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

[Andrea Di Biagio]

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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