From: Rich Felker <dalias@libc.org>
To: musl@lists.openwall.com
Subject: Re: for information, gcc-4.2.3 miscompiles musl math
Date: Sat, 21 Nov 2015 14:51:44 -0500 [thread overview]
Message-ID: <20151121195144.GF3818@brightrain.aerifal.cx> (raw)
In-Reply-To: <20151121194132.GF23362@port70.net>
On Sat, Nov 21, 2015 at 08:41:32PM +0100, Szabolcs Nagy wrote:
> * Rich Felker <dalias@libc.org> [2015-11-21 14:25:48 -0500]:
>
> > On Sat, Nov 21, 2015 at 06:24:18PM +0100, u-uy74@aetey.se wrote:
> > > Good to be aware of:
> > > gcc-4.2.3 miscompiles musl math since at least 1.1.6,
> > > tested while targeting i486,
> > > 1.0.x seems to have been alright.
> > >
> > > The symptom is that sin(larger-than-2*pi) yields large values
> > > like "sin(8.000000) = 21.709544".
> > > Looks like the argument reduction logic has changed in a way
> > > which is not compatible with gcc-4.2.3.
> >
> > Are you using configure or a hand-written config.mak? configure sets
> > up a big hammer, -ffloat-store, when -fexcess-precision=standard is
> > not supported (i.e. on old gcc), which hopefully suffices to make this
> > code work, but it's possible it doesn't always do the job.
> >
>
> i think this change might be it:
> http://git.musl-libc.org/cgit/musl/commit/?id=0ce946cf808274c2d6e5419b139e130c8ad4bd30
>
> the new code avoids an extra store,
> but then i rely on the evaluation
> being in long double.
>
> with -ffloat-store this breaks,
> adding extra store rounds at the
> wrong place.
Hmm, which places does it add the stores around? Could you fix it with
an explicit conversion to double_t? That might be nice to harden
against broken compilers without penalizing correct ones.
Rich
next prev parent reply other threads:[~2015-11-21 19:51 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-11-21 17:24 u-uy74
2015-11-21 19:25 ` Rich Felker
2015-11-21 19:41 ` Szabolcs Nagy
2015-11-21 19:49 ` Rich Felker
2015-11-21 19:56 ` Szabolcs Nagy
2015-11-21 19:51 ` Rich Felker [this message]
2015-11-21 20:03 ` Szabolcs Nagy
2015-11-21 20:15 ` Rich Felker
2015-11-21 21:54 ` Szabolcs Nagy
2015-11-21 20:32 ` u-uy74
2015-11-21 20:11 ` u-uy74
2015-11-21 20:18 ` Rich Felker
2015-11-21 20:30 ` Alexander Monakov
2015-11-21 21:08 ` u-uy74
2015-11-22 4:00 ` Isaac Dunham
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