[PATCH] D107449: [RISCV] Improve codegen for shuffles with LHS/RHS splats

[Fraser Cormack via Phabricator via llvm-commits]

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Shuffles which are broken into separate halves reveal splats in which
a half is accessed via one index; such operations can be optimized to
use "vrgather.vi".

This optimization could be achieved by adding extra patterns to match
`vrgather_vv_vl` which uses a splat as an index operand, but this patch
instead identifies splat earlier. This way, future optimizations can
build on top of the data gathered here, e.g., to splat-gather dominant
indices and insert any leftovers.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D107449

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

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