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, iseqsig,
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.
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.
Like the ‘? :’ operator, this 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. On the
other hand, __builtin_choose_expr differs from ‘? :’ in that the
first operand must be a compile-time constant, and the other operands are not
subject to the ‘? :’ type constraints and promotions.
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 the complex type corresponding to the real
floating type that would be chosen for the corresponding real integer 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 the expression exp 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 expression to test.
The expression is not evaluated, side-effects are discarded. 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.
Any expression that has side-effects makes the function return 0.
A return of 0 does not indicate that the expression is not a constant,
but merely that GCC cannot prove it is a constant within the constraints
of the active set of optimization options.
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.
type __builtin_counted_by_ref (ptr) ¶The built-in function __builtin_counted_by_ref checks whether the array
object pointed by the pointer ptr has another object associated with it
that represents the number of elements in the array object through the
counted_by attribute (i.e. the counted-by object). If so, returns a
pointer to the corresponding counted-by object.
If such counted-by object does not exist, returns a null pointer.
This built-in function is only available in C for now.
The argument ptr must be a pointer to an array. The type of the returned value is a pointer type pointing to the corresponding type of the counted-by object or a void pointer type in case of a null pointer being returned.
For example:
struct foo1 {
int counter;
struct bar1 array[] __attribute__((counted_by (counter)));
} *p;
struct foo2 {
int other;
struct bar2 array[];
} *q;
the following call to the built-in
__builtin_counted_by_ref (p->array)
returns:
&p->counter with type int *.
However, the following call to the built-in
__builtin_counted_by_ref (q->array)
returns a null pointer to void.
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
relevant when -fassociative-math is active.
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.
It is also relevant when -ffp-contract=fast is active;
it will prevent contraction between expressions.
float x0 = a * b + c; float x1 = __builtin_assoc_barrier (a * b) + c;
means that, with -ffp-contract=fast, x0 may be optimized to
use a fused multiply-add instruction 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 a constant size estimate of an object pointed to by ptr. See Object Size Checking, for a detailed description of the function.
size_t __builtin_dynamic_object_size (const void * ptr, int type) ¶Similar to __builtin_object_size except that the return value
need not be a constant. See Object Size Checking, for a detailed
description of the function.
int __builtin_classify_type (arg) ¶int __builtin_classify_type (type) ¶The __builtin_classify_type returns a small integer with a category
of arg argument’s type, like void type, integer type, enumeral type,
boolean type, pointer type, reference type, offset type, real type, complex
type, function type, method type, record type, union type, array type,
string type, bit-precise integer type, vector type, etc. When the argument
is an expression, for backwards compatibility reason the argument is promoted
like arguments passed to ... in varargs function, so some classes are
never returned in certain languages. Alternatively, the argument of the
built-in function can be a typename, such as the typeof specifier.
int a[2]; __builtin_classify_type (a) == __builtin_classify_type (int[5]); __builtin_classify_type (a) == __builtin_classify_type (void*); __builtin_classify_type (typeof (a)) == __builtin_classify_type (int[5]);
The first comparison will never be true, as a is implicitly converted to pointer. The last two comparisons will be true as they classify pointers in the second case and arrays in the last case.
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.
int __builtin_ffsg (...) ¶Similar to __builtin_ffs, except the argument is type-generic
signed integer (standard, extended or bit-precise). No integral argument
promotions are performed on the argument.
int __builtin_clzg (...) ¶Similar to __builtin_clz, except the argument is type-generic
unsigned integer (standard, extended or bit-precise) and there is
optional second argument with int type. No integral argument promotions
are performed on the first argument. If two arguments are specified,
and first argument is 0, the result is the second argument. If only
one argument is specified and it is 0, the result is undefined.
int __builtin_ctzg (...) ¶Similar to __builtin_ctz, except the argument is type-generic
unsigned integer (standard, extended or bit-precise) and there is
optional second argument with int type. No integral argument promotions
are performed on the first argument. If two arguments are specified,
and first argument is 0, the result is the second argument. If only
one argument is specified and it is 0, the result is undefined.
int __builtin_clrsbg (...) ¶Similar to __builtin_clrsb, except the argument is type-generic
signed integer (standard, extended or bit-precise). No integral argument
promotions are performed on the argument.
int __builtin_popcountg (...) ¶Similar to __builtin_popcount, except the argument is type-generic
unsigned integer (standard, extended or bit-precise). No integral argument
promotions are performed on the argument.
int __builtin_parityg (...) ¶Similar to __builtin_parity, except the argument is type-generic
unsigned integer (standard, extended or bit-precise). No integral argument
promotions are performed on the argument.
type __builtin_stdc_bit_ceil (type arg) ¶The __builtin_stdc_bit_ceil function is available only
in C. It is type-generic, the argument can be any unsigned integer
(standard, extended or bit-precise). No integral argument promotions are
performed on the argument. It is equivalent to
arg <= 1 ? (type) 1
: (type) 2 << (prec - 1 - __builtin_clzg ((type) (arg - 1)))
where prec is bit width of type, except that side-effects
in arg are evaluated just once.
type __builtin_stdc_bit_floor (type arg) ¶The __builtin_stdc_bit_floor function is available only
in C. It is type-generic, the argument can be any unsigned integer
(standard, extended or bit-precise). No integral argument promotions are
performed on the argument. It is equivalent to
arg == 0 ? (type) 0
: (type) 1 << (prec - 1 - __builtin_clzg (arg))
where prec is bit width of type, except that side-effects
in arg are evaluated just once.
unsigned int __builtin_stdc_bit_width (type arg) ¶The __builtin_stdc_bit_width function is available only
in C. It is type-generic, the argument can be any unsigned integer
(standard, extended or bit-precise). No integral argument promotions are
performed on the argument. It is equivalent to
(unsigned int) (prec - __builtin_clzg (arg, prec))
where prec is bit width of type.
unsigned int __builtin_stdc_count_ones (type arg) ¶The __builtin_stdc_count_ones function is available only
in C. It is type-generic, the argument can be any unsigned integer
(standard, extended or bit-precise). No integral argument promotions are
performed on the argument. It is equivalent to
(unsigned int) __builtin_popcountg (arg)
unsigned int __builtin_stdc_count_zeros (type arg) ¶The __builtin_stdc_count_zeros function is available only
in C. It is type-generic, the argument can be any unsigned integer
(standard, extended or bit-precise). No integral argument promotions are
performed on the argument. It is equivalent to
(unsigned int) __builtin_popcountg ((type) ~arg)
unsigned int __builtin_stdc_first_leading_one (type arg) ¶The __builtin_stdc_first_leading_one function is available only
in C. It is type-generic, the argument can be any unsigned integer
(standard, extended or bit-precise). No integral argument promotions are
performed on the argument. It is equivalent to
__builtin_clzg (arg, -1) + 1U
unsigned int __builtin_stdc_first_leading_zero (type arg) ¶The __builtin_stdc_first_leading_zero function is available only
in C. It is type-generic, the argument can be any unsigned integer
(standard, extended or bit-precise). No integral argument promotions are
performed on the argument. It is equivalent to
__builtin_clzg ((type) ~arg, -1) + 1U
unsigned int __builtin_stdc_first_trailing_one (type arg) ¶The __builtin_stdc_first_trailing_one function is available only
in C. It is type-generic, the argument can be any unsigned integer
(standard, extended or bit-precise). No integral argument promotions are
performed on the argument. It is equivalent to
__builtin_ctzg (arg, -1) + 1U
unsigned int __builtin_stdc_first_trailing_zero (type arg) ¶The __builtin_stdc_first_trailing_zero function is available only
in C. It is type-generic, the argument can be any unsigned integer
(standard, extended or bit-precise). No integral argument promotions are
performed on the argument. It is equivalent to
__builtin_ctzg ((type) ~arg, -1) + 1U
unsigned int __builtin_stdc_has_single_bit (type arg) ¶The __builtin_stdc_has_single_bit function is available only
in C. It is type-generic, the argument can be any unsigned integer
(standard, extended or bit-precise). No integral argument promotions are
performed on the argument. It is equivalent to
(_Bool) (__builtin_popcountg (arg) == 1)
unsigned int __builtin_stdc_leading_ones (type arg) ¶The __builtin_stdc_leading_ones function is available only
in C. It is type-generic, the argument can be any unsigned integer
(standard, extended or bit-precise). No integral argument promotions are
performed on the argument. It is equivalent to
(unsigned int) __builtin_clzg ((type) ~arg, prec)
unsigned int __builtin_stdc_leading_zeros (type arg) ¶The __builtin_stdc_leading_zeros function is available only
in C. It is type-generic, the argument can be any unsigned integer
(standard, extended or bit-precise). No integral argument promotions are
performed on the argument. It is equivalent to
(unsigned int) __builtin_clzg (arg, prec)
unsigned int __builtin_stdc_trailing_ones (type arg) ¶The __builtin_stdc_trailing_ones function is available only
in C. It is type-generic, the argument can be any unsigned integer
(standard, extended or bit-precise). No integral argument promotions are
performed on the argument. It is equivalent to
(unsigned int) __builtin_ctzg ((type) ~arg, prec)
unsigned int __builtin_stdc_trailing_zeros (type arg) ¶The __builtin_stdc_trailing_zeros function is available only
in C. It is type-generic, the argument can be any unsigned integer
(standard, extended or bit-precise). No integral argument promotions are
performed on the argument. It is equivalent to
(unsigned int) __builtin_ctzg (arg, prec)
type1 __builtin_stdc_rotate_left (type1 arg1, type2 arg2) ¶The __builtin_stdc_rotate_left function is available only
in C. It is type-generic, the first argument can be any unsigned integer
(standard, extended or bit-precise) and second argument any signed or
unsigned integer or char. No integral argument promotions are
performed on the arguments. It is equivalent to
(type1) ((arg1 << (arg2 % prec))
| (arg1 >> ((-(unsigned type2) arg2) % prec)))
where prec is bit width of type1, except that side-effects
in arg1 and arg2 are evaluated just once. The behavior is
undefined if arg2 is negative.
type1 __builtin_stdc_rotate_right (type1 arg1, type2 arg2) ¶The __builtin_stdc_rotate_right function is available only
in C. It is type-generic, the first argument can be any unsigned integer
(standard, extended or bit-precise) and second argument any signed or
unsigned integer or char. No integral argument promotions are
performed on the arguments. It is equivalent to
(type1) ((arg1 >> (arg2 % prec))
| (arg1 << ((-(unsigned type2) arg2) % prec)))
where prec is bit width of type1, except that side-effects
in arg1 and arg2 are evaluated just once. The behavior is
undefined if arg2 is negative.
double __builtin_powi (double, int) ¶float __builtin_powif (float, int) ¶long double __builtin_powil (long 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.
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.