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Message-ID: <alpine.LNX.2.20.13.2003222136510.2534@monopod.intra.ispras.ru>
Date: Sun, 22 Mar 2020 21:51:09 +0300 (MSK)
From: Alexander Monakov <amonakov@...ras.ru>
To: musl@...ts.openwall.com
Subject: Re: Q: dealing with missing removal of excess precision
On Sun, 22 Mar 2020, Rich Felker wrote:
> > Nack for sqrt 'unsigned short' fix, I recommend to consider
> > 'unsigned fpsr = 0' and "+a" constraint, the resulting assembly is
> > much better (GCC doesn't seem to do a good job optimizing the 'unsigned short'
> > variant at all).
>
> Sorry, I forgot to disassemble and check after making that change, and
> indeed it is very bad.
>
> How about just leaving it as-is? The value is masked such that upper
> bits don't matter, and "whatever the register happened to contain
> before" is a valid albeit ugly output from inline asm -- it doesn't
> admit any compiler transformations that would cause inconsistent value
> to be observed or other problems.
Yes, I guess it's acceptable.
> > Actually, may I ask why the initial commit did not mention that it relies
> > on this nontrivial property?
>
> The need for fixup of double was only realized later, in commit
> 809556e60a3359f646946879dd94c4760e5b8e84. It was discussed at the time
> that no action was needed for single, but it seems since there was no
> change there wasn't any mention of it in log.
Are you sure? single-precision sqrtf received a change just two days
prior to that in commit e0a54e6725 ("correct rounding for i387 sqrtf function")
which is the same day as when Stephen Canon supplied his answer in
https://stackoverflow.com/questions/9678224/is-there-any-way-to-get-correct-rounding-with-the-i387-fsqrt-instruction
(and also the same day you asked the question)
Alexander
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