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

[Stefan Ehmann]

On Friday 15 December 2006 06:43, Scott Long wrote:
> Steve Kargl wrote:
> > On Fri, Dec 15, 2006 at 02:50:30PM +1030, Daniel O'Connor wrote:
> >>On Friday 15 December 2006 05:50, Scott Long wrote:
> >>>Yes, the industry moves fast, but that's no reason to fool ourselves
> >>>into thinking that the FSF will support GCC 4.2 a day after they release
> >>>4.3 and start working on 4.4.  Your point above about the lifespan of
> >>>FreeBSD 7.x is a valid one, and I agree that it should be a
> >>>consideration.  Vendor support is a myth and should not be a
> >>>consideration.
> >>
> >>Not to mention it is *trivial* to install a compiler using ports or
> >> packages.
> >>
> >>If you are serious about high performance computing installing a new
> >> compiler is about the lowest barrier you'll find.
> >
> > Actually, 4.1.x will produce much worse code than 3.4.6.
> > You can search the gcc mail listings for extensive comparison
> > by Clinton Whaley (the author of math/atlas) for details.
>
> Has this been fixed in GCC 4.2?  If the FSF claims to have fixed it,
> has it been actually verified?  I thought that gcc 4 was supposed to
> solve the world's problems with vectorization.

I've been playing around with optimizations for a small cpu-intensive program 
(only integer, no FP) for a course some time ago and tested different gcc 
versions. gcc-3.4 (with -O3 -march=pentium4) won over gcc-4.0 there.

My new test setup:
FreeBSD 6.2-RC1
gcc version 3.4.6 [FreeBSD] 20060305 (base system)
gcc version 4.1.2 20061013 (prerelease) (lang/gcc41 package)
gcc version 4.2.0 20061014 (experimental) (lang/gcc42 package)

CPU: AMD Athlon(TM) XP 2700+ (2166.44-MHz 686-class CPU)
Instructions counted with
pmcstat -C -p k7-retired-instructions

Settings/Compiler           | gcc-3.4 | gcc-4.1 | gcc-4.2
----------------------------+---------+---------+---------
-O2                         |  13.1bn |  13.8bn |  13.5bn
-O2 -funroll-loops          |   9.6bn |   9.3bn |   9.2bn
-O2 -march=athlon-xp -fun.. |   9.7bn |  10.6bn |  10.7bn
-O3                         |  11.5bn |   9.5bn |   9.6bn
-O3 -funroll-loops          |   8.4bn |   9.2bn |   9.4bn
-O3 -march=athlon-xp -fun.. |   8.8bn |  10.6bn |  11.1bn


I'm aware that testing with a single program is not too meaningful, but it 
might give a hint at least.