[LLVMdev] RFB: Would like to flip the vector shuffle legality flag

[Kuperstein, Michael M]

Thanks for the heads up, Chandler!
I'll also try to get some numbers locally.

Michael

-----Original Message-----
From: Andrea Di Biagio [mailto:andrea.dibiagio at gmail.com] 
Sent: Friday, January 23, 2015 14:19
To: Chandler Carruth
Cc: Simon Pilgrim; qcolombet at apple.com; Sanjay Patel; LLVM Developers Mailing List; Kuperstein, Michael M
Subject: Re: RFB: Would like to flip the vector shuffle legality flag

Hi Chandler,

On Fri, Jan 23, 2015 at 8:15 AM, Chandler Carruth <chandlerc at gmail.com> wrote:
> Greetings LLVM hackers and x86 vector shufflers!
>
> I would like to flip on another chunk of the new vector shuffling, 
> specifically the logic to mark ~all shuffles as "legal".
>
> This can be tested today with the flag 
> "-x86-experimental-vector-shuffle-legality". I would essentially like 
> to make this the default (by removing the "false" path). Doing this 
> will allow me to completely delete the old vector shuffle lowering.
>
> I've got the patches prepped and ready to go, but it will likely have 
> a significant impact on performance. Notably, a bunch of the remaining 
> domain crossing bugs I'm seeing are due to this. The key thing to 
> realize is that vector shuffle combining is *much* more powerful when 
> we think all of these are legal, and so we combine away bad shuffles 
> that would trigger domain crosses.

That's good news!
Also, I really like your idea of making all shuffles legal by default.
I remember I did some experiments disabling the checks for legal shuffles in the DAGCombiner to see how well the new shuffle lowering coped with 'overly' aggressive shuffle combining. I was surprised to see that from eyeballing the generated code it looked much cleaner (although I didn't test it extensively). Our target is btver2, so I also didn't look at what could have been codegen for targets with no
AVX/SSE4.1 where there might be fewer opportunities to match a shuffle with a single target instruction during legalization.

>
> All of my benchmarks have come back performance neutral overall with a 
> few benchmarks improving. However, there may be some regressions that 
> folks want to track down first. I'd really like to get those reported 
> and prioritize among the vector shuffle work so we can nuke several 
> *thousand* lines of code from X86ISelLowering.cpp. =D

I'll see if I can get some numbers from our internal codebase and help with reporting potential regressions.

Thanks,
Andrea

>
> Thanks!
> -Chandler
>
>
> PS: If you're feeling adventurous, the next big mode flip flag I want 
> to see changed is -x86-experimental-vector-widening-legalization, but 
> this is a much more deep change to the entire vector legalization 
> strategy, so I want to do it second and separately.
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