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From: Rich Felker <dalias@libc.org>
To: musl@lists.openwall.com
Subject: Re: [musl] [PATCH] math: move i386 sqrt to C
Date: Sat, 21 Mar 2020 13:57:43 -0400	[thread overview]
Message-ID: <20200321175743.GK11469@brightrain.aerifal.cx> (raw)
In-Reply-To: <20200321175351.GJ11469@brightrain.aerifal.cx>

On Sat, Mar 21, 2020 at 01:53:51PM -0400, Rich Felker wrote:
> 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?

And, I think I do. Changing it to:

    ux.i.m ^= (fpsr & 0x200) + 0x300;

yields an offset of +0x300 (^0x300) or -0x300 (^0x500). This looks
like it should work theoretically, and indeed it passes libc-test.

Rich

  reply	other threads:[~2020-03-21 17:57 UTC|newest]

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-05 16:35 math patches for moving bare asm to C inline asm Alexander Monakov
2020-01-05 16:36 ` [PATCH] math: move x86_64 fabs, fabsf to C with " Alexander Monakov
2020-01-05 20:05   ` Rich Felker
2020-01-05 21:32     ` Alexander Monakov
2020-01-05 22:43       ` Rich Felker
2020-01-06  8:17         ` Alexander Monakov
2020-01-06  8:40 ` [PATCH] math: move more x86-family fabs functions to C Alexander Monakov
2020-03-21 17:06   ` [musl] " Rich Felker
2020-01-06 16:50 ` [PATCH] math: move trivial x86-family sqrt " Alexander Monakov
2020-01-06 17:43 ` [PATCH] math: move i386 sqrtf " Alexander Monakov
2020-01-06 18:32   ` Pascal Cuoq
2020-01-09 15:55   ` Alexander Monakov
2020-01-09 17:00     ` Rich Felker
2020-01-09 21:00       ` Szabolcs Nagy
2020-01-09 22:00         ` Rich Felker
2020-01-09 23:18           ` Szabolcs Nagy
2020-01-10  2:07             ` Rich Felker
2020-01-10  9:17               ` Szabolcs Nagy
2020-01-14 17:59         ` [musl] " Alexander Monakov
2020-01-14 18:47           ` Szabolcs Nagy
2020-01-07 13:06 ` [PATCH] math: move i386 sqrt " Alexander Monakov
2020-01-08  7:26   ` Rich Felker
2020-03-21 17:53   ` [musl] " Rich Felker
2020-03-21 17:57     ` Rich Felker [this message]
2020-03-21 20:30       ` Alexander Monakov
2020-01-11 15:06 ` [PATCH] math: move x86_64 (l)lrint(f) functions " Alexander Monakov
2020-01-11 15:23 ` [PATCH] math: move more x86-family lrint " Alexander Monakov
2020-01-11 16:07   ` Rich Felker
2020-01-11 16:22     ` Rich Felker
2020-01-14 11:54 ` [musl] [PATCH] math: move x86-family rint " Alexander Monakov
2020-01-14 18:17 ` [musl] Q: dealing with missing removal of excess precision Alexander Monakov
2020-01-14 18:50   ` Szabolcs Nagy
2020-01-14 18:58     ` Rich Felker
2020-01-14 19:53       ` Alexander Monakov
2020-02-06 14:51         ` Rich Felker
2020-02-06 17:15           ` Alexander Monakov
2020-02-06 17:46             ` Rich Felker
2020-02-06 19:03               ` Rich Felker
2020-02-06 20:02                 ` Rich Felker
2020-02-06 22:08                   ` Szabolcs Nagy
2020-02-22 19:59             ` Rich Felker
2020-02-22 20:21               ` Alexander Monakov
2020-02-23  0:19                 ` Rich Felker
2020-02-23 16:14                   ` Alexander Monakov
2020-03-20 18:12                     ` Rich Felker
2020-03-22  1:19                       ` Rich Felker
2020-03-22 17:40                         ` Alexander Monakov
2020-03-22 17:53                           ` Rich Felker
2020-03-22 18:51                             ` Alexander Monakov
2020-03-22 19:10                               ` Rich Felker
2020-03-22 19:46                                 ` Alexander Monakov
2020-01-14 20:41 ` [musl] [PATCH] math: move x86-family remainder functions to C Alexander Monakov
2020-01-15  6:54   ` Szabolcs Nagy
2020-01-15 15:44 ` [musl] [PATCH] math: move x86-family fmod " Alexander Monakov
2020-01-16 21:00 ` [musl] [PATCH] math: add x86_64 remquol Alexander Monakov

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