Re: [Caml-list] Timing Ocaml

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

> Reading that the bytecode interpreter for Ocaml runs 2/3 as fast
> when compiled with VC 6 compared to gcc, has anybody done any
> timing comparisons with VisualStudio.Net, Intel C++ 5.x or
> Intel C++ 6.0?

As others mentioned, the reason why gcc does a better job on the Caml
bytecode interpreter is not that gcc generates better code all by
itself (it doesn't), but that it supports "computed gotos" as a C
language extension.  The bytecode interpreter takes advantage of this
feature by replacing opcodes with the addresses of the code fragments that
execute them, saving a significant amount of time in the bytecode
interpretation loop.

Microsoft's C compilers don't support this extension, and I doubt
Intel's compilers do, at least under Windows.  (Although I seem to
remember that Intel's compiler for Linux implements gcc extensions.)

Someone else mentioned the explicit register declarations in the
bytecode interpreter.  This is another gcc-specific extension, but
actually the bytecode interpreter uses them to work around the poor
register allocation performed by gcc (it fails to guess correctly
which local variables of the bytecode interpreter are most critical
and should end up in registers).  So, it's really a gcc feature used
to work around a gcc deficiency :-)  Other C compilers might actually
get the registers right by themselves.

- Xavier Leroy
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