Polyhedron Fortran benchmark - compiler comparison: Good results for GCC 4.6

[Tobias Burnus <burnus at net-b dot de>]

 Hello all,

given the optimization work which has gone into 4.6, I thought I would 
run the Polyhedron Fortran test suite. The system is
- Intel Core2 Duo CPU  E8400  @ 3.00GHz
- CentOS release 5.5 (Final)  [x86-64]

Tested compilers:
- GCC 4.6 (recent nightly build)
- Intel Fortran compiler (ifort) 11.1
- PathScale compiler (pathf95) 3.2.99
- Portland Group compiler (pgf90) 10.1

In case of GCC, I have two 'normal' compilations (gfortran64 and 
gfortranIMF2) and two using -flto -fwhole-program (gfortranLTO and 
gfortranIMF). Additionally, for gfortranIMF and gfortranIMF, LD_PRELOAD 
is used to preload Intel's libimf library, which replaces some libm 
functions.

Result in a nutshell:
- On geometric average, all compilers do not differ much - for single 
benchmarks, the difference can be rather large. On geometric means 
maximal difference is 10%

- GCC (using LTO and GLIBC's libm), ifort and pathf95's performance is 
almost the same (< 1.5% difference) - pgi is about 6% slower.
- Using Intel's libimf, GCC is (on geometric mean) the fastest compiler 
(3% faster than ifort and pathf95), without it is the slowest of the 
three (by <1.5%)

The results are up at 
http://users.physik.fu-berlin.de/~tburnus/gcc-trunk/benchmark/iff/

Regarding the slow down of rnflow due to LTO, cf. PR
Regarding the gas_dyn slowness, cf. PR 31067

Thanks to all who made it possible!

Tobias

PS: I really miss a GCC vectorization library; I could have used in 
addition -mveclibabi=svml, but I have decided that there are already 
enough results for GCC.