Which functions are affected by -fno-math-errno?

Question

I have been excited by this post: https://stackoverflow.com/a/57674631/2492801 and I consider using -fno-math-errno. But I would like to be sure that I do not harm the behaviour of the software I am working on.

Therefore I have checked the (rather large) codebase to see where errno is being used and I wanted to decide whether these usages interfere with -fno-math-errno. But how to do that? The documentation says:

-fno-math-errno

Do not set errno after calling math functions that are executed with a single instruction, e.g., sqrt...

But how can I know which math functions are executed with a single instruction? Is this documented somewhere? Where?

It seems as if the codebase I use relies on errno especially when calling strtol and when working with streams. I guess that strtol is not executed with a single instruction. Is it considered to be a math function at all? How can I be sure?

Answer 1

You can find list of functions affected by -fno-math-errno in GCC's builtins.def (search for "ERRNO"). It seems that only some functions from math.h header (cos, sin, exp, etc.) are affected. Treatment of other standard functions that use errno (strtol, etc.) will not change under this flag.