vector unsigned short,
                              vector unsigned short);
vector signed int vec_mule (vector signed short, vector signed short);

vector unsigned short vec_mulo (vector unsigned char,
                                vector unsigned char);
vector signed short vec_mulo (vector signed char, vector signed char);
vector unsigned int vec_mulo (vector unsigned short,
                              vector unsigned short);
vector signed int vec_mulo (vector signed short, vector signed short);

vector float vec_nmsub (vector float, vector float, vector float);

vector float vec_nor (vector float, vector float);
vector signed int vec_nor (vector signed int, vector signed int);
vector unsigned int vec_nor (vector unsigned int, vector unsigned int);
vector signed short vec_nor (vector signed short, vector signed short);
vector unsigned short vec_nor (vector unsigned short,
                               vector unsigned short);
vector signed char vec_nor (vector signed char, vector signed char);
vector unsigned char vec_nor (vector unsigned char,
                              vector unsigned char);

vector float vec_or (vector float, vector float);
vector float vec_or (vector float, vector signed int);
vector float vec_or (vector signed int, vector float);
vector signed int vec_or (vector signed int, vector signed int);
vector unsigned int vec_or (vector signed int, vector unsigned int);
vector unsigned int vec_or (vector unsigned int, vector signed int);
vector unsigned int vec_or (vector unsigned int, vector unsigned int);
vector signed short vec_or (vector signed short, vector signed short);
vector unsigned short vec_or (vector signed short,
                              vector unsigned short);
vector unsigned short vec_or (vector unsigned short,
                              vector signed short);
vector unsigned short vec_or (vector unsigned short,
                              vector unsigned short);
vector signed char vec_or (vector signed char, vector signed char);
vector unsigned char vec_or (vector signed char, vector unsigned char);
vector unsigned char vec_or (vector unsigned char, vector signed char);
vector unsigned char vec_or (vector unsigned char,
                             vector unsigned char);

vector signed char vec_pack (vector signed short, vector signed short);
vector unsigned char vec_pack (vector unsigned short,
                               vector unsigned short);
vector signed short vec_pack (vector signed int, vector signed int);
vector unsigned short vec_pack (vector unsigned int,
                                vector unsigned int);

vector signed short vec_packpx (vector unsigned int,
                                vector unsigned int);

vector unsigned char vec_packs (vector unsigned short,
                                vector unsigned short);
vector signed char vec_packs (vector signed short, vector signed short);

vector unsigned short vec_packs (vector unsigned int,
                                 vector unsigned int);
vector signed short vec_packs (vector signed int, vector signed int);

vector unsigned char vec_packsu (vector unsigned short,
                                 vector unsigned short);
vector unsigned char vec_packsu (vector signed short,
                                 vector signed short);
vector unsigned short vec_packsu (vector unsigned int,
                                  vector unsigned int);
vector unsigned short vec_packsu (vector signed int, vector signed int);

vector float vec_perm (vector float, vector float,
                       vector unsigned char);
vector signed int vec_perm (vector signed int, vector signed int,
                            vector unsigned char);
vector unsigned int vec_perm (vector unsigned int, vector unsigned int,
                              vector unsigned char);
vector signed short vec_perm (vector signed short, vector signed short,
                              vector unsigned char);
vector unsigned short vec_perm (vector unsigned short,
                                vector unsigned short,
                                vector unsigned char);
vector signed char vec_perm (vector signed char, vector signed char,
                             vector unsigned char);
vector unsigned char vec_perm (vector unsigned char,
                               vector unsigned char,
                               vector unsigned char);

vector float vec_re (vector float);

vector signed char vec_rl (vector signed char, vector unsigned char);
vector unsigned char vec_rl (vector unsigned char,
                             vector unsigned char);
vector signed short vec_rl (vector signed short, vector unsigned short);

vector unsigned short vec_rl (vector unsigned short,
                              vector unsigned short);
vector signed int vec_rl (vector signed int, vector unsigned int);
vector unsigned int vec_rl (vector unsigned int, vector unsigned int);

vector float vec_round (vector float);

vector float vec_rsqrte (vector float);

vector float vec_sel (vector float, vector float, vector signed int);
vector float vec_sel (vector float, vector float, vector unsigned int);
vector signed int vec_sel (vector signed int, vector signed int,
                           vector signed int);
vector signed int vec_sel (vector signed int, vector signed int,
                           vector unsigned int);
vector unsigned int vec_sel (vector unsigned int, vector unsigned int,
                             vector signed int);
vector unsigned int vec_sel (vector unsigned int, vector unsigned int,
                             vector unsigned int);
vector signed short vec_sel (vector signed short, vector signed short,
                             vector signed short);
vector signed short vec_sel (vector signed short, vector signed short,
                             vector unsigned short);
vector unsigned short vec_sel (vector unsigned short,
                               vector unsigned short,
                               vector signed short);
vector unsigned short vec_sel (vector unsigned short,
                               vector unsigned short,
                               vector unsigned short);
vector signed char vec_sel (vector signed char, vector signed char,
                            vector signed char);
vector signed char vec_sel (vector signed char, vector signed char,
                            vector unsigned char);
vector unsigned char vec_sel (vector unsigned char,
                              vector unsigned char,
                              vector signed char);
vector unsigned char vec_sel (vector unsigned char,
                              vector unsigned char,
                              vector unsigned char);

vector signed char vec_sl (vector signed char, vector unsigned char);
vector unsigned char vec_sl (vector unsigned char,
                             vector unsigned char);
vector signed short vec_sl (vector signed short, vector unsigned short);

vector unsigned short vec_sl (vector unsigned short,
                              vector unsigned short);
vector signed int vec_sl (vector signed int, vector unsigned int);
vector unsigned int vec_sl (vector unsigned int, vector unsigned int);

vector float vec_sld (vector float, vector float, const char);
vector signed int vec_sld (vector signed int, vector signed int,
                           const char);
vector unsigned int vec_sld (vector unsigned int, vector unsigned int,
                             const char);
vector signed short vec_sld (vector signed short, vector signed short,
                             const char);
vector unsigned short vec_sld (vector unsigned short,
                               vector unsigned short, const char);
vector signed char vec_sld (vector signed char, vector signed char,
                            const char);
vector unsigned char vec_sld (vector unsigned char,
                              vector unsigned char,
                              const char);

vector signed int vec_sll (vector signed int, vector unsigned int);
vector signed int vec_sll (vector signed int, vector unsigned short);
vector signed int vec_sll (vector signed int, vector unsigned char);
vector unsigned int vec_sll (vector unsigned int, vector unsigned int);
vector unsigned int vec_sll (vector unsigned int,
                             vector unsigned short);
vector unsigned int vec_sll (vector unsigned int, vector unsigned char);

vector signed short vec_sll (vector signed short, vector unsigned int);
vector signed short vec_sll (vector signed short,
                             vector unsigned short);
vector signed short vec_sll (vector signed short, vector unsigned char);

vector unsigned short vec_sll (vector unsigned short,
                               vector unsigned int);
vector unsigned short vec_sll (vector unsigned short,
                               vector unsigned short);
vector unsigned short vec_sll (vector unsigned short,
                               vector unsigned char);
vector signed char vec_sll (vector signed char, vector unsigned int);
vector signed char vec_sll (vector signed char, vector unsigned short);
vector signed char vec_sll (vector signed char, vector unsigned char);
vector unsigned char vec_sll (vector unsigned char,
                              vector unsigned int);
vector unsigned char vec_sll (vector unsigned char,
                              vector unsigned short);
vector unsigned char vec_sll (vector unsigned char,
                              vector unsigned char);

vector float vec_slo (vector float, vector signed char);
vector float vec_slo (vector float, vector unsigned char);
vector signed int vec_slo (vector signed int, vector signed char);
vector signed int vec_slo (vector signed int, vector unsigned char);
vector unsigned int vec_slo (vector unsigned int, vector signed char);
vector unsigned int vec_slo (vector unsigned int, vector unsigned char);

vector signed short vec_slo (vector signed short, vector signed char);
vector signed short vec_slo (vector signed short, vector unsigned char);

vector unsigned short vec_slo (vector unsigned short,
                               vector signed char);
vector unsigned short vec_slo (vector unsigned short,
                               vector unsigned char);
vector signed char vec_slo (vector signed char, vector signed char);
vector signed char vec_slo (vector signed char, vector unsigned char);
vector unsigned char vec_slo (vector unsigned char, vector signed char);

vector unsigned char vec_slo (vector unsigned char,
                              vector unsigned char);

vector signed char vec_splat (vector signed char, const char);
vector unsigned char vec_splat (vector unsigned char, const char);
vector signed short vec_splat (vector signed short, const char);
vector unsigned short vec_splat (vector unsigned short, const char);
vector float vec_splat (vector float, const char);
vector signed int vec_splat (vector signed int, const char);
vector unsigned int vec_splat (vector unsigned int, const char);

vector signed char vec_splat_s8 (const char);

vector signed short vec_splat_s16 (const char);

vector signed int vec_splat_s32 (const char);

vector unsigned char vec_splat_u8 (const char);

vector unsigned short vec_splat_u16 (const char);

vector unsigned int vec_splat_u32 (const char);

vector signed char vec_sr (vector signed char, vector unsigned char);
vector unsigned char vec_sr (vector unsigned char,
                             vector unsigned char);
vector signed short vec_sr (vector signed short, vector unsigned short);

vector unsigned short vec_sr (vector unsigned short,
                              vector unsigned short);
vector signed int vec_sr (vector signed int, vector unsigned int);
vector unsigned int vec_sr (vector unsigned int, vector unsigned int);

vector signed char vec_sra (vector signed char, vector unsigned char);
vector unsigned char vec_sra (vector unsigned char,
                              vector unsigned char);
vector signed short vec_sra (vector signed short,
                             vector unsigned short);
vector unsigned short vec_sra (vector unsigned short,
                               vector unsigned short);
vector signed int vec_sra (vector signed int, vector unsigned int);
vector unsigned int vec_sra (vector unsigned int, vector unsigned int);

vector signed int vec_srl (vector signed int, vector unsigned int);
vector signed int vec_srl (vector signed int, vector unsigned short);
vector signed int vec_srl (vector signed int, vector unsigned char);
vector unsigned int vec_srl (vector unsigned int, vector unsigned int);
vector unsigned int vec_srl (vector unsigned int,
                             vector unsigned short);
vector unsigned int vec_srl (vector unsigned int, vector unsigned char);

vector signed short vec_srl (vector signed short, vector unsigned int);
vector signed short vec_srl (vector signed short,
                             vector unsigned short);
vector signed short vec_srl (vector signed short, vector unsigned char);

vector unsigned short vec_srl (vector unsigned short,
                               vector unsigned int);
vector unsigned short vec_srl (vector unsigned short,
                               vector unsigned short);
vector unsigned short vec_srl (vector unsigned short,
                               vector unsigned char);
vector signed char vec_srl (vector signed char, vector unsigned int);
vector signed char vec_srl (vector signed char, vector unsigned short);
vector signed char vec_srl (vector signed char, vector unsigned char);
vector unsigned char vec_srl (vector unsigned char,
                              vector unsigned int);
vector unsigned char vec_srl (vector unsigned char,
                              vector unsigned short);
vector unsigned char vec_srl (vector unsigned char,
                              vector unsigned char);

vector float vec_sro (vector float, vector signed char);
vector float vec_sro (vector float, vector unsigned char);
vector signed int vec_sro (vector signed int, vector signed char);
vector signed int vec_sro (vector signed int, vector unsigned char);
vector unsigned int vec_sro (vector unsigned int, vector signed char);
vector unsigned int vec_sro (vector unsigned int, vector unsigned char);

vector signed short vec_sro (vector signed short, vector signed char);
vector signed short vec_sro (vector signed short, vector unsigned char);

vector unsigned short vec_sro (vector unsigned short,
                               vector signed char);
vector unsigned short vec_sro (vector unsigned short,
                               vector unsigned char);
vector signed char vec_sro (vector signed char, vector signed char);
vector signed char vec_sro (vector signed char, vector unsigned char);
vector unsigned char vec_sro (vector unsigned char, vector signed char);

vector unsigned char vec_sro (vector unsigned char,
                              vector unsigned char);

void vec_st (vector float, int, float *);
void vec_st (vector float, int, vector float *);
void vec_st (vector signed int, int, int *);
void vec_st (vector signed int, int, unsigned int *);
void vec_st (vector unsigned int, int, unsigned int *);
void vec_st (vector unsigned int, int, vector unsigned int *);
void vec_st (vector signed short, int, short *);
void vec_st (vector signed short, int, vector unsigned short *);
void vec_st (vector signed short, int, vector signed short *);
void vec_st (vector unsigned short, int, unsigned short *);
void vec_st (vector unsigned short, int, vector unsigned short *);
void vec_st (vector signed char, int, signed char *);
void vec_st (vector signed char, int, unsigned char *);
void vec_st (vector signed char, int, vector signed char *);
void vec_st (vector unsigned char, int, unsigned char *);
void vec_st (vector unsigned char, int, vector unsigned char *);

void vec_ste (vector signed char, int, unsigned char *);
void vec_ste (vector signed char, int, signed char *);
void vec_ste (vector unsigned char, int, unsigned char *);
void vec_ste (vector signed short, int, short *);
void vec_ste (vector signed short, int, unsigned short *);
void vec_ste (vector unsigned short, int, void *);
void vec_ste (vector signed int, int, unsigned int *);
void vec_ste (vector signed int, int, int *);
void vec_ste (vector unsigned int, int, unsigned int *);
void vec_ste (vector float, int, float *);

void vec_stl (vector float, int, vector float *);
void vec_stl (vector float, int, float *);
void vec_stl (vector signed int, int, vector signed int *);
void vec_stl (vector signed int, int, int *);
void vec_stl (vector signed int, int, unsigned int *);
void vec_stl (vector unsigned int, int, vector unsigned int *);
void vec_stl (vector unsigned int, int, unsigned int *);
void vec_stl (vector signed short, int, short *);
void vec_stl (vector signed short, int, unsigned short *);
void vec_stl (vector signed short, int, vector signed short *);
void vec_stl (vector unsigned short, int, unsigned short *);
void vec_stl (vector unsigned short, int, vector signed short *);
void vec_stl (vector signed char, int, signed char *);
void vec_stl (vector signed char, int, unsigned char *);
void vec_stl (vector signed char, int, vector signed char *);
void vec_stl (vector unsigned char, int, unsigned char *);
void vec_stl (vector unsigned char, int, vector unsigned char *);

vector signed char vec_sub (vector signed char, vector signed char);
vector unsigned char vec_sub (vector signed char, vector unsigned char);

vector unsigned char vec_sub (vector unsigned char, vector signed char);

vector unsigned char vec_sub (vector unsigned char,
                              vector unsigned char);
vector signed short vec_sub (vector signed short, vector signed short);
vector unsigned short vec_sub (vector signed short,
                               vector unsigned short);
vector unsigned short vec_sub (vector unsigned short,
                               vector signed short);
vector unsigned short vec_sub (vector unsigned short,
                               vector unsigned short);
vector signed int vec_sub (vector signed int, vector signed int);
vector unsigned int vec_sub (vector signed int, vector unsigned int);
vector unsigned int vec_sub (vector unsigned int, vector signed int);
vector unsigned int vec_sub (vector unsigned int, vector unsigned int);
vector float vec_sub (vector float, vector float);

vector unsigned int vec_subc (vector unsigned int, vector unsigned int);

vector unsigned char vec_subs (vector signed char,
                               vector unsigned char);
vector unsigned char vec_subs (vector unsigned char,
                               vector signed char);
vector unsigned char vec_subs (vector unsigned char,
                               vector unsigned char);
vector signed char vec_subs (vector signed char, vector signed char);
vector unsigned short vec_subs (vector signed short,
                                vector unsigned short);
vector unsigned short vec_subs (vector unsigned short,
                                vector signed short);
vector unsigned short vec_subs (vector unsigned short,
                                vector unsigned short);
vector signed short vec_subs (vector signed short, vector signed short);

vector unsigned int vec_subs (vector signed int, vector unsigned int);
vector unsigned int vec_subs (vector unsigned int, vector signed int);
vector unsigned int vec_subs (vector unsigned int, vector unsigned int);

vector signed int vec_subs (vector signed int, vector signed int);

vector unsigned int vec_sum4s (vector unsigned char,
                               vector unsigned int);
vector signed int vec_sum4s (vector signed char, vector signed int);
vector signed int vec_sum4s (vector signed short, vector signed int);

vector signed int vec_sum2s (vector signed int, vector signed int);

vector signed int vec_sums (vector signed int, vector signed int);

vector float vec_trunc (vector float);

vector signed short vec_unpackh (vector signed char);
vector unsigned int vec_unpackh (vector signed short);
vector signed int vec_unpackh (vector signed short);

vector signed short vec_unpackl (vector signed char);
vector unsigned int vec_unpackl (vector signed short);
vector signed int vec_unpackl (vector signed short);

vector float vec_xor (vector float, vector float);
vector float vec_xor (vector float, vector signed int);
vector float vec_xor (vector signed int, vector float);
vector signed int vec_xor (vector signed int, vector signed int);
vector unsigned int vec_xor (vector signed int, vector unsigned int);
vector unsigned int vec_xor (vector unsigned int, vector signed int);
vector unsigned int vec_xor (vector unsigned int, vector unsigned int);
vector signed short vec_xor (vector signed short, vector signed short);
vector unsigned short vec_xor (vector signed short,
                               vector unsigned short);
vector unsigned short vec_xor (vector unsigned short,
                               vector signed short);
vector unsigned short vec_xor (vector unsigned short,
                               vector unsigned short);
vector signed char vec_xor (vector signed char, vector signed char);
vector unsigned char vec_xor (vector signed char, vector unsigned char);

vector unsigned char vec_xor (vector unsigned char, vector signed char);

vector unsigned char vec_xor (vector unsigned char,
                              vector unsigned char);

vector signed int vec_all_eq (vector signed char, vector unsigned char);

vector signed int vec_all_eq (vector signed char, vector signed char);
vector signed int vec_all_eq (vector unsigned char, vector signed char);

vector signed int vec_all_eq (vector unsigned char,
                              vector unsigned char);
vector signed int vec_all_eq (vector signed short,
                              vector unsigned short);
vector signed int vec_all_eq (vector signed short, vector signed short);

vector signed int vec_all_eq (vector unsigned short,
                              vector signed short);
vector signed int vec_all_eq (vector unsigned short,
                              vector unsigned short);
vector signed int vec_all_eq (vector signed int, vector unsigned int);
vector signed int vec_all_eq (vector signed int, vector signed int);
vector signed int vec_all_eq (vector unsigned int, vector signed int);
vector signed int vec_all_eq (vector unsigned int, vector unsigned int);

vector signed int vec_all_eq (vector float, vector float);

vector signed int vec_all_ge (vector signed char, vector unsigned char);

vector signed int vec_all_ge (vector unsigned char, vector signed char);

vector signed int vec_all_ge (vector unsigned char,
                              vector unsigned char);
vector signed int vec_all_ge (vector signed char, vector signed char);
vector signed int vec_all_ge (vector signed short,
                              vector unsigned short);
vector signed int vec_all_ge (vector unsigned short,
                              vector signed short);
vector signed int vec_all_ge (vector unsigned short,
                              vector unsigned short);
vector signed int vec_all_ge (vector signed short, vector signed short);

vector signed int vec_all_ge (vector signed int, vector unsigned int);
vector signed int vec_all_ge (vector unsigned int, vector signed int);
vector signed int vec_all_ge (vector unsigned int, vector unsigned int);

vector signed int vec_all_ge (vector signed int, vector signed int);
vector signed int vec_all_ge (vector float, vector float);

vector signed int vec_all_gt (vector signed char, vector unsigned char);

vector signed int vec_all_gt (vector unsigned char, vector signed char);

vector signed int vec_all_gt (vector unsigned char,
                              vector unsigned char);
vector signed int vec_all_gt (vector signed char, vector signed char);
vector signed int vec_all_gt (vector signed short,
                              vector unsigned short);
vector signed int vec_all_gt (vector unsigned short,
                              vector signed short);
vector signed int vec_all_gt (vector unsigned short,
                              vector unsigned short);
vector signed int vec_all_gt (vector signed short, vector signed short);

vector signed int vec_all_gt (vector signed int, vector unsigned int);
vector signed int vec_all_gt (vector unsigned int, vector signed int);
vector signed int vec_all_gt (vector unsigned int, vector unsigned int);

vector signed int vec_all_gt (vector signed int, vector signed int);
vector signed int vec_all_gt (vector float, vector float);

vector signed int vec_all_in (vector float, vector float);

vector signed int vec_all_le (vector signed char, vector unsigned char);

vector signed int vec_all_le (vector unsigned char, vector signed char);

vector signed int vec_all_le (vector unsigned char,
                              vector unsigned char);
vector signed int vec_all_le (vector signed char, vector signed char);
vector signed int vec_all_le (vector signed short,
                              vector unsigned short);
vector signed int vec_all_le (vector unsigned short,
                              vector signed short);
vector signed int vec_all_le (vector unsigned short,
                              vector unsigned short);
vector signed int vec_all_le (vector signed short, vector signed short);

vector signed int vec_all_le (vector signed int, vector unsigned int);
vector signed int vec_all_le (vector unsigned int, vector signed int);
vector signed int vec_all_le (vector unsigned int, vector unsigned int);

vector signed int vec_all_le (vector signed int, vector signed int);
vector signed int vec_all_le (vector float, vector float);

vector signed int vec_all_lt (vector signed char, vector unsigned char);

vector signed int vec_all_lt (vector unsigned char, vector signed char);

vector signed int vec_all_lt (vector unsigned char,
                              vector unsigned char);
vector signed int vec_all_lt (vector signed char, vector signed char);
vector signed int vec_all_lt (vector signed short,
                              vector unsigned short);
vector signed int vec_all_lt (vector unsigned short,
                              vector signed short);
vector signed int vec_all_lt (vector unsigned short,
                              vector unsigned short);
vector signed int vec_all_lt (vector signed short, vector signed short);

vector signed int vec_all_lt (vector signed int, vector unsigned int);
vector signed int vec_all_lt (vector unsigned int, vector signed int);
vector signed int vec_all_lt (vector unsigned int, vector unsigned int);

vector signed int vec_all_lt (vector signed int, vector signed int);
vector signed int vec_all_lt (vector float, vector float);

vector signed int vec_all_nan (vector float);

vector signed int vec_all_ne (vector signed char, vector unsigned char);

vector signed int vec_all_ne (vector signed char, vector signed char);
vector signed int vec_all_ne (vector unsigned char, vector signed char);

vector signed int vec_all_ne (vector unsigned char,
                              vector unsigned char);
vector signed int vec_all_ne (vector signed short,
                              vector unsigned short);
vector signed int vec_all_ne (vector signed short, vector signed short);

vector signed int vec_all_ne (vector unsigned short,
                              vector signed short);
vector signed int vec_all_ne (vector unsigned short,
                              vector unsigned short);
vector signed int vec_all_ne (vector signed int, vector unsigned int);
vector signed int vec_all_ne (vector signed int, vector signed int);
vector signed int vec_all_ne (vector unsigned int, vector signed int);
vector signed int vec_all_ne (vector unsigned int, vector unsigned int);

vector signed int vec_all_ne (vector float, vector float);

vector signed int vec_all_nge (vector float, vector float);

vector signed int vec_all_ngt (vector float, vector float);

vector signed int vec_all_nle (vector float, vector float);

vector signed int vec_all_nlt (vector float, vector float);

vector signed int vec_all_numeric (vector float);

vector signed int vec_any_eq (vector signed char, vector unsigned char);

vector signed int vec_any_eq (vector signed char, vector signed char);
vector signed int vec_any_eq (vector unsigned char, vector signed char);

vector signed int vec_any_eq (vector unsigned char,
                              vector unsigned char);
vector signed int vec_any_eq (vector signed short,
                              vector unsigned short);
vector signed int vec_any_eq (vector signed short, vector signed short);

vector signed int vec_any_eq (vector unsigned short,
                              vector signed short);
vector signed int vec_any_eq (vector unsigned short,
                              vector unsigned short);
vector signed int vec_any_eq (vector signed int, vector unsigned int);
vector signed int vec_any_eq (vector signed int, vector signed int);
vector signed int vec_any_eq (vector unsigned int, vector signed int);
vector signed int vec_any_eq (vector unsigned int, vector unsigned int);

vector signed int vec_any_eq (vector float, vector float);

vector signed int vec_any_ge (vector signed char, vector unsigned char);

vector signed int vec_any_ge (vector unsigned char, vector signed char);

vector signed int vec_any_ge (vector unsigned char,
                              vector unsigned char);
vector signed int vec_any_ge (vector signed char, vector signed char);
vector signed int vec_any_ge (vector signed short,
                              vector unsigned short);
vector signed int vec_any_ge (vector unsigned short,
                              vector signed short);
vector signed int vec_any_ge (vector unsigned short,
                              vector unsigned short);
vector signed int vec_any_ge (vector signed short, vector signed short);

vector signed int vec_any_ge (vector signed int, vector unsigned int);
vector signed int vec_any_ge (vector unsigned int, vector signed int);
vector signed int vec_any_ge (vector unsigned int, vector unsigned int);

vector signed int vec_any_ge (vector signed int, vector signed int);
vector signed int vec_any_ge (vector float, vector float);

vector signed int vec_any_gt (vector signed char, vector unsigned char);

vector signed int vec_any_gt (vector unsigned char, vector signed char);

vector signed int vec_any_gt (vector unsigned char,
                              vector unsigned char);
vector signed int vec_any_gt (vector signed char, vector signed char);
vector signed int vec_any_gt (vector signed short,
                              vector unsigned short);
vector signed int vec_any_gt (vector unsigned short,
                              vector signed short);
vector signed int vec_any_gt (vector unsigned short,
                              vector unsigned short);
vector signed int vec_any_gt (vector signed short, vector signed short);

vector signed int vec_any_gt (vector signed int, vector unsigned int);
vector signed int vec_any_gt (vector unsigned int, vector signed int);
vector signed int vec_any_gt (vector unsigned int, vector unsigned int);

vector signed int vec_any_gt (vector signed int, vector signed int);
vector signed int vec_any_gt (vector float, vector float);

vector signed int vec_any_le (vector signed char, vector unsigned char);

vector signed int vec_any_le (vector unsigned char, vector signed char);

vector signed int vec_any_le (vector unsigned char,
                              vector unsigned char);
vector signed int vec_any_le (vector signed char, vector signed char);
vector signed int vec_any_le (vector signed short,
                              vector unsigned short);
vector signed int vec_any_le (vector unsigned short,
                              vector signed short);
vector signed int vec_any_le (vector unsigned short,
                              vector unsigned short);
vector signed int vec_any_le (vector signed short, vector signed short);

vector signed int vec_any_le (vector signed int, vector unsigned int);
vector signed int vec_any_le (vector unsigned int, vector signed int);
vector signed int vec_any_le (vector unsigned int, vector unsigned int);

vector signed int vec_any_le (vector signed int, vector signed int);
vector signed int vec_any_le (vector float, vector float);

vector signed int vec_any_lt (vector signed char, vector unsigned char);

vector signed int vec_any_lt (vector unsigned char, vector signed char);

vector signed int vec_any_lt (vector unsigned char,
                              vector unsigned char);
vector signed int vec_any_lt (vector signed char, vector signed char);
vector signed int vec_any_lt (vector signed short,
                              vector unsigned short);
vector signed int vec_any_lt (vector unsigned short,
                              vector signed short);
vector signed int vec_any_lt (vector unsigned short,
                              vector unsigned short);
vector signed int vec_any_lt (vector signed short, vector signed short);

vector signed int vec_any_lt (vector signed int, vector unsigned int);
vector signed int vec_any_lt (vector unsigned int, vector signed int);
vector signed int vec_any_lt (vector unsigned int, vector unsigned int);

vector signed int vec_any_lt (vector signed int, vector signed int);
vector signed int vec_any_lt (vector float, vector float);

vector signed int vec_any_nan (vector float);

vector signed int vec_any_ne (vector signed char, vector unsigned char);

vector signed int vec_any_ne (vector signed char, vector signed char);
vector signed int vec_any_ne (vector unsigned char, vector signed char);

vector signed int vec_any_ne (vector unsigned char,
                              vector unsigned char);
vector signed int vec_any_ne (vector signed short,
                              vector unsigned short);
vector signed int vec_any_ne (vector signed short, vector signed short);

vector signed int vec_any_ne (vector unsigned short,
                              vector signed short);
vector signed int vec_any_ne (vector unsigned short,
                              vector unsigned short);
vector signed int vec_any_ne (vector signed int, vector unsigned int);
vector signed int vec_any_ne (vector signed int, vector signed int);
vector signed int vec_any_ne (vector unsigned int, vector signed int);
vector signed int vec_any_ne (vector unsigned int, vector unsigned int);

vector signed int vec_any_ne (vector float, vector float);

vector signed int vec_any_nge (vector float, vector float);

vector signed int vec_any_ngt (vector float, vector float);

vector signed int vec_any_nle (vector float, vector float);

vector signed int vec_any_nlt (vector float, vector float);

vector signed int vec_any_numeric (vector float);

vector signed int vec_any_out (vector float, vector float);


5.46 Pragmas Accepted by GCC

GCC supports several types of pragmas, primarily in order to compile code originally written for other compilers. Note that in general we do not recommend the use of pragmas; See section 5.25 Declaring Attributes of Functions, for further explanation.

5.46.1 ARM Pragmas  
5.46.2 RS/6000 and PowerPC Pragmas  
5.46.3 Darwin Pragmas  
5.46.4 Solaris Pragmas  
5.46.5 Tru64 Pragmas  


5.46.1 ARM Pragmas

The ARM target defines pragmas for controlling the default addition of long_call and short_call attributes to functions. See section 5.25 Declaring Attributes of Functions, for information about the effects of these attributes.

long_calls
Set all subsequent functions to have the long_call attribute.

no_long_calls
Set all subsequent functions to have the short_call attribute.

long_calls_off
Do not affect the long_call or short_call attributes of subsequent functions.


5.46.2 RS/6000 and PowerPC Pragmas

The RS/6000 and PowerPC targets define one pragma for controlling whether or not the longcall attribute is added to function declarations by default. This pragma overrides the `-mlongcall' option, but not the longcall and shortcall attributes. See section 3.17.10 IBM RS/6000 and PowerPC Options, for more information about when long calls are and are not necessary.

longcall (1)
Apply the longcall attribute to all subsequent function declarations.

longcall (0)
Do not apply the longcall attribute to subsequent function declarations.


5.46.3 Darwin Pragmas

The following pragmas are available for all architectures running the Darwin operating system. These are useful for compatibility with other Mac OS compilers.

mark tokens...
This pragma is accepted, but has no effect.

options align=alignment
This pragma sets the alignment of fields in structures. The values of alignment may be mac68k, to emulate m68k alignment, or power, to emulate PowerPC alignment. Uses of this pragma nest properly; to restore the previous setting, use reset for the alignment.

segment tokens...
This pragma is accepted, but has no effect.

unused (var [, var]...)
This pragma declares variables to be possibly unused. GCC will not produce warnings for the listed variables. The effect is similar to that of the unused attribute, except that this pragma may appear anywhere within the variables' scopes.


5.46.4 Solaris Pragmas

For compatibility with the SunPRO compiler, the following pragma is supported.

redefine_extname oldname newname

This pragma gives the C function oldname the assembler label newname. The pragma must appear before the function declaration. This pragma is equivalent to the asm labels extension (see section 5.37 Controlling Names Used in Assembler Code). The preprocessor defines __PRAGMA_REDEFINE_EXTNAME if the pragma is available.


5.46.5 Tru64 Pragmas

For compatibility with the Compaq C compiler, the following pragma is supported.

extern_prefix string

This pragma renames all subsequent function and variable declarations such that string is prepended to the name. This effect may be terminated by using another extern_prefix pragma with the empty string.

This pragma is similar in intent to to the asm labels extension (see section 5.37 Controlling Names Used in Assembler Code) in that the system programmer wants to change the assembly-level ABI without changing the source-level API. The preprocessor defines __PRAGMA_EXTERN_PREFIX if the pragma is available.


5.47 Unnamed struct/union fields within structs/unions.

For compatibility with other compilers, GCC allows you to define a structure or union that contains, as fields, structures and unions without names. For example:

 
struct {
  int a;
  union {
    int b;
    float c;
  };
  int d;
} foo;

In this example, the user would be able to access members of the unnamed union with code like `foo.b'. Note that only unnamed structs and unions are allowed, you may not have, for example, an unnamed int.

You must never create such structures that cause ambiguous field definitions. For example, this structure:

 
struct {
  int a;
  struct {
    int a;
  };
} foo;

It is ambiguous which a is being referred to with `foo.a'. Such constructs are not supported and must be avoided. In the future, such constructs may be detected and treated as compilation errors.


5.48 Thread-Local Storage

Thread-local storage (TLS) is a mechanism by which variables are allocated such that there is one instance of the variable per extant thread. The run-time model GCC uses to implement this originates in the IA-64 processor-specific ABI, but has since been migrated to other processors as well. It requires significant support from the linker (ld), dynamic linker (ld.so), and system libraries (`libc.so' and `libpthread.so'), so it is not available everywhere.

At the user level, the extension is visible with a new storage class keyword: __thread. For example:

 
__thread int i;
extern __thread struct state s;
static __thread char *p;

The __thread specifier may be used alone, with the extern or static specifiers, but with no other storage class specifier. When used with extern or static, __thread must appear immediately after the other storage class specifier.

The __thread specifier may be applied to any global, file-scoped static, function-scoped static, or static data member of a class. It may not be applied to block-scoped automatic or non-static data member.

When the address-of operator is applied to a thread-local variable, it is evaluated at run-time and returns the address of the current thread's instance of that variable. An address so obtained may be used by any thread. When a thread terminates, any pointers to thread-local variables in that thread become invalid.

No static initialization may refer to the address of a thread-local variable.

In C++, if an initializer is present for a thread-local variable, it must be a constant-expression, as defined in 5.19.2 of the ANSI/ISO C++ standard.

See ELF Handling For Thread-Local Storage for a detailed explanation of the four thread-local storage addressing models, and how the run-time is expected to function.

5.48.1 ISO/IEC 9899:1999 Edits for Thread-Local Storage  
5.48.2 ISO/IEC 14882:1998 Edits for Thread-Local Storage  


5.48.1 ISO/IEC 9899:1999 Edits for Thread-Local Storage

The following are a set of changes to ISO/IEC 9899:1999 (aka C99) that document the exact semantics of the language extension.


5.48.2 ISO/IEC 14882:1998 Edits for Thread-Local Storage

The following are a set of changes to ISO/IEC 14882:1998 (aka C++98) that document the exact semantics of the language extension.


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