Re: [Qemu-devel] [PATCH 0/7] tcg: conditional set and move opcodes

[Richard Henderson]

On 12/18/2009 07:40 AM, malc wrote:
> After fixing a bug (crop was done after reading the cr) i run some
> openssl speed benchmarks, and, at least here on an MPC7447A, got a
> speed degradation, tiny but consistent.

Well, you could try rendering the setcond with branches instead of 
logical operations.  You'll still gain the benefit of not having ended 
the TCG basic block, and forced the stores of globals to their slots etc 
etc.

> IN:
> 0x40082295:  movzbl (%eax),%eax
> 0x40082298:  cmp    $0x3d,%al
> 0x4008229a:  setne  %dl
> 0x4008229d:  test   %al,%al
> 0x4008229f:  je     0x400822d2
>
> OP after liveness analysis:
>   mov_i32 tmp2,eax
>   qemu_ld8u tmp0,tmp2,$0xffffffff
>   mov_i32 eax,tmp0
>   movi_i32 tmp1,$0x3d
>   mov_i32 tmp0,eax
>   nopn $0x2,$0x2
>   sub_i32 cc_dst,tmp0,tmp1
>   movi_i32 tmp13,$0xff
>   and_i32 tmp4,cc_dst,tmp13
>   movi_i32 tmp13,$0x0
>   setcond_i32 tmp0,tmp4,tmp13,ne
>   movi_i32 tmp14,$0xff
>   and_i32 tmp13,tmp0,tmp14
>
> ....
>
> OUT: [size=204]
> 0x601051b0:  lwz     r14,0(r27)
> 0x601051b4:  lbzx    r14,0,r14
> 0x601051b8:  mr      r15,r14
> 0x601051bc:  addi    r15,r15,-61
> 0x601051c0:  andi.   r15,r15,255
> 0x601051c4:  cmpwi   cr6,r15,0
> 0x601051c8:  crnot   4*cr7+eq,4*cr6+eq
> 0x601051cc:  mfcr    r0
> 0x601051d0:  rlwinm  r15,r0,31,31,31
> 0x601051d4:  andi.   r15,r15,255
>
> ...
>
> So the fact that setcond produces 0/1 was never communicated to the
> tcg, not that i would claim that it's possible at all...

It isn't.

And anyway, if you look at the opcodes generated without the setcond 
patch you'll see that and 255 in there as well.  Some more surgery on 
the i386 translator could probably get rid of that.  All I replaced were 
sequences of

  brcond c1,c2,$lab_true
  movi dest,0
  br $lab_over
  movi dest,1



r~