[PATCH] D146711: [RISCV] Lower shuffles as vmerge + vslide{up,down} where possible

[Luke Lau via Phabricator via llvm-commits]

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Currently, shuffles are lowered as selects if each destination element
is choosing between either source vector's element at the same index.
This extends the logic to allow the source vectors to be slid up or down
before merging.

For example,
shufflevector <4 x i8> v1, <4 x i8> v2, <0, 1, 4, 5>
can be expressed as
select v1, (slideup v2, 2), <0, 0, 1, 1>

This pattern shows up with shuffles that insert subvectors
(SK_InsertSubvector)


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D146711

Files:
  llvm/lib/Target/RISCV/RISCVISelLowering.cpp
  llvm/test/CodeGen/RISCV/rvv/fixed-vectors-int-shuffles.ll
  llvm/test/CodeGen/RISCV/rvv/fixed-vectors-shufflevector-vnsrl.ll
  llvm/test/CodeGen/RISCV/rvv/vector-deinterleave-fixed.ll

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