Flags -g and -O give very different results

[Michael P Friedlander]

Hi Folks,

The behavior of a numerical code I'm working on varies
drastically depending on whether I've compiled it with
the  -g  or  -O  flags.

The code's behavior under -g is much more stable, and I'm
wondering if the -O flag is exposing a bug that I need to
fix.  Are there some gcc flags that I should try that might
guide me in finding the problem?  (I've already tried the
obvious  -Wall  which gives no warnings.)

In case it helps, I'm using both gcc 4.1.0 (SUSE Linux)
and on gcc 4.0.1 (Mac PPC) which give similar behavior.

Thanks!
--Michael