[Bug c/48966] New: gcc 4.5 gets wrong answer at -O2

[geir at cray dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48966

           Summary: gcc 4.5 gets wrong answer at -O2
           Product: gcc
           Version: 4.5.3
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: g...@cray.com


The following test gets the wrong answer for optimization level -O2 (and -O3)
for GCC 4.5.3.  The correct answer occurs when -O1 is used.  The problem does
occur in GCC 4.6.0:


$ cat ptest.c

#include <stdio.h>

double d0 = 0.49999999999999999;
double d1 = 0.99320549518091861;
double d2 = 0.00674833952334470;
double d3 = 0.00679450481908144;

int main() {
   double      tmp;
   int         answer = 0;

   tmp = d3 - d2/(d1-d0) - d0;
   answer = (*(((int *)&tmp)+1) >> 31) & 1;
   printf("answer = %d\n",answer);
   return answer;
}

$ gcc --version
gcc (GCC) 4.5.3 20110428 (Cray Inc.)
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ gcc -O1 ptest.c && ./a.out
answer = 1
$ gcc -O2 ptest.c && ./a.out
answer = 0       <----------- Wrong answer!!!
$ module swap gcc/4.5.3 gcc/4.6.0
$ gcc --version
gcc (GCC) 4.6.0 20110325 (Cray Inc.)
Copyright (C) 2011 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

geir@kaibab:/lus/scratch/geir/ISU/ISU3475> gcc -O2 ptest.c && ./a.out
answer = 1
$

The processor being used is:

$ grep "model name" /proc/cpuinfo
model name    : 6-Core AMD Opteron(tm) Processor 23 (D0)
model name    : 6-Core AMD Opteron(tm) Processor 23 (D0)
model name    : 6-Core AMD Opteron(tm) Processor 23 (D0)
model name    : 6-Core AMD Opteron(tm) Processor 23 (D0)
model name    : 6-Core AMD Opteron(tm) Processor 23 (D0)
model name    : 6-Core AMD Opteron(tm) Processor 23 (D0)
$