Re: [Qemu-devel] [PATCH RFC 00/11] AREG0 elimination

[Aurelien Jarno <aurelien@aurel32.net>]

On Sat, May 14, 2011 at 10:35:20PM +0300, Blue Swirl wrote:
> Here's a RFC series for eliminating AREG0.
> 
> Blue Swirl (11):
>   Move user emulator stuff from cpu-exec.c to user-exec.c
>   Delete unused tb_invalidate_page_range
> 
> The above should be OK to commit.
> 
>   cpu_loop_exit: avoid using AREG0
>   Delegate setup of TCG temporaries to targets
> 
> These two are not, unless the overall plan is OK.
> 
>   TCG: fix negative frame offset calculations
>   TCG/x86: use stack for TCG temps
>   TCG/Sparc64: use stack for TCG temps
> 
> But these three should be OK. I've tested lightly x86_64 and Sparc64 hosts.
> 
>   Add CONFIG_TARGET_NEEDS_AREG0
>   Don't compile legacy qemu_ld/st functions if target doesn't need them
> 
> Should be OK, though the latter patch only touches x86.
> 
>   Add new qemu_ld and qemu_st functions
>   sparc: use new qemu_ld and qemu_st functions
> 
> The last two compile but QEMU segfaults. I just made a naive
> conversion for getting comments.
> 

What is the goal behing removing TCG_AREG0? If it is speed improvement,
can you please provide some benchmarks?

The env register is used very often (basically for every load/store, but
also a lot of helpers), so it makes sense to reserve a register for it.

For what I understand from your patch series, you prefer to pass this
register explicitly to TCG functions. This basically means this TCG
global will be loaded to host register as soon as it is used, but also
regularly, as globals are saved back to their canonical location before
an helper or a load/store.

So it seems that this patch series will just allowing the "env register"
to change over time, though it will not spare one more register for the 
TCG code, and it will emit longer TCG code to regularly reload the env
global into a host register.

In any case at then end benchmarks is what are need to decided, TCG has
always shown that performance improvements doesn't match the improvement
analysis.

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