[PATCH] D142264: [RISCV] Combine extract_vector_elt followed by VFMV_S_F_VL.

[Craig Topper via Phabricator via llvm-commits]

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If we're extracting an element and inserting into a undef vector
with the same number of elements, we can use the original vector.

This pattern occurs around reductions that have been cascaded
together.

This can be generalized to wider/narrow vectors by using
insert_subvector/extract_subvector, but we don't have lit tests
for that case currently.

We can also support non-undef before by using a slide or vmv.v.v


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D142264

Files:
  llvm/lib/Target/RISCV/RISCVISelLowering.cpp
  llvm/test/CodeGen/RISCV/rvv/fixed-vectors-reduction-fp-vp.ll
  llvm/test/CodeGen/RISCV/rvv/fixed-vectors-reduction-fp.ll
  llvm/test/CodeGen/RISCV/rvv/vreductions-fp-vp.ll

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