4 ms·

Don't CPU compilers in general, not just ARM ones, avoid using FMA for normal arithmetic unless you use -ffast-math? I don't think FMAs are inaccurate per se, b

Don't CPU compilers in general, not just ARM ones, avoid using FMA for normal arithmetic unless you use -ffast-math? I don't think FMAs are inaccurate per se, but they will produce differently-rounded results than multiplications and additions, and moreover you have to do a bunch of reassociation to make good use of them. C and C++ expect precise math by default. GLSL and the like are different.

PennRobotics 5y ago
I think for this particular instruction, -ffp-contract=fast
qayxc 5y ago
> 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 https://godbolt.org/z/e9EcfjdeY
- fulafel 5y ago
  Your link uses -ffast-math with gcc.
  - qayxc 5y ago
    The result stays more or less the same without it, still using FMA: https://godbolt.org/z/6MWcahsfo https://godbolt.org/z/6MWcahsfo
celrod 5y ago
https://godbolt.org/z/v67Tc3jEc https://godbolt.org/z/v67Tc3jEc gcc and icc will use fma under `-O3`, but Clang won't.
- thxg 5y ago
  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).