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From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] patch for qemu with newer gcc-3.4.x (support repz
	retq optimization for amd processors correctly)
Date: Wed, 9 Nov 2005 19:45:24 +0000
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To: qemu-devel@nongnu.org
Cc: Igor Kovalenko <garrison@mail.ru>

> Notice the 'repz mov' sequence, which seems to be undocumented
> instruction. It seems to work somehow but chokes valgrind decoder.
> The following patch (against current CVS) fixes this problem,

This patch is incorrect.

It could match any number of other instructions that happen to end in 0xf3. eg

   0:   c7 45 00 00 00 00 f3    movl   $0xf3000000,0x0(%ebp)
   7:   c3                      ret

IIRC the "rep; ret" sequence is to avoid a pipeline stall on Athlon CPUs.  Try 
tuning for a different CPU.

Paul

> Index: dyngen.c
> ===================================================================
> RCS file: /cvsroot/qemu/qemu/dyngen.c,v
> retrieving revision 1.40
> diff -u -r1.40 dyngen.c
> --- dyngen.c    27 Apr 2005 19:55:58 -0000      1.40
> +++ dyngen.c    9 Nov 2005 19:12:38 -0000
> @@ -1387,6 +1387,12 @@
>               error("empty code for %s", name);
>           if (p_end[-1] == 0xc3) {
>               len--;
> +            /* This can be 'rep ; ret' optimized return sequence,
> +             * need to check further and strip the 'rep' prefix
> +             */
> +            if (len != 0 && p_end[-2] == 0xf3) {
> +                len--;
> +            }
>           } else {
>               error("ret or jmp expected at the end of %s", name);
>           }