Re: c++/6914: -O2 and -O give different results for the same valid FP code

[Tim Prince <tprince@computer.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