This section documents miscellaneous built-in functions available in GCC.
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_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 indeterminate 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 a
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 zero, one or two; one means that the prefetch is preparing for a write to the memory address, two means that the prefetch is preparing for a shared read (expected to be read by at least one other processor before it is written if written at all) 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.
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.
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.