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Message-ID: <20151121195642.GG23362@port70.net>
Date: Sat, 21 Nov 2015 20:56:43 +0100
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: for information, gcc-4.2.3 miscompiles musl math
* Rich Felker <dalias@...c.org> [2015-11-21 14:49:38 -0500]:
> On Sat, Nov 21, 2015 at 08:41:32PM +0100, Szabolcs Nagy wrote:
> > * Rich Felker <dalias@...c.org> [2015-11-21 14:25:48 -0500]:
> >
> > > On Sat, Nov 21, 2015 at 06:24:18PM +0100, u-uy74@...ey.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.
> >
> > i didnt think about old toolchains
> > when i made that change.
> >
> > this also affects rounding functions
> > (but i386 has asm for most of them)
> >
> > i will think about it if i can change
> > the code so it does not break with
> > -ffloat-store.
>
> Alternatively we could see if removing -ffloat-store fixes it and
> avoids introducing any horrible bugs. That would be cleaner, wouldn't
> it?
>
i removed all the volatile hacks and i think
those are needed without -ffloat-store or
-fexcess-precision=standard when i do want
the rounding.
i dont know how many places are affected
because i tried to rewrite things to avoid
the need for extra stores.
it might be worth a try but there is no warranty..
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