Re: Flags -g and -O give very different results

[Tim Prince <timothyprince at sbcglobal dot net>]

Michael P Friedlander wrote:
> Thanks for pointing out various options, Andrew.
>
> > > 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.)
> >
> > If your code does something different with/without -g, then that's a
> > bug in gcc.  -g shouldn't make any difference to the behaviour of your
> > program.
>
> -g makes no difference.
>
> > If your code does somethig different with -O, that's possibly a gcc
> > bug but it's probably a bug in your code.
>
> -OO  and  -O  give different results.
>
> > If you use -fno-strict-aliasing and that makes a difference with -O,
> > then that's definitely a bug in your code.
>
> I think you nailed it!  With -fno-strict-aliasing,  -O0  and  -O  give 
> identical results.  I tried -fstrict-aliasing  with -Wstrict-aliasing=2, 
> but gcc doesn't issue any warnings.
>
> Any pointers for how to track this sort of thing down?  What kind of 
> things should I look for?
-fstrict-aliasing asserts that your code complies with the C standard on 
typed aliasing.  Pointers to objects of incompatible type must not 
modify the same storage locations.  Code which violated this restriction 
was common, prior to the time when gcc introduced this option.