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

["Jim C. Brown" <jma5@umd.edu>]

On Thu, Nov 10, 2005 at 01:33:55AM +0000, Julian Seward wrote:
> 
> The use of gcc to generate the back end in QEMU's early days was a 
> clever way to get the project up and running quickly.  But surely
> now it would be better to transition to a handwritten backend, so
> as to be independent future changes in gcc, and generally more robust?
> 
> J
> 

Yes, Paul Brook is working on it.

> On Wednesday 09 November 2005 19:51, Igor Kovalenko wrote:
> > 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 :)
> 
> 
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