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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