[llvm-dev] BSWAP matching in codegen

[Jim Lewis via llvm-dev]

>> Are you sure there isn't any test coverage?  As far as I can tell, the tests from https://reviews.llvm.org/rL133503 are still in the tree.

I looked at those, but none of them include the full pattern that decomposes into bswap and rol. I debugged through the X86 bswap.ll test and verified none of those cases make it through MatchBSwapHWord (they get handled in MatchBSwapHWordLow instead).

>> I don't think there is any existing code to match this particular pattern in instcombine.  I guess it wouldn't be hard to add, though.

Yes, I guess the InstCombiner::MatchBSwap() function is very similar but not identical. It looks for patterns that match the bswap exactly, whereas the DAGCombiner version is looking for halfword swap patterns that decompose to a bswap and a 16-bit rotate.

Jim



On 12/8/16, 12:31 PM, "Friedman, Eli" <efriedma at codeaurora.org<mailto:efriedma at codeaurora.org>> wrote:

On 12/8/2016 10:01 AM, Jim Lewis via llvm-dev wrote:
I’m looking into problems with the function DAGCombiner::MatchBSwapHWord and had a couple questions on how to proceed (I’m new here, and to llvm). It looks like there was never any test coverage for this code, and it doesn’t match the patterns it claims to be looking for. I found a fix for this in our fork, but the fix is even worse in that it matches a whole bunch of invalid patterns as well as the canonical patterns. I have simple test cases that break both the existing code and our local fix.

Are you sure there isn't any test coverage?  As far as I can tell, the tests from https://reviews.llvm.org/rL133503 are still in the tree.


I see equivalent matching code upstream in InstCombiner::MatchBSwap()  (InstCombineAndOrXor.cpp), but not an easy way to re-use it AFAICT. Questions:

--Is this functionality even worth fixing in DAGCombiner (ie, what is the probability that this complex pattern would re-emerge in codegen after the InstCombiner transform, weighed against the obviously error-prone nature of the matching code)?

I don't think there is any existing code to match this particular pattern in instcombine.  I guess it wouldn't be hard to add, though.

-Eli


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