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* Re: optimization/9840: Incorrect optimization for floating-point aliasing to int
@ 2003-02-24 21:59 bangerth
0 siblings, 0 replies; 2+ messages in thread
From: bangerth @ 2003-02-24 21:59 UTC (permalink / raw)
To: gcc-bugs, gcc-prs, gsasha, nobody
Synopsis: Incorrect optimization for floating-point aliasing to int
State-Changed-From-To: open->closed
State-Changed-By: bangerth
State-Changed-When: Mon Feb 24 21:58:59 2003
State-Changed-Why:
Not a bug. You are violating aliasing rules, so the compiler
may generate whatever code it likes. If you need to play
type games, check the manual for ways to do that.
http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=9840
^ permalink raw reply [flat|nested] 2+ messages in thread
* optimization/9840: Incorrect optimization for floating-point aliasing to int
@ 2003-02-24 21:26 gsasha
0 siblings, 0 replies; 2+ messages in thread
From: gsasha @ 2003-02-24 21:26 UTC (permalink / raw)
To: gcc-gnats
>Number: 9840
>Category: optimization
>Synopsis: Incorrect optimization for floating-point aliasing to int
>Confidential: no
>Severity: serious
>Priority: medium
>Responsible: unassigned
>State: open
>Class: wrong-code
>Submitter-Id: net
>Arrival-Date: Mon Feb 24 21:26:00 UTC 2003
>Closed-Date:
>Last-Modified:
>Originator: Alex Gontmakher
>Release: gcc version 3.2.2 (Mandrake Linux 9.1 3.2.2-2mdk)
>Organization:
>Environment:
machine: Athlon MP 1800
>Description:
The following code worked with previous versions of GCC, producing either 1 or 0. Now it produces 2.
A workaround is to uncomment the printf() line - obviously causes it to perform the initialization of the float prior to executing the ifs.
The problem happens only at optimization level O2 or higher.
>How-To-Repeat:
#include <iostream>
int f()
{
int *p;
double x = 1.0;
/*fprintf(stderr,"");*/
/* NOTE: this check assumes IEEE floating point format */
p = (int *)&x;
if (*p == 0)
return 0;
else if (*p == 0x3ff00000)
return 1;
else
return 2;
}
int main()
{
std::cout << f() << std::endl;
}
>Fix:
I didn't read the optimization code for this, but it should probably notice that p aliases &x, and perform the initialization of x prior to reading *p.
>Release-Note:
>Audit-Trail:
>Unformatted:
^ permalink raw reply [flat|nested] 2+ messages in thread
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2003-02-24 21:59 optimization/9840: Incorrect optimization for floating-point aliasing to int bangerth
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