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Date: -- (:)
From: Blair Zajac <blair@o...>
Subject: Re: [Caml-list] Timing Ocaml
Xavier Leroy wrote:
> 
> > 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.

Thanks for the info.

And do you recommend a particular program or set of programs to run
to get a general relative performance number for each compiler, or
does it really matter?

Blair

-- 
Blair Zajac <blair@orcaware.com>
Web and OS performance plots - http://www.orcaware.com/orca/
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