Re: [Qemu-devel] [PATCH v5 11/19] target/i386: optimize cross-page direct jumps in softmmu

[Richard Henderson]

On 04/28/2017 06:56 PM, Alex Bennée wrote:
> Richard Henderson <address@hidden> writes:
>
> > From: "Emilio G. Cota" <address@hidden>
> >
> > Instead of unconditionally exiting to the exec loop, use the
> > gen_jr helper to jump to the target if it is valid.
> >
> > Perf impact: see next commit's log.
> >
> > Reviewed-by: Richard Henderson <address@hidden>
> > Signed-off-by: Emilio G. Cota <address@hidden>
> > Message-Id: <address@hidden>
> > Signed-off-by: Richard Henderson <address@hidden>
> > ---
> >   target/i386/translate.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/target/i386/translate.c b/target/i386/translate.c
> > index f0e48dc..ea113fe 100644
> > --- a/target/i386/translate.c
> > +++ b/target/i386/translate.c
> > @@ -2154,9 +2154,9 @@ static inline void gen_goto_tb(DisasContext *s, int 
> > tb_num, target_ulong eip)
> >           gen_jmp_im(eip);
> >           tcg_gen_exit_tb((uintptr_t)s->tb + tb_num);
> >       } else {
> > -        /* jump to another page: currently not optimized */
> > +        /* jump to another page */
> >           gen_jmp_im(eip);
> > -        gen_eob(s);
> > +        gen_jr(s, cpu_tmp0);
>
>
> I had to look up what was going on with cpu_tmp0 there. Is there a
> particular reason i386 has these global temps with implied setting
> rules? It does seem somewhat hacky.

It's mostly hysterical raisins, and that no one has rewritten it yet.

> Given cmp_tmp0 seems to be a heavily used across i386 I guess it keeps
> to the style of the translator :-/

Yeah.  :-/


r~