### 6.51 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.

Outside strict ISO C mode (`-ansi`, `-std=c90` or `-std=c99`), the functions `_exit`, `alloca`, `bcmp`, `bzero`, `dcgettext`, `dgettext`, `dremf`, `dreml`, `drem`, `exp10f`, `exp10l`, `exp10`, `ffsll`, `ffsl`, `ffs`, `fprintf_unlocked`, `fputs_unlocked`, `gammaf`, `gammal`, `gamma`, `gammaf_r`, `gammal_r`, `gamma_r`, `gettext`, `index`, `isascii`, `j0f`, `j0l`, `j0`, `j1f`, `j1l`, `j1`, `jnf`, `jnl`, `jn`, `lgammaf_r`, `lgammal_r`, `lgamma_r`, `mempcpy`, `pow10f`, `pow10l`, `pow10`, `printf_unlocked`, `rindex`, `scalbf`, `scalbl`, `scalb`, `signbit`, `signbitf`, `signbitl`, `signbitd32`, `signbitd64`, `signbitd128`, `significandf`, `significandl`, `significand`, `sincosf`, `sincosl`, `sincos`, `stpcpy`, `stpncpy`, `strcasecmp`, `strdup`, `strfmon`, `strncasecmp`, `strndup`, `toascii`, `y0f`, `y0l`, `y0`, `y1f`, `y1l`, `y1`, `ynf`, `ynl` and `yn` may be handled as built-in functions. All these functions have corresponding versions prefixed with `__builtin_`, which may be used even in strict C90 mode.

The ISO C99 functions `_Exit`, `acoshf`, `acoshl`, `acosh`, `asinhf`, `asinhl`, `asinh`, `atanhf`, `atanhl`, `atanh`, `cabsf`, `cabsl`, `cabs`, `cacosf`, `cacoshf`, `cacoshl`, `cacosh`, `cacosl`, `cacos`, `cargf`, `cargl`, `carg`, `casinf`, `casinhf`, `casinhl`, `casinh`, `casinl`, `casin`, `catanf`, `catanhf`, `catanhl`, `catanh`, `catanl`, `catan`, `cbrtf`, `cbrtl`, `cbrt`, `ccosf`, `ccoshf`, `ccoshl`, `ccosh`, `ccosl`, `ccos`, `cexpf`, `cexpl`, `cexp`, `cimagf`, `cimagl`, `cimag`, `clogf`, `clogl`, `clog`, `conjf`, `conjl`, `conj`, `copysignf`, `copysignl`, `copysign`, `cpowf`, `cpowl`, `cpow`, `cprojf`, `cprojl`, `cproj`, `crealf`, `creall`, `creal`, `csinf`, `csinhf`, `csinhl`, `csinh`, `csinl`, `csin`, `csqrtf`, `csqrtl`, `csqrt`, `ctanf`, `ctanhf`, `ctanhl`, `ctanh`, `ctanl`, `ctan`, `erfcf`, `erfcl`, `erfc`, `erff`, `erfl`, `erf`, `exp2f`, `exp2l`, `exp2`, `expm1f`, `expm1l`, `expm1`, `fdimf`, `fdiml`, `fdim`, `fmaf`, `fmal`, `fmaxf`, `fmaxl`, `fmax`, `fma`, `fminf`, `fminl`, `fmin`, `hypotf`, `hypotl`, `hypot`, `ilogbf`, `ilogbl`, `ilogb`, `imaxabs`, `isblank`, `iswblank`, `lgammaf`, `lgammal`, `lgamma`, `llabs`, `llrintf`, `llrintl`, `llrint`, `llroundf`, `llroundl`, `llround`, `log1pf`, `log1pl`, `log1p`, `log2f`, `log2l`, `log2`, `logbf`, `logbl`, `logb`, `lrintf`, `lrintl`, `lrint`, `lroundf`, `lroundl`, `lround`, `nearbyintf`, `nearbyintl`, `nearbyint`, `nextafterf`, `nextafterl`, `nextafter`, `nexttowardf`, `nexttowardl`, `nexttoward`, `remainderf`, `remainderl`, `remainder`, `remquof`, `remquol`, `remquo`, `rintf`, `rintl`, `rint`, `roundf`, `roundl`, `round`, `scalblnf`, `scalblnl`, `scalbln`, `scalbnf`, `scalbnl`, `scalbn`, `snprintf`, `tgammaf`, `tgammal`, `tgamma`, `truncf`, `truncl`, `trunc`, `vfscanf`, `vscanf`, `vsnprintf` and `vsscanf` are handled as built-in functions except in strict ISO C90 mode (`-ansi` or `-std=c90`).

There are also built-in versions of the ISO C99 functions `acosf`, `acosl`, `asinf`, `asinl`, `atan2f`, `atan2l`, `atanf`, `atanl`, `ceilf`, `ceill`, `cosf`, `coshf`, `coshl`, `cosl`, `expf`, `expl`, `fabsf`, `fabsl`, `floorf`, `floorl`, `fmodf`, `fmodl`, `frexpf`, `frexpl`, `ldexpf`, `ldexpl`, `log10f`, `log10l`, `logf`, `logl`, `modfl`, `modf`, `powf`, `powl`, `sinf`, `sinhf`, `sinhl`, `sinl`, `sqrtf`, `sqrtl`, `tanf`, `tanhf`, `tanhl` and `tanl` 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 C94 functions `iswalnum`, `iswalpha`, `iswcntrl`, `iswdigit`, `iswgraph`, `iswlower`, `iswprint`, `iswpunct`, `iswspace`, `iswupper`, `iswxdigit`, `towlower` and `towupper` are handled as built-in functions except in strict ISO C90 mode (`-ansi` or `-std=c90`).

The ISO C90 functions `abort`, `abs`, `acos`, `asin`, `atan2`, `atan`, `calloc`, `ceil`, `cosh`, `cos`, `exit`, `exp`, `fabs`, `floor`, `fmod`, `fprintf`, `fputs`, `frexp`, `fscanf`, `isalnum`, `isalpha`, `iscntrl`, `isdigit`, `isgraph`, `islower`, `isprint`, `ispunct`, `isspace`, `isupper`, `isxdigit`, `tolower`, `toupper`, `labs`, `ldexp`, `log10`, `log`, `malloc`, `memchr`, `memcmp`, `memcpy`, `memset`, `modf`, `pow`, `printf`, `putchar`, `puts`, `scanf`, `sinh`, `sin`, `snprintf`, `sprintf`, `sqrt`, `sscanf`, `strcat`, `strchr`, `strcmp`, `strcpy`, `strcspn`, `strlen`, `strncat`, `strncmp`, `strncpy`, `strpbrk`, `strrchr`, `strspn`, `strstr`, `tanh`, `tan`, `vfprintf`, `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. In the same fashion, GCC provides `fpclassify`, `isfinite`, `isinf_sign` and `isnormal` built-ins used with `__builtin_` prefixed. The `isinf` and `isnan` builtins appear both with and without the `__builtin_` prefix.

— 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 not considered to be compatible with another `enum` type even if both are compatible with the same integer type; this is what the C standard specifies. For example, `enum {foo, bar}` is not 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 = (x);                                       \
>                   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 an integer constant expression, 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 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, including the case where `__builtin_constant_p` returns 1 because EXPRESSION can be folded to a constant but EXPRESSION contains operands that would not otherwise be permitted in a static initializer (for example, `0 && foo ()`). 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 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_trap (void)

> This function causes the program to exit abnormally. GCC implements this function by using a target-dependent mechanism (such as intentionally executing an illegal instruction) or by calling `abort`. The mechanism used may vary from release to release so you should not rely on any particular implementation.

— Built-in Function: void __builtin_unreachable (void)

> If control flow reaches the point of the `__builtin_unreachable`, the program is undefined. It is useful in situations where the compiler cannot deduce the unreachability of the code.

> One such case is immediately following an `asm` statement that will either never terminate, or one that transfers control elsewhere and never returns. In this example, without the `__builtin_unreachable`, GCC would issue a warning that control reaches the end of a non-void function. It would also generate code to return after the `asm`.

>               int f (int c, int v)
>               {
>                 if (c)
>                   {
>                     return v;
>                   }
>                 else
>                   {
>                     asm("jmp error_handler");
>                     __builtin_unreachable ();
>                   }
>               }

> Because the `asm` statement unconditionally transfers control out of the function, control will never reach the end of the function body. The `__builtin_unreachable` is in fact unreachable and communicates this fact to the compiler.

> Another use for `__builtin_unreachable` is following a call a function that never returns but that is not declared `__attribute__((noreturn))`, as in this example:

>               void function_that_never_returns (void);

>               int g (int c)
>               {
>                 if (c)
>                   {
>                     return 1;
>                   }
>                 else
>                   {
>                     function_that_never_returns ();
>                     __builtin_unreachable ();
>                   }
>               }

— Built-in Function: void __builtin___clear_cache (char *begin, char *end)

> This function is used to flush the processor's instruction cache for the region of memory between begin inclusive and end exclusive. Some targets require that the instruction cache be flushed, after modifying memory containing code, in order to obtain deterministic behavior.

> If the target does not require instruction cache flushes, `__builtin___clear_cache` has no effect. Otherwise either instructions are emitted in-line to clear the instruction cache or a call to the `__clear_cache` function in libgcc is made.

— 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: int __builtin_fpclassify (int, int, int, int, int, ...)

> This built-in implements the C99 fpclassify functionality. The first five int arguments should be the target library's notion of the possible FP classes and are used for return values. They must be constant values and they must appear in this order: `FP_NAN`, `FP_INFINITE`, `FP_NORMAL`, `FP_SUBNORMAL` and `FP_ZERO`. The ellipsis is for exactly one floating point value to classify. GCC treats the last argument as type-generic, which means it does not do default promotion from float to 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.

— Built-in Function: _Decimal32 __builtin_infd32 (void)

> Similar to `__builtin_inf`, except the return type is `_Decimal32`.

— Built-in Function: _Decimal64 __builtin_infd64 (void)

> Similar to `__builtin_inf`, except the return type is `_Decimal64`.

— Built-in Function: _Decimal128 __builtin_infd128 (void)

> Similar to `__builtin_inf`, except the return type is `_Decimal128`.

— Built-in Function: float __builtin_inff (void)

> Similar to `__builtin_inf`, except the return type is `float`. This function is suitable for implementing the ISO C99 macro `INFINITY`.

— Built-in Function: long double __builtin_infl (void)

> Similar to `__builtin_inf`, except the return type is `long double`.

— Built-in Function: int __builtin_isinf_sign (...)

> Similar to `isinf`, except the return value will be negative for an argument of `-Inf`. Note while the parameter list is an ellipsis, this function only accepts exactly one floating point argument. GCC treats this parameter as type-generic, which means it does not do default promotion from float to 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 all of which would have been consumed by strtol, is evaluated early enough that it is considered a compile-time constant.

— Built-in Function: _Decimal32 __builtin_nand32 (const char *str)

> Similar to `__builtin_nan`, except the return type is `_Decimal32`.

— Built-in Function: _Decimal64 __builtin_nand64 (const char *str)

> Similar to `__builtin_nan`, except the return type is `_Decimal64`.

— Built-in Function: _Decimal128 __builtin_nand128 (const char *str)

> Similar to `__builtin_nan`, except the return type is `_Decimal128`.

— 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`.

— Built-in Function: int __builtin_ffs (unsigned int x)

> Returns one plus the index of the least significant 1-bit of x, or if x is zero, returns zero.

— Built-in Function: int __builtin_clz (unsigned int x)

> Returns the number of leading 0-bits in x, starting at the most significant bit position. If x is 0, the result is undefined.

— Built-in Function: int __builtin_ctz (unsigned int x)

> Returns the number of trailing 0-bits in x, starting at the least significant bit position. If x is 0, the result is undefined.

— Built-in Function: int __builtin_popcount (unsigned int x)

> Returns the number of 1-bits in x.

— Built-in Function: int __builtin_parity (unsigned int x)

> Returns the parity of x, i.e. the number of 1-bits in x modulo 2.

— Built-in Function: int __builtin_ffsl (unsigned long)

> Similar to `__builtin_ffs`, except the argument type is `unsigned long`.

— Built-in Function: int __builtin_clzl (unsigned long)

> Similar to `__builtin_clz`, except the argument type is `unsigned long`.

— Built-in Function: int __builtin_ctzl (unsigned long)

> Similar to `__builtin_ctz`, except the argument type is `unsigned long`.

— Built-in Function: int __builtin_popcountl (unsigned long)

> Similar to `__builtin_popcount`, except the argument type is `unsigned long`.

— Built-in Function: int __builtin_parityl (unsigned long)

> Similar to `__builtin_parity`, except the argument type is `unsigned long`.

— Built-in Function: int __builtin_ffsll (unsigned long long)

> Similar to `__builtin_ffs`, except the argument type is `unsigned long long`.

— Built-in Function: int __builtin_clzll (unsigned long long)

> Similar to `__builtin_clz`, except the argument type is `unsigned long long`.

— Built-in Function: int __builtin_ctzll (unsigned long long)

> Similar to `__builtin_ctz`, except the argument type is `unsigned long long`.

— Built-in Function: int __builtin_popcountll (unsigned long long)

> Similar to `__builtin_popcount`, except the argument type is `unsigned long long`.

— Built-in Function: int __builtin_parityll (unsigned long long)

> Similar to `__builtin_parity`, except the argument type is `unsigned long long`.

— Built-in Function: double __builtin_powi (double, int)

> Returns the first argument raised to the power of the second. Unlike the `pow` function no guarantees about precision and rounding are made.

— Built-in Function: float __builtin_powif (float, int)

> Similar to `__builtin_powi`, except the argument and return types are `float`.

— Built-in Function: long double __builtin_powil (long double, int)

> Similar to `__builtin_powi`, except the argument and return types are `long double`.

— Built-in Function: int32_t __builtin_bswap32 (int32_t x)

> Returns x with the order of the bytes reversed; for example, `0xaabbccdd` becomes `0xddccbbaa`. Byte here always means exactly 8 bits.

— Built-in Function: int64_t __builtin_bswap64 (int64_t x)

> Similar to `__builtin_bswap32`, except the argument and return types are 64-bit.