c/7857: Different behaviour if compiled with optimization!

[jcas@netcabo.pt]

>Number:         7857
>Category:       c
>Synopsis:       Different behaviour if compiled with optimization!
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Sep 08 07:46:00 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator:     Jose Santos
>Release:        3.10
>Organization:
>Environment:
Windows XP
>Description:
If code is compiled with any optimization (-O1 to -O3) it has a different behaviour than with compiled with no optimizations.
The code is related with precision errors.With optimizations there are no precision errors, without optimization there's a precision error!
>How-To-Repeat:
Just run the file with -O3 and without -O3 and see that in first case it runs fine(requires a couple of seconds, eg: 15s), in second case it enters an infinite loop(and just ends after looping all integers back to 0)
>Fix:
Don't known...perhaps don't use the sqrt function ?
>Release-Note:
>Audit-Trail:
>Unformatted:
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Content-Type: text/plain; name="bug.c"
Content-Disposition: inline; filename="bug.c"

#include <stdio.h>
#include <math.h>

/*
Compile with -O3 and runs fine
without there's a precision error.
SQ's value has the correct value with -O3 (+1 than without -O3 for n==46611179)

E-mail me at jcas@netcabo.pt
*/
#define NUMPAIRS 10

int main(){
  long long n=2, count = 0;
  for(n=2; count<NUMPAIRS; n++){
    long long n2 =n*n;
    //double sq = sqrt(1+8.0*n2);
    long long sq = sqrt(1+8*n2);
    long long k = -(1 - sq)/2;    
    long long k2 = k*k;  	
    if(n==46611179)
      printf("%lld, %lld\n", sq, k);      
    if(2*n2==k2+k){
      printf("%10lld %10lld\n", n, k);
      count++;
    }
  }
  return 0;
}