Re: [Qemu-devel] [PATCH 5/7] tcg-i386: Implement deposit operation.

["Edgar E. Iglesias" <edgar.iglesias@gmail.com>]

On Tue, Jan 25, 2011 at 08:13:53AM -0800, Richard Henderson wrote:
> On 01/25/2011 04:27 AM, Edgar E. Iglesias wrote:
> > I've tested this patch a bit and got mixed results. I tested with patched
> > CRIS and MicroBlaze translators. The patch works OK (it doesn't break
> > anything) for the usecases I had but I saw a bit of a slowdown with
> > MicroBlaze (compare to not using deposit at all).
> > 
> > I suspect that the fast 8 and 16 bit x86 deposits are giving me a slight
> > speedup with CRIS. But MicroBlaze uses one bit fields into bit 2 and
> > 31. Those seem to be slower with deposit than with other tcg sequences.
> > 
> > I would have guessed that at worst, this patch would be equally fast
> > as any TCG sequence. Am I missing something?
> 
> With or without the i386 tcg-target.c changes?
> 
> If without, then I'm stumped, since it looks like identical tcg ops
> being emitted.

It's with the tcg-target patch.

> It with, then perhaps SHLD is slower than I thought.  I see that GCC
> lists this insn as "vector decoded" for AMD cores, as opposed to
> "direct decoded".  If this insn is indeed microcoded on some hosts
> then maybe the i386 tcg-target patch isn't such a great idea.

OK, I see. Maybe we should try to emit an insn sequence more similar
to what tcg was emitting (for the non 8 & 16-bit deposits)?
That ought too at least give similar results as before for those and
give us a speedup for the byte and word moves.

> That said, there are still other tcg targets which support this 
> operation directly.  I would be shocked if you measured a slowdown
> with these changes on a ppc host, for instance.

Yep, agreed.

Cheers