Re: [Qemu-devel] [PATCH 2/7] tcg: Optimize add2 + sub2

[Aurelien Jarno <aurelien@aurel32.net>]

On Thu, Sep 27, 2012 at 04:28:47PM -0700, Richard Henderson wrote:
> On 09/27/2012 04:20 PM, Aurelien Jarno wrote:
> > I understand that we can't easily insert an instruction, so the
> > limitation comes from here, but is it really something happening often?
> 
> It will certainly appear sometimes.  E.g. s390x has an add immediate
> instruction that does exactly: r1 += imm16 << 32.
> 
> Or did you mean specifically the full constant being folded?  That
> would happen quite a bit more often.  That you can see with most any
> 64-bit RISC guest when they attempt to generate a constant from 
> addition primitives instead of logical primitives.
> 
> For a 32-bit host, we've already decomposed logical primitives to 32-bit
> operations.  And we can constant-fold through all of those.  But when
> addition comes into play, we can't constant-fold through add2.
> 

I tried this patch on an i386 host running an x86_64 target, but it
even fails to start seabios, there is probably a wrong logic somewhere
in the patch.

For the first add2 that seemed to have work correctly, this patch
optimized 0.2% of them. I am not sure it worth it as is.

I think optimizing add2, and in general all *2 ops is a good idea, but
we should be able to do more agressive optimization. Maybe, a bit like
Blue was suggesting, add2 should always be followed by a nop, so we can
do more optimizations?

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
aurelien@aurel32.net                 http://www.aurel32.net