Re: [Qemu-devel] patch for qemu with newer gcc-3.4.x (support repz retq optimization for amd processors correctly)

[Igor Kovalenko]

Paul Brook wrote:
> > 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);
> >           }

OK I missed that...
Then a discussion about gcc-4 turns into something much more interesting :)

--
Kind regards,
Igor V. Kovalenko