Re: [musl] [PATCH] configure: prevent compilers from turning a * b + c into fma(a, b, c)

[Alexander Monakov <amonakov@ispras.ru>]

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On Wed, 28 Aug 2024, Alex Rønne Petersen wrote:

> I've seen Clang do this for expressions in the fma() implementation itself,
> which of course led to infinite recursion. This happened when targeting
> arm-linux-musleabi with full soft float mode and -march=armv8-a. I imagine
> it's possible for GCC to do similar silliness.

musl passes -std=c99 to the compiler, and in all GCC releases so far* that
disables FMA contraction (as opposed to -std=gnu99 or whichever -std=gnuXX
is enabled by default, where unrestricted contraction is implicitly enabled,
i.e. the non-standard and dangerous -ffp-contract=fast mode).

Clang respects #pragma STDC FP_CONTRACT OFF, so that is available as
a smaller hammer than disabling fma across the board. Breaking up
contractable expression in fma*.c will work too.

[*] maybe modulo bugs in old releases where the backend doesn't respect
-ffp-contract=off and which Glibc worked around with -mno-fused-madd.

Alexander