optimization/5264: Possible optimization bug at -O2 on HP-UX

[luke@stat.umn.edu]

>Number:         5264
>Category:       optimization
>Synopsis:       Possible optimization bug at -O2 on HP-UX
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Thu Jan 03 07:06:00 PST 2002
>Closed-Date:
>Last-Modified:
>Originator:     Luke Tierney
>Release:        gcc version 2.95.2 19991024 (release)
>Organization:
>Environment:
HP-UX bass B.11.00 A 9000/780 2007851940
>Description:
A possible optimization bug at -O2 on HP-UX found in building
R (Gnu S).  Simplified program:
 ---------------(opttest.c)--------------------------------------------#include <stdio.h>

double x[] = { 1.0, 2.0}, y[] = { 1.0, 2.0 }, z[2];

main()
{
  int i, i1, i2, n, n1, n2;

  i1 = i2 = 0;
  n = n1 = n2 = 2;

  for (i = 0; i < n; i++) {
    z[i] = x[i1] + y[i2];
    i1 = (++i1 == n1) ? 0 : i1;
    i2 = (++i2 == n2) ? 0 : i2;
  }
  printf("z[1] = %g\n", z[1]);
}
----------------------------------------------------------------------

Without optimization or with just -O it works fine:

tierney@bass [125]% gcc -o opttest opttest.c
ltierney@bass [126]% ./opttest 
z[1] = 4ltierney@bass [127]% gcc -O -o opttest opttest.c
ltierney@bass [128]% ./opttest
z[1] = 4

But with -O2 it gets the wrong answer:

ltierney@bass [129]% gcc -O2 -o opttest opttest.c
ltierney@bass [130]% ./opttest
z[1] = 2

Please let me know if you need additional info.


>How-To-Repeat:
On HP-UX compare output of

gcc -O2 -o opttest opttest.c; ./opttest
gcc -O -o opttest opttest.c; ./opttest
>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted:
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