Re: [Qemu-devel] [PATCH 09/10] target/i386: optimize indirect branches with TCG's jr op

["Emilio G. Cota" <cota@braap.org>]

On Wed, Apr 12, 2017 at 11:43:45 +0800, Paolo Bonzini wrote:
> 
> 
> On 12/04/2017 09:17, Emilio G. Cota wrote:
> > 
> > The fact that NBench is not very sensitive to changes here is a
> > little surprising, especially given the significant improvements for
> > ARM shown in the previous commit. I wonder whether the compiler is doing
> > a better job compiling the x86_64 version (I'm using gcc 5.4.0), or I'm simply
> > missing some i386 instructions to which the jr optimization should
> > be applied.
> 
> Maybe it is "ret"?  That would be a straightforward "bx lr" on ARM, but
> it is missing in your i386 patch.

Yes I missed that. I added this fix-up:

diff --git a/target/i386/translate.c b/target/i386/translate.c
index aab5c13..f2b5a0f 100644
--- a/target/i386/translate.c
+++ b/target/i386/translate.c
@@ -6430,7 +6430,7 @@ static target_ulong disas_insn(CPUX86State *env, DisasContext *s,
         /* Note that gen_pop_T0 uses a zero-extending load.  */
         gen_op_jmp_v(cpu_T0);
         gen_bnd_jmp(s);
-        gen_eob(s);
+        gen_jr(s, cpu_T0);
         break;
     case 0xc3: /* ret */
         ot = gen_pop_T0(s);
@@ -6438,7 +6438,7 @@ static target_ulong disas_insn(CPUX86State *env, DisasContext *s,
         /* Note that gen_pop_T0 uses a zero-extending load.  */
         gen_op_jmp_v(cpu_T0);
         gen_bnd_jmp(s);
-        gen_eob(s);
+        gen_jr(s, cpu_T0);
         break;
     case 0xca: /* lret im */
         val = cpu_ldsw_code(env, s->pc);

Any other instructions I should look into? Perhaps lret/lret im?

Anyway, nbench does not improve much with the above. The reason seems to be
that it's full of direct jumps (visible with -d in_asm). Also tried softmmu
to see whether these jumps are in-page or not: peak improvement is ~8%, so
I guess most of them are in-page. See http://imgur.com/EKRrYUz

I'm running new tests on a server with no other users and which has
frequency scaling disabled. This should help get less noisy numbers,
since I'm having trouble replicating my own results :> (I used my desktop
machine until now). Will post these numbers tomorrow (running overnight
SPECint both train and set sizes).

Thanks,

		Emilio