On Thu, Aug 12, 2010 at 09:51:42AM +0200, Steven Bosscher wrote: > On Thu, Aug 12, 2010 at 8:46 AM, Ralf W. Grosse-Kunstleve > <r...@yahoo.com> wrote: > > Hi Vladimir, > > > > Thanks for the feedback! Very interesting. > > > > > >> Intel optimization compiler team (besides researchers) is much bigger than > >>whole GCC community. > > > > That's a surprise to me. I have to say that the GCC community has done > > amazing > > work, as you came within factor 1.4 (gfortran) and 1.6 (g++ compiling > > converted > > code) > > of ifort performance, which is close enough for our purposes, and I think > > those > > of many people. > > Well, I think a ratio of gfortran/ifort=1.4 isn't so great, really. If > you look at one of the popular Fortran benchmarks (Polyhedron, > http://www.polyhedron.com/pb05-linux-f90bench_p40html), the ratio was > less than 1.2 for gfortran 4.3 vs. ifort 11 on an Intel iCore7. > > Can you tell how you obtained the performance numbers you are using? > There may be a few compiler flags you could add to reduce that ratio > of 1.4 to something better. >
Without knowing the compiler options, the results of any benchmark are meaningless. For various versions of gfortran, I find the following average of 5 executions in seconds: # A B C D E F G H # gfc43 9.808 9.374 9.314 9.832 9.620 9.526 9.022 9.156 # gfc44 9.806 9.440 9.222 9.810 9.414 9.320 8.980 9.152 # gfc45 9.672 9.530 9.250 9.744 9.400 9.204 8.960 8.992 # gfc4x 9.814 9.358 8.622 9.810 Note1 9.172 8.958 9.022 # # A = -march=native -O # B = -march=native -O2 # C = -march=native -O3 # D = -march=native -O -ffast-math # E = -march=native -O2 -ffast-math # F = -march=native -O -funroll-loops # G = -march=native -O2 -funroll-loops # H = -march=native -O3 -funroll-loops # # Note 1: STOP DLAMC1 failure (10) # # gfc43 --> 4.3.6 20100728 (prerelease) # gfc44 --> 4.4.5 20100728 (prerelease) # gfc45 --> 4.5.1 20100728 (prerelease) # gfc4x --> 4.6.0 20100810 (experimental) # I'll note that G is my normal FFLAGS setting along with -ftree-vectorize. Column D and E above highlights why I consider -ffast-math to be an evil option. For this benchmark the math is neither fast nor is it particularly safe with gfc4x. -- Steve
