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Date: Sat, 21 Nov 2015 14:49:38 -0500
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: for information, gcc-4.2.3 miscompiles musl math

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?

Rich

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