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Date: -- (:)
From: Dmitry Bely <dbely@m...>
Subject: Re: [Caml-list] Timing Ocaml
Xavier Leroy <xavier.leroy@inria.fr> writes:

>> 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.)

Thank a lot for the explanation. But why then not to use inline asm for
MSVC, something like that:

#if defined(__GNUC__) && __GNUC__ >= 2
#define indirect_goto(addr) goto (addr)
#elif defined(_MSC_VER)
#define indirect_goto(addr) \
  { void* a = addr; __asm jmp dword ptr a; }
#endif

Hope to hear from you soon,
Dmitry


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