Re: [Qemu-devel] [PATCH 0/4] target-ppc: create TCG slots for registers based on CPU

[Nathan Froyd <froydnj@codesourcery.com>]

On Sat, Mar 28, 2009 at 11:54:43PM +0100, Aurelien Jarno wrote:
> On Sat, Mar 28, 2009 at 02:30:13PM -0700, Nathan Froyd wrote:
> > I am not a TCG expert, but there are several loops in TCG over all
> > globals and it seems like those loops would go faster if they didn't
> > have to consider registers that would never be touched.  If this patch
> > series makes no difference in TCG's performance, then I'd be glad to
> > have an explanation of why that's the case.
> 
> Do you actually have run a benchmark with those changes? TCG is
> sometimes a bit strange, and some optimizations does not change the
> execution speed, while others improve it a lot. It is very difficult to
> predict what will give a gain or not.
> 
> Suggestions of benchmarks: gzip/bzip2 on a big file using user emulation
> or a compilation in system emulation.

Benchmarking?  Pffft. ;)

A benchmarking session with qemu-ppc and bzip2/bunzip2 on ~400MB files
and a 603e emulated CPU suggests that these changes are not terribly
beneficial (maybe 1% improvement, if that).  I don't imagine that a
similarly stressful benchmark in system emulation would be much
different.  Consider the patch series withdrawn.

-Nathan