You can use the built-in function __builtin_types_compatible_p to determine whether two types are the same.

This built-in function returns 1 if the unqualified versions of the types type1 and type2 (which are types, not expressions) are compatible, 0 otherwise. The result of this built-in function can be used in integer constant expressions.

This built-in function ignores top level qualifiers (e.g., const, volatile). For example, int is equivalent to const int.

The type int[] and int[5] are compatible. On the other hand, int and char * are not compatible, even if the size of their types, on the particular architecture are the same. Also, the amount of pointer indirection is taken into account when determining similarity. Consequently, short * is not similar to short **. Furthermore, two types that are typedefed are considered compatible if their underlying types are compatible.

An enum type is not considered to be compatible with another enum type even if both are compatible with the same integer type; this is what the C standard specifies. For example,enum {foo, bar} is not similar to enum {hot, dog}.

You would typically use this function in code whose execution varies depending on the arguments' types. For example:

          #define foo(x)                                                  \
            ({                                                           \
              typeof (x) tmp = (x);                                       \
              if (__builtin_types_compatible_p (typeof (x), long double)) \
                tmp = foo_long_double (tmp);                              \
              else if (__builtin_types_compatible_p (typeof (x), double)) \
                tmp = foo_double (tmp);                                   \
              else if (__builtin_types_compatible_p (typeof (x), float))  \
                tmp = foo_float (tmp);                                    \
              else                                                        \
                abort ();                                                 \
              tmp;                                                        \
            })
     

Note: This construct is only available for C.

You can use the built-in function __builtin_choose_expr to evaluate code depending on the value of a constant expression. This built-in function returns exp1 if const_exp, which is a constant expression that must be able to be determined at compile time, is nonzero. Otherwise it returns 0.

This built-in function is analogous to the `? :' operator in C, except that the expression returned has its type unaltered by promotion rules. Also, the built-in function does not evaluate the expression that was not chosen. For example, if const_exp evaluates to true, exp2 is not evaluated even if it has side-effects.

This built-in function can return an lvalue if the chosen argument is an lvalue.

If exp1 is returned, the return type is the same as exp1's type. Similarly, if exp2 is returned, its return type is the same as exp2.

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.

You can use the built-in function __builtin_constant_p to determine if a value is known to be constant at compile-time and hence that GCC can perform constant-folding on expressions involving that value. The argument of the function is the value to test. The function returns the integer 1 if the argument is known to be a compile-time constant and 0 if it is not known to be a compile-time constant. A return of 0 does not indicate that the value is not a constant, but merely that GCC cannot prove it is a constant with the specified value of the -O option.

You would typically use this function in an embedded application where memory was a critical resource. If you have some complex calculation, you may want it to be folded if it involves constants, but need to call a function if it does not. For example:

          #define Scale_Value(X)      \
            (__builtin_constant_p (X) \
            ? ((X) * SCALE + OFFSET) : Scale (X))
     

You may use this built-in function in either a macro or an inline function. However, if you use it in an inlined function and pass an argument of the function as the argument to the built-in, GCC will never return 1 when you call the inline function with a string constant or compound literal (see Compound Literals) and will not return 1 when you pass a constant numeric value to the inline function unless you specify the -O option.

You may also use __builtin_constant_p in initializers for static data. For instance, you can write

          static const int table[] = {
             __builtin_constant_p (EXPRESSION) ? (EXPRESSION) : -1,
             /* ... */
          };
     

This is an acceptable initializer even if EXPRESSION is not a constant expression. GCC must be more conservative about evaluating the built-in in this case, because it has no opportunity to perform optimization.

Previous versions of GCC did not accept this built-in in data initializers. The earliest version where it is completely safe is 3.0.1.

You may use __builtin_expect to provide the compiler with branch prediction information. In general, you should prefer to use actual profile feedback for this (-fprofile-arcs), as programmers are notoriously bad at predicting how their programs actually perform. However, there are applications in which this data is hard to collect.

The return value is the value of exp, which should be an integral expression. The semantics of the built-in are that it is expected that exp == c. For example:

          if (__builtin_expect (x, 0))
            foo ();
     

would indicate that we do not expect to call foo, since we expect x to be zero. Since you are limited to integral expressions for exp, you should use constructions such as

          if (__builtin_expect (ptr != NULL, 1))
            error ();
     

when testing pointer or floating-point values.

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.

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.

This function is used to minimize cache-miss latency by moving data into a cache before it is accessed. You can insert calls to __builtin_prefetch into code for which you know addresses of data in memory that is likely to be accessed soon. If the target supports them, data prefetch instructions will be generated. If the prefetch is done early enough before the access then the data will be in the cache by the time it is accessed.

The value of addr is the address of the memory to prefetch. There are two optional arguments, rw and locality. The value of rw is a compile-time constant one or zero; one means that the prefetch is preparing for a write to the memory address and zero, the default, means that the prefetch is preparing for a read. The value locality must be a compile-time constant integer between zero and three. A value of zero means that the data has no temporal locality, so it need not be left in the cache after the access. A value of three means that the data has a high degree of temporal locality and should be left in all levels of cache possible. Values of one and two mean, respectively, a low or moderate degree of temporal locality. The default is three.

          for (i = 0; i < n; i++)
            {
              a[i] = a[i] + b[i];
              __builtin_prefetch (&a[i+j], 1, 1);
              __builtin_prefetch (&b[i+j], 0, 1);
              /* ... */
            }
     

Data prefetch does not generate faults if addr is invalid, but the address expression itself must be valid. For example, a prefetch of p->next will not fault if p->next is not a valid address, but evaluation will fault if p is not a valid address.

If the target does not support data prefetch, the address expression is evaluated if it includes side effects but no other code is generated and GCC does not issue a warning.

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.

Similar to __builtin_huge_val, except the return type is float.

Similar to __builtin_huge_val, except the return type is long double.

Similar to __builtin_huge_val, except a warning is generated if the target floating-point format does not support infinities.

Similar to __builtin_inf, except the return type is _Decimal32.

Similar to __builtin_inf, except the return type is _Decimal64.

Similar to __builtin_inf, except the return type is _Decimal128.

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

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

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.

Similar to __builtin_nan, except the return type is _Decimal32.

Similar to __builtin_nan, except the return type is _Decimal64.

Similar to __builtin_nan, except the return type is _Decimal128.

Similar to __builtin_nan, except the return type is float.

Similar to __builtin_nan, except the return type is long double.

Similar to __builtin_nan, except the significand is forced to be a signaling NaN. The nans function is proposed by WG14 N965.

Similar to __builtin_nans, except the return type is float.

Similar to __builtin_nans, except the return type is long double.

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

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

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

Returns the number of 1-bits in x.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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