Date: Sat, 21 Nov 2015 20:41:32 +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 14:25:48 -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. > > 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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