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gcc and icc will use fma under `-O3`, but Clang won't.

gcc will avoid FMA if you specify `-std=c17` (or another C standard, instead of the default "GNU" variant). Note that it has to avoid FMA in that case because fusing (a * b) + c would not comply with the C17/IEEE-754 spec (see -fexcess-precision in the manual).

> Don't CPU compilers in general, not just ARM ones, avoid using FMA for normal arithmetic unless you use -ffast-math?

Doesn't seem o be the case: MSVC avoids FMA, while GCC prefers it: https://godbolt.org/z/e9EcfjdeY

Your link uses -ffast-math with gcc.

The result stays more or less the same without it, still using FMA: https://godbolt.org/z/6MWcahsfo

I think for this particular instruction, -ffp-contract=fast