如何检查 __builtin_ 函数在 gcc 上是否可用

Question

我想知道 gcc 是否有一种方法来检查那些很棒的东西的存在 __builtin_MY_DESIRED_FUNCTIONs

例如，我想使用 __builtin_nan 并确保它可用于我的程序并且在编译期间不会失败。

我会更具体一点：在 clang 上有 __has_builtin "checker" 所以我们可以写成

#if __has_builtin(__builtin_nan)

但是我找不到 gcc 的模拟。

也许我可以只依赖 gcc，比如 "Oh, I'm on gcc now, just let's assume all of those __builtin_ are here like in example below..."

#if __GNUC__
double mynan = __builtin_nan("0");
#endif

可能它会起作用，直到有人放置这个“-fno-builtin”编译标志。

Answer 1

不，您必须使用 __GNUC__ 和 __GNUC_MINOR__（如果您使用此类 gcc 版本，则 __GNUC_PATCHLEVEL__）来测试每个版本特定的内置函数（gcc 版本可以是找到 here)

例如：

/* __builtin_mul_overflow_p added in gcc 7.4 */
#if (__GNUC__ > 7) || \
         ((__GNUC__ == 7) && (__GNUC_MINOR__ > 3))
#define BUILTIN_MUL_OVERFLOW_EXIST
#endif

#ifdef BUILTIN_MUL_OVERFLOW_EXIST
int c = __builtin_mul_overflow_p (3, 2, 3) ? 0 : 3 * 2;
#endif

here.

中有一个针对您所询问的问题的未解决错误

Answer 2

好消息！一个 __has_builtin was added in GCC 10 (see change notes):

The special operator __has_builtin (operand) may be used in constant integer contexts and in preprocessor ‘#if’ and ‘#elif’ expressions to test whether the symbol named by its operand is recognized as a built-in function by GCC in the current language and conformance mode. It evaluates to a constant integer with a nonzero value if the argument refers to such a function, and to zero otherwise. The operator may also be used in preprocessor ‘#if’ and ‘#elif’ expressions. The __has_builtin operator by itself, without any operand or parentheses, acts as a predefined macro so that support for it can be tested in portable code. Thus, the recommended use of the operator is as follows:

#if defined __has_builtin
#  if __has_builtin (__builtin_object_size)
#    define builtin_object_size(ptr) __builtin_object_size (ptr, 2)
#  endif
#endif
#ifndef builtin_object_size
#  define builtin_object_size(ptr)   ((size_t)-1)
#endif

如何检查 __builtin_ 函数在 gcc 上是否可用

How to check __builtin_ function is available on gcc

c

gcc

macros

built-in