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Message-ID: <20250817030956.GV1827@brightrain.aerifal.cx>
Date: Sat, 16 Aug 2025 23:09:56 -0400
From: Rich Felker <dalias@...c.org>
To: Alex Rønne Petersen <alex@...xrp.com>
Cc: musl@...ts.openwall.com
Subject: Re: Re: [PATCH] configure: prevent gcc and clang from
performing floating point contraction
On Sat, Aug 16, 2025 at 07:37:35PM +0200, Alex Rønne Petersen wrote:
> On Fri, May 30, 2025, at 00:42, Alex Rønne Petersen wrote:
> > The default setting for -ffp-contract is implementation-defined, so make
> > -ffp-contract=off explicit. This matters on clang in particular where
> > contraction is on by default even in standard C mode. Because of this, clang
> > will turn these expressions into llvm.fmuladd.* intrinsic calls in the
> > generated LLVM IR. This means we're completely at the mercy of the backend in
> > regards to whether those intrinsics get lowered to an fma() call or simply
> > fmul + fadd instructions. Similarly, optimization passes are allowed to perform
> > transformations on those intrinsic calls that fmul + fadd would not permit.
> >
> > Older gcc versions had a similar issue and required -mno-fused-madd, hence set
> > this as well.
> >
> > With zig cc -target arm-linux-musleabi, we've seen a case where clang turned
> > a * b + c into an fma() library call in musl's fma() implementation itself,
> > resulting in infinite recursion. This patch also fixes that.
> > ---
> > configure | 9 +++++++++
> > 1 file changed, 9 insertions(+)
> >
> > diff --git a/configure b/configure
> > index bc9fbe48..aa890101 100755
> > --- a/configure
> > +++ b/configure
> > @@ -355,6 +355,15 @@ tryflag CFLAGS_C99FSE -fexcess-precision=standard \
> > || { test "$ARCH" = i386 && tryflag CFLAGS_C99FSE -ffloat-store ; }
> > tryflag CFLAGS_C99FSE -frounding-math
> >
> > +#
> > +# Explicitly disable floating point contraction because the default
> > +# setting is implementation-defined, and clang is known to have it
> > +# on by default even in standard C mode. The same was true in older
> > +# gcc versions, hence -mno-fused-add as well.
> > +#
> > +tryflag CFLAGS_C99FSE -mno-fused-madd
> > +tryflag CFLAGS_C99FSE -ffp-contract=off
> > +
> > #
> > # Semantically we want to insist that our sources follow the
> > # C rules for type-based aliasing, but most if not all real-world
> > --
> > 2.48.1
>
> Friendly ping.
Thanks. I'm looking back at this now and I thought we were using
#pragma STDC FP_CONTRACT OFF and compilers just weren't honoring it,
but it seems we're not. Is this not supported by gcc and/or clang?
Because this is what we really should be doing, with the CFLAGS just
as a fallback in case a broken compiler doesn't honor it.
Rich
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