[LLVMdev] Please benchmark new x86 vector shuffle lowering, planning to make it the default very soon!

[Quentin Colombet]

On Sep 9, 2014, at 3:35 PM, Chandler Carruth <chandlerc at gmail.com> wrote:

> First off, thanks for the *fantastic* testing and investigation. =]
> 
> On Tue, Sep 9, 2014 at 3:01 PM, Quentin Colombet <qcolombet at apple.com> wrote:
> Hi Chandler,
> 
> Here is a test case for the biggest offender (oourafft.c).
> To reproduce:
> llc -mcpu=core-avx-i -x86-experimental-vector-shuffle-lowering=true repro.ll
> llc -mcpu=core-avx-i -x86-experimental-vector-shuffle-lowering=false repro.ll
> 
> The main problem is that we miss:
> 	vmovsd	(%rdi,%rcx,8), %xmm2
> 	vmovlhps	%xmm2, %xmm2, %xmm2 ## xmm2 = xmm2[0,0]
> =>
> 	vmovddup	(%rdi,%rcx,8), %xmm2
> 
> I do not know how problematic is that (it seems we catch up on the performance with just the previous transformation),
> 
> Actually, this is awesome, because this was also the main problem I saw. I already wrote the fix, and just need to fix up test case fixes and submit it. =]

Great!

> 
> I think blendps is the other big missing piece as mentioned.
>  
> but we also miss:
> 	vsubpd	%xmm1, %xmm0, %xmm2
> 	vaddpd	%xmm1, %xmm0, %xmm0
> 	vshufpd	$2, %xmm0, %xmm2, %xmm0 ## xmm0 = xmm2[0],xmm0[1]
> =>
> 	vaddsubpd	%xmm1, %xmm0, %xmm0
> 
> I’ll look into the other regressions.
> 
> Maybe wait until i can land the duplicate move support and the blendps support? I'd rather see what the results are after that.
> 
> There is also some AVX specific stuff that I've left FIXMEs fore that I could probably address to pull it up a bit.
> 
> FWIW, I've got the main test-suite reproducing your results for x86, but I don't currently have a nice reproduction for SPEC, so digging into those would help somewhat more.

Understood.

I’ll wait for the patches to support the duplicate move before doing that :).

Shot me an email when they land, just to be sure ;).

Thanks,
-Quentin
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