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

[Andrea Di Biagio]

On Wed, Oct 1, 2014 at 1:52 AM, Chandler Carruth <chandlerc at google.com> wrote:
> This has been added in r218724.
Thanks Chandler!

> 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.

Nice.
Again, thanks for working on this!

-Andrea

>
> 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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>>> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>> >
>>
>>
>