From: Szabolcs Nagy <nsz@port70.net>
To: musl@lists.openwall.com
Subject: Re: FE Exception triggered by comparison
Date: Wed, 27 Feb 2019 18:08:53 +0100 [thread overview]
Message-ID: <20190227170853.GF21289@port70.net> (raw)
In-Reply-To: <20190227164225.GV23599@brightrain.aerifal.cx>
* Rich Felker <dalias@libc.org> [2019-02-27 11:42:25 -0500]:
> 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.
yeah, a target may have slow fpreg->greg moves to do the bit checks,
but fast single instruction check on the fpreg and then the builtin
would be preferred.
next prev parent reply other threads:[~2019-02-27 17:08 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
2019-02-27 17:08 ` Szabolcs Nagy [this message]
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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