[bonzini]

Stuff like Kahan summation breaks horribly with -ffast-math, because if you assume that sum is associative the error term, which is ((s+x)-s)-x, simplifies to zero.

https://en.m.wikipedia.org/wiki/Kahan_summation_algorithm

[all-fakes]

Yeah, that's the obvious counterexample, but that's why I said "more often than not". The statement I was questioning was that fast-math is "normally not more correct."

[kevin_thibedeau]

That's where you dust off volatile.

[nestorD]

Sadly true. That's the only portable way I found to implement something akind to a Kahan sum so that no compiler / flag would destroy it.