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GCC won't use FMA without fast-math though. Even when AVX is otherwise enabled.

Sure it will:

> -ffp-contract=fast enables floating-point expression contraction such as forming of fused multiply-add operations if the target has native support for them

> The default is -ffp-contract=off for C in a standards compliant mode (-std=c11 or similar), -ffp-contract=fast otherwise.

https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#ind...

Oh, wow, forgot about fp-contract. It says it is off in C by default, what about C++?

Read closer, it defaults to fast, not off

I would have expected to be a bug in the documentation? Why would they turn FMA off for standard compliant C mode, but not for standard compliant C++ mode?

But the documentation does appear to be correct: https://godbolt.org/z/3bvP136oc

Crazy.

it defaults to off for standard-compliant mode. Which in my mind was the default mode as that's what we use everywhere I have worked in the last 15 years. But of course that's not the case.

In any case, according to the sibling comment, the default is 'fast' even in std-compliant mode in C++, which I find very surprising. I'm not very familiar with that corner of the standard, but it must be looser than the equivalent wording in the C standard.

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