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Message-ID: <20250311173517.GD1827@brightrain.aerifal.cx>
Date: Tue, 11 Mar 2025 13:35:17 -0400
From: Rich Felker <dalias@...c.org>
To: Victor Stinner <vstinner@...hon.org>
Cc: musl@...ts.openwall.com
Subject: Re: Bug in fma(): should return negative zero
On Tue, Mar 11, 2025 at 01:09:23PM -0400, Rich Felker wrote:
> On Tue, Mar 11, 2025 at 05:19:26PM +0100, Victor Stinner wrote:
> > Hi,
> >
> > On Tue, Mar 11, 2025 at 2:25 PM Rich Felker <dalias@...c.org> wrote:
> > > What archs have you tested it on?
> >
> > I tested on x86-64.
> >
> > > It doesn't happen for me and the general C code looks like it handles
> > > this case correctly,
> >
> > Oh. I can reproduce the issue with my C example on:
> >
> > * Fedora 41: musl 1.2.5-1, musl-gcc version "gcc (GCC) 14.2.1"
> > * Alpine Linux v3.21: musl-1.2.5-r9, gcc version 14.2.0.
> >
> > Example on Fedora:
> > ---
> > $ musl-gcc x.c -o x && ./x
> > fma(+1e-300, -1e-300, +0) = +0
> > ---
> >
> > The result is wrong (+0), it must be "-0": fma(x, y, z) = (+1e-300) *
> > (-1e-300) + (+0) = -0.
>
> Indeed, the correct result is -0 and +0 is wrong. But there is no bug
> in musl. The bug is in the version of gcc used to compile musl. Using
> gcc 13.2.1 on Alpine 3.20 and building musl myself, fma correctly
> returns -0. But the musl binary shipped with Alpine 3.20 has been
> miscompiled so that the "return x*y;" on line 56 incorrectly just
> branches to line 53's "return x*y+z;".
>
> I'm not sure what the buggy version of gcc is, but I would guess it's
> a new regression in gcc 14. So I guess we need to track this down and
> report it. My guess is that someone wrongly thought they could infer
> from the branches on the bit representation of the float that it's
> zero, and thereby "of course adding zero is a no-op" (no! no! NO!) so
> they folded the two code paths to the same thing...
Ah, no, I should have checked. This is a bug in musl that was fixed by
nsz last year in git, but the fix hasn't made it into any release.
Rich
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