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

[Artyom Tarasenko]

Hi Aurelien,

On Fri, Jul 31, 2015 at 5:43 PM, Aurelien Jarno <address@hidden> 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) ?

Artyom

-- 
Regards,
Artyom Tarasenko

SPARC and PPC PReP under qemu blog: http://tyom.blogspot.com/search/label/qemu