gcc - GCC 的 __builtin_isnan 是否做了 isnan 以外的事情？

Question

在 GCC built-ins description ， 它说:

GCC provides built-in versions of the ISO C99 floating-point comparison macros that avoid raising exceptions for unordered operands. They have the same names as the standard macros ( isgreater, isgreaterequal, isless, islessequal, islessgreater, and isunordered) , with _builtin prefixed. We intend for a library implementor to be able to simply #define each standard macro to its built-in equivalent. In the same fashion, GCC provides fpclassify, isfinite, isinf_sign, isnormal and signbit built-ins used with _builtin prefixed. The isinf and isnan built-in functions appear both with and without the _builtin prefix.

所以，我不太能解析这个。浮点比较什么时候应该引发异常？ C标准是否要求他们这样做？授权他们不？什么都不强制？并且 - 做 __builtin_isnan()行为不同于 isnan() ?

Answer 1

此处异常(exception)，GCC 文档引用 IEEE 754 floating point exceptions .如果你做类似的事情

a < b

并且操作数之一是 NaN，将引发 FP 异常(无效)。这意味着 FPU 中的某个位将保持设置状态，直到它被程序员明确清除。改为使用 isgreater/ isless/等等。程序员可以避免触发FP异常。