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
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