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Your compiler does many optimizations that break numerical reproducibility, especially in floats. I reviewed a PR the other day that wrote X=AB+(CD)+E;

And when I checked 3 different compilers, each of them chose a different way to use FMAs.

Even with integer math, you can get different numerical results via UB (e.g. expressions with signed overflow one way and not another).

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Floating point reproducibility and cross platform determinism requires strict adherence to the IEEE standard and disabling of fused instructions.

The point being that there are many other places where reproducibility fails to hold, especially when optimizations are involved. The standard doesn't mandate a way to disable contraction, nor the existence (or absence) of -ffast-math and others. They're simply different, legal compilations within the broad scope allowed under the standard.

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