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Date: Sat, 21 Nov 2015 14:25:48 -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 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 do not notice any problems while compiling musl with gcc-5.2, nor
> have a compelling reason to insist on using gcc-4.2.3 (somebody else
> might have though, gcc-4.2.3 is the last one under gpl 2).
I thought 4.2.1 was the last.
If you don't want to look into it further yourself I'll see if someone
else interested in old toolchains can or try to get around to it
myself. I really don't want to introduce more hacks for these broken
compilers though. If people really still want to use them, we should
probably just find a cheap way to fix the compiler, like patching it
not to perform any optimizations whatsoever on floating point
expressions.
Rich
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