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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 C Dialect Options) 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).

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, llabs and imaxabs are handled as built-in functions except in strict ISO C89 mode. There are also built-in versions of the ISO C99 functions cosf, cosl, fabsf, fabsl, sinf, sinl, sqrtf, and sqrtl, that are recognized in any mode since ISO C89 reserves these names for the purpose to which ISO C99 puts them. All these functions have corresponding versions prefixed with __builtin_.

The ISO C89 functions abs, cos, fabs, fprintf, fputs, labs, memcmp, memcpy, memset, printf, sin, sqrt, strcat, strchr, strcmp, strcpy, strcspn, strlen, strncat, strncmp, strncpy, strpbrk, strrchr, strspn, and strstr 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.

int __builtin_types_compatible_p (type1, type2) Built-in Function

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.

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

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.


          #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.  */                     \

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.

int __builtin_constant_p (exp) Built-in Function
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 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.

long __builtin_expect (long exp, long c) Built-in Function
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.

void __builtin_prefetch (const void *addr, ...) Built-in Function
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.