[MaxBarraclough]

> -ffast-math is a shotgun that affects a lot of things

Interesting point. GCC and MSVC both seem to have (incompatible) intrinsic functions, for what that's worth.

https://gcc.gnu.org/onlinedocs/gcc-4.8.5/gcc/X86-Built-in-Fu...

https://docs.microsoft.com/en-us/previous-versions/visualstu...

[mmozeiko]

That is not quite true. You don't need to use GCC __builtin functions for this. GCC supports SSE1 intrinsics like _mm_rsqrt_ss exactly same as MSVC - it is declared in xmmintrin.h header. Just include it and _mm_rsqrt_ss/ps will be available for you in gcc and msvc.