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Re: [Qemu-ppc] [PATCH 2/2] tcg-ppc: use new return-argument ld/st helper

From: Paolo Bonzini
Subject: Re: [Qemu-ppc] [PATCH 2/2] tcg-ppc: use new return-argument ld/st helpers
Date: Thu, 05 Sep 2013 17:41:32 +0200
User-agent: 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
> http://lists.nongnu.org/archive/html/qemu-devel/2013-09/msg00025.html
> 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
separate patch.


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