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

[Chandler Carruth]

This has been added in r218724.

Based on the feedback here and from Quentin, I'm going to email the list
shortly with a heads-up, and then flip the default over to the new shuffle
lowering.

On Mon, Sep 29, 2014 at 10:48 PM, Chandler Carruth <chandlerc at google.com>
wrote:

> Wow. Somehow, I forgot about vbroadcast and vpbroadcast. =[ Sorry about
> that. I'll fix those.
>
> On Fri, Sep 26, 2014 at 3:39 AM, Andrea Di Biagio <
> andrea.dibiagio at gmail.com> wrote:
>
>> Hi Chandler,
>>
>> Here is another test.
>>
>> When looking at the AVX codegen, I noticed that, when using the new
>> shuffle lowering, we no longer emit a single vbroadcastss in the case
>> where the shuffle performs a splat of a scalar float loaded from
>> memory.
>>
>> For example:
>> (with -mcpu=corei7-avx -x86-experimental-vector-shuffle-lowering)
>>    vmovss (%rdi), %xmm0
>>    vpermilps $0, %xmm0, %xmm0 # xmm0 = xmm0[0,0,0,0]
>>
>> Instead of:
>> (with -mcpu=corei7-avx)
>>   vbroadcastss (%rdi), %xmm0
>>
>> I have attached a small reproducible for it.
>>
>> Basically, the old shuffle lowering logic calls function
>> 'NormalizeVectorShuffle' to handle shuffles that perform a splat
>> operation.
>> On AVX, function 'NormalizeVectorShuffle' tries to lower a splat where
>> the splat value comes from a load into a X86ISD::VBROADCAST dag node.
>> Later on, during instruction selection, we emit a single avx_broadcast
>> for the load+splat sequence (basically, we end up folding the load in
>> the operand of the vbroadcastss).
>>
>> What happens is that the new shuffle lowering doesn't emit a
>> vbroadcast node in this case and eventually we end up selecting the
>> sequence of vmovss+vpermilps.
>>
>> I hope this helps.
>> Andrea
>>
>> On Tue, Sep 23, 2014 at 10:53 PM, Chandler Carruth <chandlerc at google.com>
>> wrote:
>> >
>> > On Tue, Sep 23, 2014 at 2:35 PM, Simon Pilgrim <llvm-dev at redking.me.uk>
>> > wrote:
>> >>
>> >> If you don’t want to spend time on this, I’d be happy to create a
>> >> candidate patch for review? I’ve been unclear if you were taking
>> patches for
>> >> your shuffle work prior to it becoming the default.
>> >
>> >
>> > While I'm happy to work on it, I'm even more happy to have patches. =D
>> >
>> > _______________________________________________
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>> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
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>> >
>>
>
>
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