compile choices changing results

[Julie Stern]

So, do my answers below point to the gcc compiler switches
-O3 -fomit-frame-pointer -funroll-loops
and should this really produces different results in the
number crunching?


On Thu, 2 Feb 2006, Neil Ferguson wrote:

> Hello -
> 
> We need more information:
> 
> - what version of which OS are you using?
          Linux and I was told gcc and cc are the same in linux.  But,
I do anticipate running on other operating systems where they won't
be the same.   So, the LINUX v. is RehHat Linux 7.3

> - what version of GCC?
          2.95.3

> - what hardware/CPU architecture are you running on?
          Gateway intel pentium III, i686

> - is 'cc' actually an alias or symlink for 'gcc', or is it another compiler 
> altogether? Even if 'cc' points to a different version of GCC, that can be 
> significant.
        Yes, there was a symbolic link to gcc.


> 
> Semi-educated guess - are you compiling for x86?
      i686 
> 
> Neil.
> 
> Julie Stern wrote:
> > Hello,
> >    I'm running some code which came with a makefile
> > containing -O3 -fomit-frame-pointer -funroll-loops
> > and the program results were drastically different from
> > running with just cc. (yes, right now its an unfair
> > comparison, cc vs. gcc -O3 -fomit-frame-pointer -funroll-loops),
> > but I didn't expect the results to be different.
> >    Can anyone explain what the compiler options actually
> > do in this case?  I can read the man pages, but don't
> > understand what it exactly does.  Can anyone explain
> > why my results would be different  (2nd and 3rd decimal place
> > different in numbers that are in scientitific notation and
> > are e06 and e37, so the differences are huge)?  Anybody
> > have opinions on cc vs. gcc that can help me decide which
> > to use?
> > 
> >     Thanks so much for help and advice.
> > 
> > --Julie
>