Re: Let's use gcc-4.2, not 4.1 -- OpenMP

On Friday 15 December 2006 21:51, David O'Brien wrote:
> On Fri, Dec 15, 2006 at 07:14:53PM +0100, Stefan Ehmann wrote:
> > > CPU: AMD Athlon(TM) XP 2700+ (2166.44-MHz 686-class CPU)
>
> ..
>
> > Settings/Compiler           | gcc-3.4 | gcc-4.1 | gcc-4.2
> > ----------------------------+---------+---------+---------
> > -O2                         |   6.46s |   6.67s |   6.38s
> > -O2 -funroll-loops          |   4.44s |   4.16s |   4.02s
> > -O2 -march=athlon-xp -fun.. |   4.39s |   4.38s |   4.26s
> > -O3                         |   6.14s |   5.23s |   5.16s
> > -O3 -funroll-loops          |   4.24s |   4.87s |   4.95s
> > -O3 -march=athlon-xp -fun.. |   4.19s |   4.90s |   5.07s
>
> A fine example that -O3 isn't always better than -O2.
> I wonder if you're blowing the L2 cache.  IIRC, all Athlon XP 2700+
> are the Thoughbread core, which has only 256KB L2.
Yes, only 256KB L2 cache here.

Results on a pentium-m with 2MB L2 cache were quite similar. With loop 
unrolling -O2 was still faster than -O3. Though not as much slower as on the 
Athlon XP.

As a side note: (stripped) gcc42 binaries were up to 200% of the size of the 
gcc34 binaries.