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Message-Id: <f54acca2-9570-44f9-87af-f5783b9daec7@app.fastmail.com>
Date: Sun, 17 Aug 2025 08:54:40 +0200
From: Alex Rønne Petersen <alex@...xrp.com>
To: "Rich Felker" <dalias@...c.org>
Cc: musl@...ts.openwall.com
Subject: Re: Re: [PATCH] configure: prevent gcc and clang from performing
floating point contraction
On Sun, Aug 17, 2025, at 05:54, Rich Felker wrote:
> On Sat, Aug 16, 2025 at 11:09:56PM -0400, Rich Felker wrote:
>> 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.
>
> To answer my own question, it looks like gcc is ignoring the pragma
> but clang (at least 20.1.0, tested on godbolt) honors it.
>
> In any case I think we should add the pragma for correctness and the
> command line option as a safety for broken compilers.
>
> Rich
Using the pragma makes sense; I wasn't actually aware that it exists. That said, I don't think I'm qualified to say where it should be applied (besides fma).
Regards,
Alex
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