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

[Michael Veksler <veksler@il.ibm.com>]

The following reply was made to PR c++/6914; it has been noted by GNATS.

From: Michael Veksler <veksler@il.ibm.com>
To: gcc-gnats@gcc.gnu.org, gcc-prs@gcc.gnu.org, veksler@il.ibm.com,
   gcc-bugs@gcc.gnu.org
Cc:  
Subject: Re: c++/6914: -O2 and -O give different results for the same valid
 FP code
Date: Mon, 03 Jun 2002 16:09:30 +0300

 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).
 
 You can read the section about optimization.