GNU bug report logs - #42147
28.0.50; pure vs side-effect-free, missing optimizations?

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Message #83 received at 42147 <at> debbugs.gnu.org (full text, mbox):

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Mattias Engdegård <mattiase <at> acm.org>
Cc: Stefan Monnier <monnier <at> iro.umontreal.ca>,
 Andrea Corallo <andrea_corallo <at> yahoo.it>, 42147 <at> debbugs.gnu.org
Subject: Re: bug#42147: 28.0.50; pure vs side-effect-free, missing
 optimizations?
Date: Fri, 3 Jul 2020 11:31:19 -0700

On 7/3/20 1:32 AM, Mattias Engdegård wrote:

> In practice, the extra precision of x87 code is so unreliable and fickle (unless the 80-bit long double is used throughout) that it's almost never worth it.

Depends on whether you want reproducibility or accuracy. We prefer the former,
it seems.

> Fortunately modern compilers generate SSE code by default

No, GCC generates x87 code by default. You need to specify -mfpmath=sse to
convince it to not generate x87 code. (Or, when you build GCC, you need to pass
--with-mfpmath=sse to 'configure'; but I think this is uncommon, at least in the
GNU/Linux world.)

Having Emacs use --with-mfpmath=sse should improve performance a bit on x86. But
more important, it should make floating point more reproducible. If I get the
time I'll look into having 'configure' add it automatically.

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