[PATCH] Teach the DAGCombiner how to fold a OR of two shufflevector into a single shufflevector node

[Hal Finkel]

----- Original Message -----
> From: "Nadav Rotem" <nrotem at apple.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "llvm-commits at cs.uiuc.edu for LLVM" <llvm-commits at cs.uiuc.edu>, "Andrea Di Biagio" <andrea.dibiagio at gmail.com>
> Sent: Wednesday, March 5, 2014 11:59:27 AM
> Subject: Re: [PATCH] Teach the DAGCombiner how to fold a OR of two shufflevector into a single shufflevector node
> 
> 
> 
> 
> On Mar 5, 2014, at 9:55 AM, Hal Finkel < hfinkel at anl.gov > wrote:
> 
> 
> 
> ----- Original Message -----
> 
> 
> From: "Nadav Rotem" < nrotem at apple.com >
> To: "Andrea Di Biagio" < andrea.dibiagio at gmail.com >
> Cc: " llvm-commits at cs.uiuc.edu for LLVM" < llvm-commits at cs.uiuc.edu >
> Sent: Wednesday, March 5, 2014 11:47:53 AM
> Subject: Re: [PATCH] Teach the DAGCombiner how to fold a OR of two
> shufflevector into a single shufflevector node
> 
> Hi Andrea,
> 
> Thanks for working on this. Lowering shuffles is really difficult
> and we have >1000 LOC in the x86 backends for lowering shuffles. It
> is really difficult to predict which shuffles are supported and
> which patterns result in slow code. Until now LLVM was *not* to
> introducing new shuffle patterns. The idea was that the users that
> generate the shuffle patterns should know if the patterns are
> supported by the target. This makes sense for users that write their
> own math libraries using the clang vector syntax. Your patch
> introduces new shuffle patterns that some backends may not know how
> to lower. I think that the code should be okay for AVX and AVX2
> because you are not introducing cross-lane shuffles. So, I suggest
> that you move this patch into the X86 backend and protect it with
> the hasAVX flags. Does that make sense?
> 
> Is this not what TLI->isShuffleMaskLegal is for? We avoid introducing
> new shuffle patterns at the IR level, especially during
> canonicalization, because we don't want to use target information at
> that stage. In the SDAG, I think that the situation is different.
> There we should use TLI->isShuffleMaskLegal (at the other associated
> legality checks) to control these kinds of optimizations. Opinions?
> 
> 
> 
> This is also a good solution. The advantage is that multiple targets
> would be able to benefit from Andrea's new optimization. The
> disadvantage is that the coverage of isShuffleMaskLegal is minimal.

Okay; I'd prefer to do that, and improve isShuffleMaskLegal and related interfaces as necessary.

 -Hal

> 
> 
> 
> 
> -Hal
> 
> 
> 
> 
> Thanks,
> Nadav
> 
> 
> 
> On Feb 26, 2014, at 7:07 AM, Andrea Di Biagio
> < andrea.dibiagio at gmail.com > wrote:
> 
> 
> 
> <patch-dagcombine.diff>
> 
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> 
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory