Re: [Qemu-devel] [PATCH v2 0/6] ARM: AREG0 conversion

[Blue Swirl]

On Tue, Mar 27, 2012 at 13:40, Laurent Desnogues
<address@hidden> wrote:
> On Mon, Mar 26, 2012 at 7:02 PM, Blue Swirl <address@hidden> wrote:
> [...]
>> At least stack protector is protecting more code than before (for
>> example TLB miss handler), but could overhead from that amount to 5%?
>>
>> Otherwise there should be just a few extra register moves here and
>> there, that should be cheap on modern processors.
>
> The extra moves might be cheap but their cost is obviously not 0:
> on top of using extra CPU core resources, code size is increased
> which results in more instruction cache misses.
>
> I didn't like the idea when we discussed it back in May, now it
> looks like we have concrete evidence the speed impact is
> measurable (though I'd like some more numbers than the rough
> 5% estimate I gave).

A clearly defined test case running on a host that does not adjust
clock frequencies would be nice. It would be interesting to find out
where exactly the slowdown comes from.

Perhaps the access helpers ({helper,_}_{ld,st}{b,w,l}_mmu) generated
by softmmu_template.h are the culprit. If so, they could be split from
other code and moved to TCG back ends. That way the interface could be
improved while keeping all other cleanups.