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Re: [Qemu-ppc] [PATCH 2/2] tcg-ppc: use new return-argument ld/st helper
Re: [Qemu-ppc] [PATCH 2/2] tcg-ppc: use new return-argument ld/st helpers
Thu, 05 Sep 2013 17:41:32 +0200
Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130805 Thunderbird/17.0.8
Il 05/09/2013 17:17, Richard Henderson ha scritto:
> On 09/05/2013 01:22 AM, Paolo Bonzini wrote:
>> These use a 32-bit load-of-immediate to save a mflr+addi+mtlr sequence.
>> Tested with a Windows 98 guest (pretty much the most recent thing I
>> could run on my PPC machine) and kvm-unit-tests's sieve.flat. The
>> speed up for sieve.flat is as high as 10% for qemu-system-i386, 25%
>> (no kidding) for qemu-system-x86_64 on my PowerBook G4.
> See also the series beginning at
> The major difference is that I use a conditional call out of the fast
> path, which lets me later just use one mflr to pass the parameter. I
> also, perhaps foolishly, got rid of the trampolines. E.g.
> 0xf57a1838: rlwinm r3,r15,24,20,27
> 0xf57a183c: rlwinm r0,r15,0,30,19
> 0xf57a1840: add r3,r3,r27
> 0xf57a1844: lwz r4,6436(r3)
> 0xf57a1848: cmpw cr7,r0,r4
> 0xf57a184c: lwz r3,6444(r3)
> 0xf57a1850: bnel- cr7,0xf57a1910
> 0xf57a1854: stwx r16,r3,r15
> 0xf57a1910: mr r3,r27
> 0xf57a1914: mr r4,r15
> 0xf57a1918: mr r5,r16
> 0xf57a191c: li r6,1
> 0xf57a1920: mflr r7
> 0xf57a1924: lis r0,4120
> 0xf57a1928: ori r0,r0,45040
> 0xf57a192c: mtctr r0
> 0xf57a1930: bctrl
> 0xf57a1934: b 0xf57a1858
> I don't see anything technically wrong with your patch. But I'd be
> interested to compare vs mine.
Sure, I'll give it a try tomorrow or in the weekend.
The G4 in my computer must simply hate the mflr/add/mtlr sequence in the
trampoline; there's no other explanation for such a huge performance
improvement. So even though I suspect that there won't be much
difference between our patches it's good to check what's better in case
your sequences are triggering something as bad. The bnel/mflr is a nice
trick to save one instruction, though!
Regarding removal of the trampolines, the extra icache cost should be a
wash now that they are half the size, but I'd still prefer it to be a