Performance loss from RedHat 7.0 to RedHat 9.0

[Toon Moene]

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 ?

-- 
Toon Moene - mailto:toon@moene.indiv.nluug.nl - phoneto: +31 346 214290
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