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 are not 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.

With the exception of built-ins that have library equivalents such as the standard C library functions discussed below, or that expand to library calls, GCC built-in functions are always expanded inline and thus do not have corresponding entry points and their address cannot be obtained. Attempting to use them in an expression other than a function call results in a compile-time error.

GCC includes built-in versions of many of the functions in the standard C library. These functions come in two forms: one whose names start with the __builtin_ prefix, and the other without. Both forms have the same type (including prototype), the same address (when their address is taken), and the same meaning as the C library functions even if you specify the -fno-builtin option see 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 is emitted.

Outside strict ISO C mode (-ansi, -std=c90, -std=c99 or -std=c11), 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, roundeven, roundevenf, roundevenl, scalbf, scalbl, scalb, signbit, signbitf, signbitl, signbitd32, signbitd64, signbitd128, significandf, significandl, significand, sincosf, sincosl, sincos, stpcpy, stpncpy, strcasecmp, strdup, strfmon, strncasecmp, strndup, strnlen, 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, modff, 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_.

There are also built-in functions __builtin_fabsfn, __builtin_fabsfnx, __builtin_copysignfn and __builtin_copysignfnx, corresponding to the TS 18661-3 functions fabsfn, fabsfnx, copysignfn and copysignfnx, for supported types _Floatn and _Floatnx.

There are also GNU extension functions clog10, clog10f and clog10l which names are reserved by ISO C99 for future use. All these functions have 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, free, 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, realloc, 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, isnormal and signbit built-ins used with __builtin_ prefixed. The isinf and isnan built-in functions appear both with and without the __builtin_ prefix. With -ffinite-math-only option the isinf and isnan built-in functions will always return 0.

GCC provides built-in versions of the ISO C99 floating-point rounding and exceptions handling functions fegetround, feclearexcept and feraiseexcept. They may not be available for all targets, and because they need close interaction with libc internal values, they may not be available for all target libcs, but in all cases they will gracefully fallback to libc calls. These built-in functions appear both with and without the __builtin_ prefix.

void *__builtin_alloca(size_t size)#

The __builtin_alloca function must be called at block scope. The function allocates an object size bytes large on the stack of the calling function. The object is aligned on the default stack alignment boundary for the target determined by the __BIGGEST_ALIGNMENT__ macro. The __builtin_alloca function returns a pointer to the first byte of the allocated object. The lifetime of the allocated object ends just before the calling function returns to its caller. This is so even when __builtin_alloca is called within a nested block.

For example, the following function allocates eight objects of n bytes each on the stack, storing a pointer to each in consecutive elements of the array a. It then passes the array to function g which can safely use the storage pointed to by each of the array elements.

void f (unsigned n)
{
  void *a [8];
  for (int i = 0; i != 8; ++i)
    a [i] = __builtin_alloca (n);

  g (a, n);   // safe
}

Since the __builtin_alloca function doesn’t validate its argument it is the responsibility of its caller to make sure the argument doesn’t cause it to exceed the stack size limit. The __builtin_alloca function is provided to make it possible to allocate on the stack arrays of bytes with an upper bound that may be computed at run time. Since C99 Variable Length Arrays offer similar functionality under a portable, more convenient, and safer interface they are recommended instead, in both C99 and C++ programs where GCC provides them as an extension. See Arrays of Variable Length, for details.

void *__builtin_alloca_with_align(size_t size, size_t alignment)#

The __builtin_alloca_with_align function must be called at block scope. The function allocates an object size bytes large on the stack of the calling function. The allocated object is aligned on the boundary specified by the argument alignment whose unit is given in bits (not bytes). The size argument must be positive and not exceed the stack size limit. The alignment argument must be a constant integer expression that evaluates to a power of 2 greater than or equal to CHAR_BIT and less than some unspecified maximum. Invocations with other values are rejected with an error indicating the valid bounds. The function returns a pointer to the first byte of the allocated object. The lifetime of the allocated object ends at the end of the block in which the function was called. The allocated storage is released no later than just before the calling function returns to its caller, but may be released at the end of the block in which the function was called.

For example, in the following function the call to g is unsafe because when overalign is non-zero, the space allocated by __builtin_alloca_with_align may have been released at the end of the if statement in which it was called.

void f (unsigned n, bool overalign)
{
  void *p;
  if (overalign)
    p = __builtin_alloca_with_align (n, 64 /* bits */);
  else
    p = __builtin_alloc (n);

  g (p, n);   // unsafe
}

Since the __builtin_alloca_with_align function doesn’t validate its size argument it is the responsibility of its caller to make sure the argument doesn’t cause it to exceed the stack size limit. The __builtin_alloca_with_align function is provided to make it possible to allocate on the stack overaligned arrays of bytes with an upper bound that may be computed at run time. Since C99 Variable Length Arrays offer the same functionality under a portable, more convenient, and safer interface they are recommended instead, in both C99 and C++ programs where GCC provides them as an extension. See Arrays of Variable Length, for details.

void *__builtin_alloca_with_align_and_max(size_t size, size_t alignment, size_t max_size)#: Similar to __builtin_alloca_with_align but takes an extra argument specifying an upper bound for size in case its value cannot be computed at compile time, for use by -fstack-usage, -Wstack-usage and -Walloca-larger-than. max_size must be a constant integer expression, it has no effect on code generation and no attempt is made to check its compatibility with size.

bool __builtin_has_attribute(type_or_expression, attribute)#

The __builtin_has_attribute function evaluates to an integer constant expression equal to true if the symbol or type referenced by the type_or_expression argument has been declared with the attribute referenced by the second argument. For an type_or_expression argument that does not reference a symbol, since attributes do not apply to expressions the built-in consider the type of the argument. Neither argument is evaluated. The type_or_expression argument is subject to the same restrictions as the argument to typeof (see Referring to a Type with typeof). The attribute argument is an attribute name optionally followed by a comma-separated list of arguments enclosed in parentheses. Both forms of attribute names—with and without double leading and trailing underscores—are recognized. See Attribute Syntax, for details. When no attribute arguments are specified for an attribute that expects one or more arguments the function returns true if type_or_expression has been declared with the attribute regardless of the attribute argument values. Arguments provided for an attribute that expects some are validated and matched up to the provided number. The function returns true if all provided arguments match. For example, the first call to the function below evaluates to true because x is declared with the aligned attribute but the second call evaluates to false because x is declared aligned (8) and not aligned (4).

__attribute__ ((aligned (8))) int x;
_Static_assert (__builtin_has_attribute (x, aligned), "aligned");
_Static_assert (!__builtin_has_attribute (x, aligned (4)), "aligned (4)");

Due to a limitation the __builtin_has_attribute function returns false for the mode attribute even if the type or variable referenced by the type_or_expression argument was declared with one. The function is also not supported with labels, and in C with enumerators.

Note that unlike the __has_attribute preprocessor operator which is suitable for use in #if preprocessing directives __builtin_has_attribute is an intrinsic function that is not recognized in such contexts.

type __builtin_speculation_safe_value(type val, type failval)#

This built-in function can be used to help mitigate against unsafe speculative execution. type may be any integral type or any pointer type.

If the CPU is not speculatively executing the code, then val is returned.
If the CPU is executing speculatively then either:
- The function may cause execution to pause until it is known that the code is no-longer being executed speculatively (in which case val can be returned, as above); or
- The function may use target-dependent speculation tracking state to cause failval to be returned when it is known that speculative execution has incorrectly predicted a conditional branch operation.

The second argument, failval, is optional and defaults to zero if omitted.

GCC defines the preprocessor macro __HAVE_BUILTIN_SPECULATION_SAFE_VALUE for targets that have been updated to support this builtin.

The built-in function can be used where a variable appears to be used in a safe way, but the CPU, due to speculative execution may temporarily ignore the bounds checks. Consider, for example, the following function:

int array[500];
int f (unsigned untrusted_index)
{
  if (untrusted_index < 500)
    return array[untrusted_index];
  return 0;
}

If the function is called repeatedly with untrusted_index less than the limit of 500, then a branch predictor will learn that the block of code that returns a value stored in array will be executed. If the function is subsequently called with an out-of-range value it will still try to execute that block of code first until the CPU determines that the prediction was incorrect (the CPU will unwind any incorrect operations at that point). However, depending on how the result of the function is used, it might be possible to leave traces in the cache that can reveal what was stored at the out-of-bounds location. The built-in function can be used to provide some protection against leaking data in this way by changing the code to:

int array[500];
int f (unsigned untrusted_index)
{
  if (untrusted_index < 500)
    return array[__builtin_speculation_safe_value (untrusted_index)];
  return 0;
}

The built-in function will either cause execution to stall until the conditional branch has been fully resolved, or it may permit speculative execution to continue, but using 0 instead of untrusted_value if that exceeds the limit.

If accessing any memory location is potentially unsafe when speculative execution is incorrect, then the code can be rewritten as

int array[500];
int f (unsigned untrusted_index)
{
  if (untrusted_index < 500)
    return *__builtin_speculation_safe_value (&array[untrusted_index], NULL);
  return 0;
}

which will cause a NULL pointer to be used for the unsafe case.

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 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.

type __builtin_call_with_static_chain(call_exp, pointer_exp)#

The call_exp expression must be a function call, and the pointer_exp expression must be a pointer. The pointer_exp is passed to the function call in the target’s static chain location. The result of builtin is the result of the function call.

Note

This builtin is only available for C.

This builtin can be used to call Go closures from C.

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 exp2.

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 is 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.

type __builtin_tgmath(functions, arguments)#

The built-in function __builtin_tgmath, available only for C and Objective-C, calls a function determined according to the rules of <tgmath.h> macros. It is intended to be used in implementations of that header, so that expansions of macros from that header only expand each of their arguments once, to avoid problems when calls to such macros are nested inside the arguments of other calls to such macros; in addition, it results in better diagnostics for invalid calls to <tgmath.h> macros than implementations using other GNU C language features. For example, the pow type-generic macro might be defined as:

#define pow(a, b) __builtin_tgmath (powf, pow, powl, \
                                    cpowf, cpow, cpowl, a, b)

The arguments to __builtin_tgmath are at least two pointers to functions, followed by the arguments to the type-generic macro (which will be passed as arguments to the selected function). All the pointers to functions must be pointers to prototyped functions, none of which may have variable arguments, and all of which must have the same number of parameters; the number of parameters of the first function determines how many arguments to __builtin_tgmath are interpreted as function pointers, and how many as the arguments to the called function.

The types of the specified functions must all be different, but related to each other in the same way as a set of functions that may be selected between by a macro in <tgmath.h>. This means that the functions are parameterized by a floating-point type t, different for each such function. The function return types may all be the same type, or they may be t for each function, or they may be the real type corresponding to t for each function (if some of the types t are complex). Likewise, for each parameter position, the type of the parameter in that position may always be the same type, or may be t for each function (this case must apply for at least one parameter position), or may be the real type corresponding to t for each function.

The standard rules for <tgmath.h> macros are used to find a common type u from the types of the arguments for parameters whose types vary between the functions; complex integer types (a GNU extension) are treated like _Complex double for this purpose (or _Complex _Float64 if all the function return types are the same _Floatn or _Floatnx type). If the function return types vary, or are all the same integer type, the function called is the one for which t is u, and it is an error if there is no such function. If the function return types are all the same floating-point type, the type-generic macro is taken to be one of those from TS 18661 that rounds the result to a narrower type; if there is a function for which t is u, it is called, and otherwise the first function, if any, for which t has at least the range and precision of u is called, and it is an error if there is no such 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 typically use this function in an embedded application where memory is 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 never returns 1 when you call the inline function with a string constant or compound literal (see Compound Literals) and does 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 are not otherwise 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.

bool __builtin_is_constant_evaluated(void)#

The __builtin_is_constant_evaluated function is available only in C++. The built-in is intended to be used by implementations of the std::is_constant_evaluated C++ function. Programs should make use of the latter function rather than invoking the built-in directly.

The main use case of the built-in is to determine whether a constexpr function is being called in a constexpr context. A call to the function evaluates to a core constant expression with the value true if and only if it occurs within the evaluation of an expression or conversion that is manifestly constant-evaluated as defined in the C++ standard. Manifestly constant-evaluated contexts include constant-expressions, the conditions of constexpr if statements, constraint-expressions, and initializers of variables usable in constant expressions. For more details refer to the latest revision of the C++ standard.

void __builtin_clear_padding(ptr)#

The built-in function __builtin_clear_padding function clears padding bits inside of the object representation of object pointed by ptr, which has to be a pointer. The value representation of the object is not affected. The type of the object is assumed to be the type the pointer points to. Inside of a union, the only cleared bits are bits that are padding bits for all the union members.

This built-in-function is useful if the padding bits of an object might have intederminate values and the object representation needs to be bitwise compared to some other object, for example for atomic operations.

For C++, ptr argument type should be pointer to trivially-copyable type, unless the argument is address of a variable or parameter, because otherwise it isn’t known if the type isn’t just a base class whose padding bits are reused or laid out differently in a derived class.

type __builtin_bit_cast(type, arg)#

The __builtin_bit_cast function is available only in C++. The built-in is intended to be used by implementations of the std::bit_cast C++ template function. Programs should make use of the latter function rather than invoking the built-in directly.

This built-in function allows reinterpreting the bits of the arg argument as if it had type type. type and the type of the arg argument need to be trivially copyable types with the same size. When manifestly constant-evaluated, it performs extra diagnostics required for std::bit_cast and returns a constant expression if arg is a constant expression. For more details refer to the latest revision of the C++ standard.

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 ();

indicates 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))
  foo (*ptr);

when testing pointer or floating-point values.

For the purposes of branch prediction optimizations, the probability that a __builtin_expect expression is true is controlled by GCC’s builtin-expect-probability parameter, which defaults to 90%.

You can also use __builtin_expect_with_probability to explicitly assign a probability value to individual expressions. If the built-in is used in a loop construct, the provided probability will influence the expected number of iterations made by loop optimizations.

long __builtin_expect_with_probability(long exp, long c, double probability)#: This function has the same semantics as __builtin_expect, but the caller provides the expected probability that exp == c. The last argument, probability, is a floating-point value in the range 0.0 to 1.0, inclusive. The probability argument must be constant floating-point expression.

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.

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 either never terminates, or one that transfers control elsewhere and never returns. In this example, without the __builtin_unreachable, GCC issues a warning that control reaches the end of a non-void function. It also generates 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 never reaches 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 ();
    }
}

type __builtin_assoc_barrier(type expr)#

This built-in inhibits re-association of the floating-point expression expr with expressions consuming the return value of the built-in. The expression expr itself can be reordered, and the whole expression expr can be reordered with operands after the barrier. The barrier is only relevant when -fassociative-math is active, since otherwise floating-point is not treated as associative.

float x0 = a + b - b;
float x1 = __builtin_assoc_barrier(a + b) - b;

means that, with -fassociative-math, x0 can be optimized to x0 = a but x1 cannot.

void *__builtin_assume_aligned(const void *exp, size_t align, ...)#

This function returns its first argument, and allows the compiler to assume that the returned pointer is at least align bytes aligned. This built-in can have either two or three arguments, if it has three, the third argument should have integer type, and if it is nonzero means misalignment offset. For example:

void *x = __builtin_assume_aligned (arg, 16);

means that the compiler can assume x, set to arg, is at least 16-byte aligned, while:

void *x = __builtin_assume_aligned (arg, 32, 8);

means that the compiler can assume for x, set to arg, that (char *) x - 8 is 32-byte aligned.

int __builtin_LINE()#: This function is the equivalent of the preprocessor __LINE__ macro and returns a constant integer expression that evaluates to the line number of the invocation of the built-in. When used as a C++ default argument for a function F, it returns the line number of the call to F.

const char *__builtin_FUNCTION()#: This function is the equivalent of the __FUNCTION__ symbol and returns an address constant pointing to the name of the function from which the built-in was invoked, or the empty string if the invocation is not at function scope. When used as a C++ default argument for a function F, it returns the name of F ‘s caller or the empty string if the call was not made at function scope.

const char *__builtin_FILE()#

This function is the equivalent of the preprocessor __FILE__ macro and returns an address constant pointing to the file name containing the invocation of the built-in, or the empty string if the invocation is not at function scope. When used as a C++ default argument for a function F, it returns the file name of the call to F or the empty string if the call was not made at function scope.

For example, in the following, each call to function foo will print a line similar to "file.c:123: foo: message" with the name of the file and the line number of the printf call, the name of the function foo, followed by the word message.

const char*
function (const char *func = __builtin_FUNCTION ())
{
  return func;
}

void foo (void)
{
  printf ("%s:%i: %s: message\n", file (), line (), function ());
}

void __builtin___clear_cache(void *begin, void *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.

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 are 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 does not fault if p->next is not a valid address, but evaluation faults 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.

size_t __builtin_object_size(const void *ptr, int type): Returns the size of an object pointed to by ptr. See Object Size Checking Built-in Functions, for a detailed description of the 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.

float __builtin_huge_valf(void)#: Similar to __builtin_huge_val, except the return type is float.

long double __builtin_huge_vall(void)#: Similar to __builtin_huge_val, except the return type is long double.

_Floatn __builtin_huge_valfn(void)#: Similar to __builtin_huge_val, except the return type is _Floatn.

_Floatnx __builtin_huge_valfnx(void)#: Similar to __builtin_huge_val, except the return type is _Floatnx.

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.

double __builtin_inf(void)#: Similar to __builtin_huge_val, except a warning is generated if the target floating-point format does not support infinities.

_Decimal32 __builtin_infd32(void)#: Similar to __builtin_inf, except the return type is _Decimal32.

_Decimal64 __builtin_infd64(void)#: Similar to __builtin_inf, except the return type is _Decimal64.

_Decimal128 __builtin_infd128(void)#: Similar to __builtin_inf, except the return type is _Decimal128.

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.

long double __builtin_infl(void)#: Similar to __builtin_inf, except the return type is long double.

_Floatn __builtin_inffn(void)#: Similar to __builtin_inf, except the return type is _Floatn.

_Floatn __builtin_inffnx(void)#: Similar to __builtin_inf, except the return type is _Floatnx.

int __builtin_isinf_sign(...)#: Similar to isinf, except the return value is -1 for an argument of -Inf and 1 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.

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.

_Decimal32 __builtin_nand32(const char *str)#: Similar to __builtin_nan, except the return type is _Decimal32.

_Decimal64 __builtin_nand64(const char *str)#: Similar to __builtin_nan, except the return type is _Decimal64.

_Decimal128 __builtin_nand128(const char *str)#: Similar to __builtin_nan, except the return type is _Decimal128.

float __builtin_nanf(const char *str)#: Similar to __builtin_nan, except the return type is float.

long double __builtin_nanl(const char *str)#: Similar to __builtin_nan, except the return type is long double.

_Floatn __builtin_nanfn(const char *str)#: Similar to __builtin_nan, except the return type is _Floatn.

_Floatnx __builtin_nanfnx(const char *str)#: Similar to __builtin_nan, except the return type is _Floatnx.

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.

_Decimal32 __builtin_nansd32(const char *str)#: Similar to __builtin_nans, except the return type is _Decimal32.

_Decimal64 __builtin_nansd64(const char *str)#: Similar to __builtin_nans, except the return type is _Decimal64.

_Decimal128 __builtin_nansd128(const char *str)#: Similar to __builtin_nans, except the return type is _Decimal128.

float __builtin_nansf(const char *str)#: Similar to __builtin_nans, except the return type is float.

long double __builtin_nansl(const char *str)#: Similar to __builtin_nans, except the return type is long double.

_Floatn __builtin_nansfn(const char *str)#: Similar to __builtin_nans, except the return type is _Floatn.

_Floatnx __builtin_nansfnx(const char *str)#: Similar to __builtin_nans, except the return type is _Floatnx.

int __builtin_issignaling(...)#: Return non-zero if the argument is a signaling NaN and zero otherwise. 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. This built-in function can work even without the non-default -fsignaling-nans option, although if a signaling NaN is computed, stored or passed as argument to some function other than this built-in in the current translation unit, it is safer to use -fsignaling-nans. With -ffinite-math-only option this built-in function will always return 0.

int __builtin_ffs(int x)#: Returns one plus the index of the least significant 1-bit of x, or if x is zero, returns zero.

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.

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.

int __builtin_clrsb(int x)#: Returns the number of leading redundant sign bits in x, i.e. the number of bits following the most significant bit that are identical to it. There are no special cases for 0 or other values.

int __builtin_popcount(unsigned int x): Returns the number of 1-bits in x.

int __builtin_parity(unsigned int x)#: Returns the parity of x, i.e. the number of 1-bits in x modulo 2.

int __builtin_ffsl(long)#: Similar to __builtin_ffs, except the argument type is long.

int __builtin_clzl(unsigned long)#: Similar to __builtin_clz, except the argument type is unsigned long.

int __builtin_ctzl(unsigned long)#: Similar to __builtin_ctz, except the argument type is unsigned long.

int __builtin_clrsbl(long)#: Similar to __builtin_clrsb, except the argument type is long.

int __builtin_popcountl(unsigned long): Similar to __builtin_popcount, except the argument type is unsigned long.

int __builtin_parityl(unsigned long)#: Similar to __builtin_parity, except the argument type is unsigned long.

int __builtin_ffsll(long long)#: Similar to __builtin_ffs, except the argument type is long long.

int __builtin_clzll(unsigned long long)#: Similar to __builtin_clz, except the argument type is unsigned long long.

int __builtin_ctzll(unsigned long long)#: Similar to __builtin_ctz, except the argument type is unsigned long long.

int __builtin_clrsbll(long long)#: Similar to __builtin_clrsb, except the argument type is long long.

int __builtin_popcountll(unsigned long long): Similar to __builtin_popcount, except the argument type is unsigned long long.

int __builtin_parityll(unsigned long long)#: Similar to __builtin_parity, except the argument type is unsigned long long.

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.

float __builtin_powif(float, int)#: Similar to __builtin_powi, except the argument and return types are float.

long double __builtin_powil(long double, int)#: Similar to __builtin_powi, except the argument and return types are long double.

uint16_t __builtin_bswap16(uint16_t x)#: Returns x with the order of the bytes reversed; for example, 0xaabb becomes 0xbbaa. Byte here always means exactly 8 bits.

uint32_t __builtin_bswap32(uint32_t x)#: Similar to __builtin_bswap16, except the argument and return types are 32-bit.

uint64_t __builtin_bswap64(uint64_t x)#: Similar to __builtin_bswap32, except the argument and return types are 64-bit.

uint128_t __builtin_bswap128(uint128_t x)#: Similar to __builtin_bswap64, except the argument and return types are 128-bit. Only supported on targets when 128-bit types are supported.

Pmode __builtin_extend_pointer(void *x)#: On targets where the user visible pointer size is smaller than the size of an actual hardware address this function returns the extended user pointer. Targets where this is true included ILP32 mode on x86_64 or Aarch64. This function is mainly useful when writing inline assembly code.

int __builtin_goacc_parlevel_id(int x)#: Returns the openacc gang, worker or vector id depending on whether x is 0, 1 or 2.

int __builtin_goacc_parlevel_size(int x)#: Returns the openacc gang, worker or vector size depending on whether x is 0, 1 or 2.