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

[Andrea Di Biagio]

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.