Date: Sat, 21 Mar 2020 13:53:51 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: [PATCH] math: move i386 sqrt to C On Tue, Jan 07, 2020 at 04:06:05PM +0300, Alexander Monakov wrote: > --- > Since union ldshape does not have a dedicated field for 32 least significant > bits of the x87 long double mantissa, keeping the original approach with > > ux.i.m -= (fpsr & 0x200) - 0x100; > > would lead to a 64-bit subtraction that is not trivial for the compiler to > optimize to 32-bit subtraction as done in the original assembly. Therefore > I have elected to change the approach and use > > ux.i.m ^= (fpsr & 0x200) + 0x200; > > which is easier to optimize to a 32-bit rather than 64-bit xor. > > Thoughts? I'm getting test failures with sqrt and this seems to be the culprit -- I don't think it's equivalent. The original version could offset the value by +0x100 or -0x100 before rounding, and offsets in the opposite direction of the rounding that already occurred. Your version can only offset it by +0x200 or -0x400. The (well, one) particular failing case is: src/math/ucb/sqrt.h:49: RU sqrt(0x1.fffffffffffffp+1023) want 0x1p+512 got 0x1.fffffffffffffp+511 ulperr -0.250 = -0x1p-1 + 0x1p-2 Here the mantissa is fffffffffffffc00 and offset by -0x400 yields: fffffffffffff800 which has exactly 53 bits and therefore does not round up like it should. I still like your approach better if there's a way to salvage it. Do you see one? Rich
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