Weird, inconsistent double bug.

[ben . wellington]

This is so so weird.
As far as I can tell, this is a g++ optimization bug. I'd love to hear
that it's not, but it certainly seems that way.

Here's the original code:
//above is some code that sets s1 and s2,
// which are both doubles.

  if(s1 < s2) {
    cerr << (s1 < s2 ) << endl;
    cerr << (s1 > s2 ) << endl;
    cerr << (s1 == s2 ) << endl;
    cerr << s1 - s2 << endl;
    return true;
  }

If I compile it with options t -Wall -G -DNDEBUG  -DLinux  the output
is blank.  It skips the inside of the loop because s1 is not less than
s2.

But if I compile it with an addition -O3 the output is:
0
0
1
0
and true is returned.  What confuses me is how can the first 0 ever be
printed.  If it enters the if statement then s1 < s2, so the first
check should always be true. But it's not. With in the look it is
indicating that s1 == s2. So how did it get in in the first place?

Stranger yet, if I compile the following code with -O3, it skips the
'if' clause entirely and works fine.  The only difference is that I
added some cerr statements.

cerr << s1 << endl;
cerr << s2 << endl;
cerr <<  (s1 < s2) << endl ;
  if(s1 < s2) {
    cerr << (s1 < s2 ) << endl;
    cerr << (s1 > s2 ) << endl;
    cerr << (s1 == s2 ) << endl;
    cerr << s1 - s2 << endl;
    return true;
  }


I'm running g++ v 4.0 on RedHat
[emnlp@c6 src]$ g++4 -v
Using built-in specs.
Target: i386-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --enable-shared --enable-threads=posix
--enable-checking=release --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions
--with-gxx-include-dir=/usr/include/c++/3.4.3 --enable-libgcj-multifile
--enable-languages=c,c++,java,f95 --enable-java-awt=gtk
--with-java-home=/usr/lib/jvm/java-1.4.2-gcj-1.4.2.0/jre
--host=i386-redhat-linux
Thread model: posix
gcc version 4.0.2 20051130 (Red Hat 4.0.2-14.EL4)
[emnlp@c6 src]$ uname -a
Linux  2.6.9-34.ELsmp #1 SMP Fri Feb 24 16:54:53 EST 2006 i686 i686
i386 GNU/Linux
[emnlp@c6 src]$

uname -a gives:
Linux c6.cs.nyu.edu 2.6.9-34.ELsmp #1 SMP Fri Feb 24 16:54:53 EST 2006
i686 i686 i386 GNU/Linux

Thanks!