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Date: Sun, 22 Mar 2020 22:46:04 +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:
> > 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)
>
> I just dug through the old gcc and glibc bz's it was mentioned on
> (52593 and 14032 resp.) and didn't find anything, but I'm pretty sure
> it was known in 2012 that it was a non-issue for single-precision.
> What I didn't understand then was that the callee was responsible for
> dropping the excess precision; I wrongly believed that "something" in
> the caller would make it not-matter/get collapsed down to nominal
> precision and rounded correctly. Commit e0a54e6725 and related commits
> were fixing that mistake, which I'd since recognized was wrong but
> hadn't yet recognized the urgency of fixing until I started seeing how
> bad it could break the compiler.
Yes, that stackoverflow thread is also from 2012. Reading the question,
it's clear that you are asking about both single and double precision
(no indication you're aware that double-rounding would be safe for floats),
then Stephen Canon answers, and soon after the aforementioned commit appears.
Anyhow - thanks for the extra historical background.
Alexander
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