From: Jamie Lokier <jamie@shareable.org>
To: Daniel Forrest <forrest@lmcg.wisc.edu>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Somewhat OT: gcc, x86, -ffast-math, and Linux
Date: Sat, 27 Mar 2004 14:24:59 +0000 [thread overview]
Message-ID: <20040327142459.GF21884@mail.shareable.org> (raw)
In-Reply-To: <200403262054.i2QKsV223748@rda07.lmcg.wisc.edu>
Daniel Forrest wrote:
> What is the -ffast-math option doing?
It enables some optimisations and mathematical transformations which
do not satisfy the properties of IEEE floating point arithmetic.
(Not that GCC's output satisfies those properties without -ffast-math
on x86, but this flag enables much looser semantics).
> How are the excess bits of precision dealt with during context
> switches?
They are preserved - they are part of the floating point context.
If there is any failure to preserve all of that context, it's a kernel bug.
> Shouldn't the same binary with the same inputs produce the same
> output on identical hardware?
Is the hardware *identical*, or are they different x86 CPUs?
Different CPUs give different results for the trigonometric functions.
GCC's manual claims that fsin, fcos and fsqrt instructions are only
used if the -funsafe-math-optimizations flag is also used, if the GCC
version is >= 2.6.1. However you may find that Glibc's <math.h> ends
up using those instructions when -ffast-math is used alone.
-- Jamie
next prev parent reply other threads:[~2004-03-27 14:25 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-03-26 20:54 Somewhat OT: gcc, x86, -ffast-math, and Linux Daniel Forrest
2004-03-26 21:26 ` Richard B. Johnson
2004-03-26 21:45 ` Andy Isaacson
2004-03-27 14:24 ` Jamie Lokier [this message]
2004-03-27 15:13 ` Jakub Jelinek
2004-03-29 8:47 ` Eric W. Biederman
2004-03-31 7:14 ` J.A. Magallon
-- strict thread matches above, loose matches on Subject: below --
2004-03-27 14:48 Nick Warne
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20040327142459.GF21884@mail.shareable.org \
--to=jamie@shareable.org \
--cc=forrest@lmcg.wisc.edu \
--cc=linux-kernel@vger.kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox