[LLVMdev] RFB: Would like to flip the vector shuffle legality flag (original) (raw)

Quentin Colombet qcolombet at apple.com
Tue Jan 27 12:58:13 PST 2015

Hi Chandler,

I ran the new flag through some of our internal benchmarks and overall this is neutral, so slightly better. I observed a couple of improvements, but also a couple of regressions. I am working on tracking those down to produce reduced test cases. No sure I would have time to do that by the end of this week though.

Cheers, -Quentin

On Jan 25, 2015, at 2:15 PM, Sanjay Patel <spatel at rotateright.com> wrote:

I ran the benchmarking subset of test-suite on a btver2 machine and optimizing for btver2 (so enabling AVX codegen).

I don't see anything outside of the noise with x86-experimental-vector-shuffle-legality=1. On Fri, Jan 23, 2015 at 5:19 AM, Andrea Di Biagio <andrea.dibiagio at gmail.com> wrote: 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.

-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150127/9f7fc94a/attachment.html>

More information about the llvm-dev mailing list