Re: Coyote Gulch test in Caml (was Re: [Caml-list] speed )

[Xavier Leroy <xavier.leroy@inria.fr>]

> http://raevnos.pennmush.org/code/almabench-ocaml.tar.gz
> It's pretty much a straight translation of the C++ version, and not very
> impressive speed-wise on my system compared to the C++ one.

Thanks a lot for the OCaml translation.  As you say, the speed of the
OCaml version is about 50% of that of the C++ version, both on Athlon
with g++, and on Alpha with the Tru64 cxx compiler.  This is both
reassuring and disappointing:

Reassuring, because our blanket performance statement "OCaml
delivers at least 50% of the performance of a decent C compiler" is
not invalidated :-)

Disappointing, because the assembly code generated by ocamlopt isn't
too ugly despite the code not being very Caml-ish in style.  In
particular, (almost) all float and ref boxing is correctly eliminated.
Given this, I was expecting maybe 75% of the performances of C++, not
50%.  Simple hand optimization (CSE, loop unrolling) doesn't affect
the speed significantly.  Apparently, the ocamlopt-generated code
offers less instruction-level parallelism than the g++-generated code
for the float computations.  Still, I haven't really understood where
the factor of 2 comes from.  

Thanks again for an interesting benchmark,

- Xavier Leroy
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