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Date: Wed, 27 Feb 2019 11:42:25 -0500
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: FE Exception triggered by comparison
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
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