[PATCH] D157417: [RISCV][SelectionDAG] Lower shuffles as bitrotates with vror.vi when possible

[Luke Lau via Phabricator via llvm-commits]

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Given a shuffle mask like <3, 0, 1, 2, 7, 4, 5, 6> for v8i8, we can
reinterpret it as a shuffle of v2i32 where the two i32s are bit rotated, and
lower it as a vror.vi (if legal with zvbb enabled).
We also need to make sure that the larger element type is a valid SEW, hence
the tests for zve32x.

X86 already did this, so I've extracted the logic for it and put it inside
ShuffleVectorSDNode so it could be reused by RISC-V. I originally tried to add
this as a generic combine in DAGCombiner.cpp, but it ended up causing worse
codegen on X86 and PPC.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D157417

Files:
  llvm/include/llvm/CodeGen/SelectionDAGNodes.h
  llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
  llvm/lib/Target/RISCV/RISCVISelLowering.cpp
  llvm/lib/Target/X86/X86ISelLowering.cpp
  llvm/test/CodeGen/RISCV/rvv/fixed-vectors-shuffle-rotate.ll

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