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Re: [Qemu-devel] [PATCH v2 00/26] tcg: rework liveness analysis and regi
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH v2 00/26] tcg: rework liveness analysis and register allocator |
Date: |
Wed, 10 Oct 2012 09:49:53 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:15.0) Gecko/20120911 Thunderbird/15.0.1 |
Il 10/10/2012 09:42, Aurelien Jarno ha scritto:
> On Wed, Oct 10, 2012 at 08:59:43AM +0200, Paolo Bonzini wrote:
>> Il 09/10/2012 21:55, Aurelien Jarno ha scritto:
>>> This patch series rework the liveness analysis and register allocator
>>> in order to generate more optimized code, by avoiding a lot of move
>>> instructions. I have measured a 9% performance improvement in user mode
>>> and 4% in system mode.
>>>
>>> The idea behind this patch series is to free registers as soon as the
>>> temps are not used anymore instead of waiting for a basic block end or
>>> an op with side effects.
>>
>> Would it make any sense to express the saves as real TCG ops? This
>> would have a couple of advantages:
>
> It depends what you mean by that. Spills are decided more or less at the
> last moment (no free registers available, clobbered registers in a
> function call).
I'm not talking of spills; only saves of dead globals and local temps.
These can be computed before the optimizer runs, right?
> If it's about inserting them in the TCG stream, as it is done at the
> last step, ie after copy propagation and dead code elimination, it's not
> really useful anymore.
>
>> - more copy propagation and dead code elimination. Something like this:
>>
>> mov_i64 cc_dst,rax
>>
>> right now is compiled as follows:
>>
>> 0x5555557ac37a: mov %rbp,(%r14) # spill rax
>> 0x5555557ac381: mov (%r14),%rbp # load rax from memory
>> 0x5555557ac38f: mov %rbp,0x98(%r14) # spill cc_dst to memory
>
> I am surprised by this kind of code, and I think there's a bug somewhere
> in TCG. With the current TCG code, given rax is not dead, it should be
> spilled only after the move of cc_dst to memory, and thus second line is
> not supposed to be emitted. With this patch series applied the second
> line should simply be removed.
Note that the above was without your series.
>> - constant propagation using constraints. This would let tcg-i386 use
>> effectively the mov $imm,(addr) instruction for spills of known-constant
>> values.
>
> This is indeed something quite frustrating and even more when the
> same immediate value is loaded multiple time. One way to do that would
> be to provide an optional tcg_out_st_immediate().
Yes, that would be simple.
Paolo
- [Qemu-devel] [PATCH v2 05/26] tcg: rework liveness analysis, (continued)
- [Qemu-devel] [PATCH v2 05/26] tcg: rework liveness analysis, Aurelien Jarno, 2012/10/09
- [Qemu-devel] [PATCH v2 01/26] tcg: add temp_dead(), Aurelien Jarno, 2012/10/09
- [Qemu-devel] [PATCH v2 04/26] tcg: sync output arguments on liveness request, Aurelien Jarno, 2012/10/09
- [Qemu-devel] [PATCH v2 13/26] tcg: synchronize globals for ops with side effects, Aurelien Jarno, 2012/10/09
- [Qemu-devel] [PATCH v2 23/26] target-sh4: rename helper flags, Aurelien Jarno, 2012/10/09
- [Qemu-devel] [PATCH v2 09/26] tcg: start with local temps in TEMP_VAL_MEM state, Aurelien Jarno, 2012/10/09
- Re: [Qemu-devel] [PATCH v2 00/26] tcg: rework liveness analysis and register allocator, Paolo Bonzini, 2012/10/10
- Re: [Qemu-devel] [PATCH v2 00/26] tcg: rework liveness analysis and register allocator, Richard Henderson, 2012/10/10