but potentially significant numerical differences for reductions.
I see the argument about unexpected performance changes, but this makes no sense whatsoever. Anyone using -ffast-math is claiming they don’t care about numerical accuracy of their results. The flag does not promise some particular non-standard outcome, but rather, says the compiler can do “whatever it wants” for certain enumerated cases. It would most-certainly be correct for the compiler to produce the same results under -ffast-math as it’s required to produce without the flag.
Anyone expecting otherwise is making a grave error in using -ffast-math in the first place.