Re: [Qemu-devel] Debian 7.8.0 SPARC64 on qemu - anything i can do to speedup the emulation?

[Aurelien Jarno <aurelien@aurel32.net>]

On 2015-08-03 10:31, Artyom Tarasenko wrote:
> Hi Aurelien,
> 
> On Fri, Jul 31, 2015 at 5:43 PM, Aurelien Jarno <aurelien@aurel32.net> wrote:
> 
> >> > It uses a lot of integer functions
> >> > based on CPU flags, so most of the time is spent computing them in
> >> > helper_compute_psr.
> >>
> >> I wonder if this can be optimized. I guess most RISC CPUs would have a
> >> similar problem. Unlike x86, the compilers usually optimize
> >> instructions on flag usage. If there is an instruction modifying flags
> >> in a code, the flags will be used for sure, so it probably makes a
> >> little sense to pospone the flag computation?
> >
> > Indeed. ARM and SH4 use one TCG temp per flag, and they can be computed
> > one by one using setcond. The optimizer and the liveness analysis then
> > get rid of the unused computation. However while it allows intra-TB
> > optimization, it prevent any other flags optimization. Therefore the
> > only way to know if it is a good idea or not is to implement it and
> > benchmark that, but using a bit more than a single biased benchmark like
> > the one from sysbench.
> >
> > Also note that the current implementation predates the introduction of
> > setcond, which is necessary to be able to compute the flags using TCG
> > code.
> 
> Thanks for explaining it, the problem is much more clear now.
> Moving to setcond is definitely worth a shot. I'd like to play with it.
> What would be the minimal entity to change without reworking the complete TCG:
>  a) one flag for one instruction,
>  b) all flags for one instruction,
>  c) one flag for all instructions,
> or d) all flags for all instructions (gradually moving to setcond is
> not possible) ?

You should with the c) option. You can look at how I done this for SH4,
starting with commit 5ed9a259c164bb9fd2a6fe8a363a4bda2e4a5461.

Aurelien

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
aurelien@aurel32.net                 http://www.aurel32.net