From: Rich Felker <dalias@libc.org>
To: musl@lists.openwall.com
Subject: Re: FE Exception triggered by comparison
Date: Wed, 27 Feb 2019 11:42:25 -0500 [thread overview]
Message-ID: <20190227164225.GV23599@brightrain.aerifal.cx> (raw)
In-Reply-To: <alpine.LNX.2.20.13.1902271917310.30425@monopod.intra.ispras.ru>
On Wed, Feb 27, 2019 at 07:32:01PM +0300, Alexander Monakov wrote:
> On Thu, 28 Feb 2019, Damian McGuckin wrote:
>
> > > Hm, no, for x86 with GCC you should not see that: the compiler knows how to
> > > expand isnan efficiently. Are you perhaps on OS X and the 'gcc' command
> > > actually invokes Clang/LLVM?
> >
> > No.
> >
> > > If not, can you show output of 'gcc -v', command-line flags you used, and
> > > the assembly you're seeing?
> >
> > gcc -O3 -S -msse4.2 -mfma mynan.c
>
> Ah, in this case you're falling victim of a problem in your Glibc version:
> while GCC is sufficiently new to know how to expand isnan efficiently,
> Glibc math.h defines isnan as a macro that redirects to __isnan that GCC
> does not recognize. Newer Glibc versions use __builtin_isnan where suitable,
> which leads to optimal assembly.
>
> (musl does not use this builtin, expanding the macro to a bit test instead)
Are there reasons we should perhaps use the __builtin versions of
these when __GNUC__ indicates they're available? I like our bit test
versions we have now, and I think they're sufficiently efficient, but
I'm open to changes if there's a good reason.
Rich
next prev parent reply other threads:[~2019-02-27 16:42 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-24 13:28 Damian McGuckin
2019-02-24 17:12 ` Markus Wichmann
2019-02-24 19:25 ` Szabolcs Nagy
2019-02-24 20:04 ` Jens Gustedt
2019-02-24 21:50 ` Damian McGuckin
2019-02-25 5:21 ` Damian McGuckin
2019-02-25 15:51 ` Markus Wichmann
2019-02-26 3:55 ` Damian McGuckin
2019-02-27 14:14 ` Alexander Monakov
2019-02-27 15:38 ` Damian McGuckin
2019-02-27 16:00 ` Alexander Monakov
2019-02-27 16:09 ` Damian McGuckin
2019-02-27 16:14 ` Markus Wichmann
2019-02-27 16:20 ` Damian McGuckin
2019-02-28 1:07 ` Damian McGuckin
2019-02-28 1:27 ` Rich Felker
2019-02-28 2:28 ` Damian McGuckin
2019-02-27 16:32 ` Alexander Monakov
2019-02-27 16:42 ` Rich Felker [this message]
2019-02-27 17:08 ` Szabolcs Nagy
2019-02-27 17:14 ` Alexander Monakov
2019-02-27 17:26 ` Rich Felker
2019-02-27 19:36 ` Szabolcs Nagy
2019-02-27 19:48 ` Alexander Monakov
2019-02-27 20:16 ` Szabolcs Nagy
2019-02-27 20:35 ` Rich Felker
2019-02-27 21:03 ` Szabolcs Nagy
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