In the GCC built-ins description, it says:
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.
So, I'm not quite able to parse this. When should floating-point comparisons raise exceptions? Does the C standard mandate they do? Mandate they don't? Doesn't mandate anything? And - does __builtin_isnan() act differently than isnan() ?