[PATCH] D133736: [RISCV] Teach lowerVECTOR_SHUFFLE to recognize some shuffles as vnsrl.

[Craig Topper via Phabricator via llvm-commits]

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Unary shuffles such as <0,2,4,6,8,10,12,14> or <1,3,5,7,9,11,13,15>
where half the elements are returned, can be lowered using vnsrl.

SelectionDAGBuilder lowers such shuffles as a build_vector of
extract_elements since the mask has less elements than the source.
To fix this, I've enable the extractSubvectorIsCheapHook to allow
DAGCombine to rebuild the shuffle using 2 extract_subvectors preceding
the shufffle.

I've gone very conservative on extractSubvectorIsCheapHook to minimize
test impact and match what we have test coverage for. This can be
improved in the future.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D133736

Files:
  llvm/lib/Target/RISCV/RISCVISelLowering.cpp
  llvm/lib/Target/RISCV/RISCVISelLowering.h
  llvm/lib/Target/RISCV/RISCVInstrInfoVVLPatterns.td
  llvm/test/CodeGen/RISCV/rvv/fixed-vectors-fp-buildvec.ll
  llvm/test/CodeGen/RISCV/rvv/fixed-vectors-shufflevector-vnsrl.ll

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