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Re: c++/6914: -O2 and -O give different results for the same valid FP code
- From: Tim Prince <tprince at computer dot org>
- To: nobody at gcc dot gnu dot org
- Cc: gcc-prs at gcc dot gnu dot org,
- Date: 3 Jun 2002 13:36:06 -0000
- Subject: Re: c++/6914: -O2 and -O give different results for the same valid FP code
- Reply-to: Tim Prince <tprince at computer dot org>
The following reply was made to PR c++/6914; it has been noted by GNATS.
From: Tim Prince <tprince@computer.org>
To: veksler@il.ibm.com, gcc-gnats@gcc.gnu.org, gcc-prs@gcc.gnu.org,
gcc-bugs@gcc.gnu.org
Cc:
Subject: Re: c++/6914: -O2 and -O give different results for the same valid FP code
Date: Mon, 3 Jun 2002 06:30:53 -0700
On Monday 03 June 2002 06:09, Michael Veksler wrote:
> http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&;
>pr=6914
>
> So here are my conclusions (after reading the assembly, and running gdb
> on it):
> Floating point on x86 is done on 80 bit registers (IEEE's double
> extended type).
> GCC generates code to exploit all 80 bits during divide. It then spills
> the result
> to memory.
>
> The second divide is not spilled to memory, and the comparison is done
> between truncated value (which lost accuracy), and a completely accurate
> 80 bit
> value.
> According to http://www.validlab.com/goldberg/paper.ps this does not
> seem right.
> According to this paper, gcc should operate on double precision (64
> bit), or at
> least give the impression that it does so (to the outside viewer). But
> gcc operates
> on a mixed 80/64 bit setting, and that seems contradictory to IEEE spirit
> (and, probably, agains ISO C rules).
>
Among the remedies available would be
a) set 53-bit rounding mode
b) choose -msse2, for appropriate targets
--
Tim Prince