[Bug optimization/11394] New: optimization leads to broken double comparison

["gccbug at chiefrocker dot net" <gcc-bugzilla at gcc dot gnu dot org>]

PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11394

           Summary: optimization leads to broken double comparison
           Product: gcc
           Version: 3.2.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: gccbug at chiefrocker dot net
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu

The following test program tests inequality of two doubles, then equality. 
The bug is that both will hold if 
- compiled with optimization (-O1 is enough) 
- first, an int was shifted into the ostream 
 
---------------------------------------------- 
#include <iostream> 
#include <cmath> 
int main() 
{ 
	double dist1 = std::sqrt((2.0 - 6)*(2.0 - 6) + (2.0 - 1)*(2.0 - 1)); 
	double dist2 = std::sqrt((5.0 - 6)*(5.0 - 6) + (5.0 - 1)*(5.0 - 1)); 
	 
	if(dist1 != dist2) 
	{ 
		std::cerr << 1 // outputting any int here makes the bug show up 
				  << ((dist1 == dist2) ? "compiler broken\n" : ""); 
	} 
} 
---------------------------------------------- 
 
Executing 
  g++ -Wall -O1 double_comparison.cxx -o double_comparison && 
./double_comparison 
gives: 
  1compiler broken 
 
The bug was verified with 3.2.3 and 3.2, too. On 2.95.3, it does not show up. 
We tried to strip the program down as far as we could, hopefully you can shed light 
on this.