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Date: -- (:)
From: Dmitry Bely <dbely@m...>
Subject: Re: [Caml-list] Timing Ocaml
Chris Hecker <checker@d6.com> writes:

>>* The GNU C compiler gcc is recommended, as the bytecode
>>   interpreter takes advantage of gcc-specific features to enhance
>>   performance.
>>What is the nature of these optimizations?
>
> GCC lets you take the address of a label.  You can see in
> byterun/interp.c that it uses a jump table instead of a switch when
> you're using GCC.
>
> At least, that's what it looks like.

I would rather say that gcc allows to force register allocation for some
specific variable, while MSVC always ignore "register" specifier.

#if defined(__GNUC__) && !defined(DEBUG)
[...]
#ifdef __i386__
#define PC_REG asm("%esi")
#define SP_REG asm("%edi")
#define ACCU_REG
#endif
[...]
#endif

/* The interpreter itself */

value interprete(code_t prog, asize_t prog_size)
{
#ifdef PC_REG
  register code_t pc PC_REG;
  register value * sp SP_REG;
  register value accu ACCU_REG;
#else
  register code_t pc;
  register value * sp;
  register value accu;
#endif

In the same time MSVC has very good optimizer and it is very strange, that
two explicit register variables lead to 30% performance gain...

Hope to hear from you soon,
Dmitry


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