[Haskell-cafe] -O2 compile option can give speed increase over -O. Fasta shootout program test runs.

Donald Bruce Stewart dons at cse.unsw.edu.au
Mon Jul 16 21:24:01 EDT 2007

> I have been playing with the Fasta program in the shootout to see if
> I can make it umm faster. Starting from dons program on this page and
> adding some timing calculations as suggested on this wiki page
> http://shootout.alioth.debian.org/gp4/benchmark.php?test=fasta&lang=ghc&id=2
> http://www.haskell.org/haskellwiki/Timing_computations
> I added different OPTIONS into the top line of the program did a
> ghc --make fasta.hs   and ran it each time with  fasta 2500000
> (This is one tenth of the shootout figure.) These runs all keep the
> existing OPTIONS of  -fbang-patterns -fexcess-precision
>   Seconds   OPTIONS Added
>   -------   -------------
>    40.5
>    40.5        -funbox-strict-fields
>    40.4                      {-# INLINE rand #-}
>    17.2        -O
>    17.0        -O  -fvia-C
>    14.4        -O  -optc-march=pentium4
>    11.5        -O2
>    11.2        -O3
>    11.5        -O3           {-# INLINE rand #-}
>    11.3        -O2 -optc-march=pentium4
> There was a bit of variation, I've averaged over two runs. This is on
> an Intel Pentium D 2.66GHz running W2K and GHC 6.6.1.
> It seems the -O2 option can give a significant speed increase relative
> to just the -O option. This is contrary to the documentation which says
> http://www.haskell.org/ghc/docs/latest/html/users_guide/options-optimise.html
> http://www.haskell.org/ghc/docs/latest/html/users_guide/faster.html
> it won't make any difference. I guess it's program, architecture and
> operating system specific, but according to these figures the -O2 option
> seems well worth a try for programs that need speed. It may be that
> we sacrifice bounds checking or something important with -O2, I don't
> know.

Yes, -O2 is getting better, as new optimisations like SpecConstr are
enabled by it. For shootout problems, I'd selectively test with -O2, and
if it is better, use that.

Good work! And yes, I see that it is currently compiled with:

    -O fbang-patterns -fexcess-precision  -fglasgow-exts  -optc-march=pentium4

if -O2 is consistently better here, then we could happily switch.

-- Don

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