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

["Alex Rønne Petersen" <alex@alexrp.com>]

On Wed, Aug 28, 2024 at 5:53 PM Alexander Monakov <amonakov@ispras.ru> wrote:
>
>
> 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).

OK, that part can definitely be left out. I only added it for good
measure; I haven't actually seen the issue occur with GCC.

>
> 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.

That pragma indeed seems to work. I suppose we could just add it to
the fma*() implementations and call it a day? I'm unsure if there are
other functions that might be affected by this.

>
> [*] 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