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: tprince@computer.org
Cc: gcc-gnats@gcc.gnu.org, gcc-prs@gcc.gnu.org, gcc-bugs@gcc.gnu.org
Subject: Re: c++/6914: -O2 and -O give different results for the same valid FP code
Date: Mon, 3 Jun 2002 18:49:24 +0300

 Tim Prince wrote:
 + Among the remedies available would be
 + a) set 53-bit rounding mode
 + b) choose -msse2, for appropriate targets
 
 After rereading the gcc manual, I found "-ffloat-store" which is the best remedy. It is described in:
 http://gcc.gnu.org/onlinedocs/gcc-3.1/gcc/Optimize-Options.html#Optimize%20Options
 
 It says "For most programs, the excess precision does only good". This statement is wrong. It is unbearable to have a perfectly valid 
 code behave differently with -O2 and -O0 on the same platform. The default 
 should be changed to -ffloat-store. Of course, when passing -funsafe-math-optimizations, gcc may also set -fno-float-store.
 
 If the decision will be to keep the current behavior, documentation should 
 be updated. It should be more clear that -float-store is critical for IEEE 
 conformance on targets like x86. Also, the bugs section should contain 
 this as a known bug (and bug it is -- gcc does not conform to the IEEE 
 standard on x86).