[DavidVoid]

It matters for reproducibility between software versions, right?

I work in audio software and we have some comparison tests that compare the audio output of a chain of audio effects with a previous result. If we make some small refactoring of the code and the compiler decides to re-organize the arithmetic operations then we might suddenly get a slightly different output. So of course we disable fast-math.

One thing we do enable though, is flushing denormals to zero. That is predictable behavior and it saves some execution time.

[recursivecaveat]

Yeah that is the killer for me. I'm not particularly attached to IEEE semantics. Unfortunately the replacement is that your results can change between any two compiles, for nearly any reason. Even if you think you don't care about tiny precision variances: consider that if you ever score and rank things with an algorithm that involves floats, the resulting order can change.