On Mon, 16 Feb 2004, Toon Moene wrote: > Jean-Marc Petit wrote: > > > I just noticed a strong decrease of efficiency in code produced by G77 > > when moving from the RedHat 7.0 version (can't recall the version, it's no > > longer on my computer) to RedHat 9.0. > > > Hardware: Athlon 1.0GHz, 512 MB RAM > > > > RedHat 7.0: > > > > compiler command: g77 -u -O3 -ffastmath -funroll-loops -m486 -o bench bench.f > > > > result of the "time" command: > > > > real 0m37.415s > > user 0m36.890s > > sys 0m0.000s > > but the following is slower: > > > RedHat 9.0: > > > > g77 -v gives: gcc version 3.2.2 20030222 (Red Hat Linux 3.2.2-5) > > > > compiler command: g77 -u -O3 -fno-automatic -funroll-loops -malign-double > > -march=athlon -ffast-math -o bench bench.f > > Note that one of the differences is the -fno-automatic compile time > option in the latter example. This might make a large difference, as it > instructs the compiler to treat all variables (even loop counts) to be > memory resident, instead of being able to live in registers. > > Why do you need -fno-automatic with gcc-3.2.2 but not with the RedHat > 7.0 compiler ? > Well, I tried wihtou the "-fno-automatic" option. It's marginally worth: real 1m4.492s user 1m4.390s sys 0m0.000s The reason I have this option is that I don't want to loose time re-creating the variables each time the program enters a subroutine. In the present case, I don't really need that, and the result is essentially the same. In some cases, I need to have it for the program to work. This replace having a SAVE statement in each and every subroutine. For example, I need this to compile and link with the PGPLOT library. Anyway, it seems this doesn't solve my problem. If you think it's worth it, I can send along my progam. Reagrds, Jean-Marc Petit
