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

[Chandler Carruth]

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
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20140929/cec7af6d/attachment.html>