[PATCH] D149667: [RISCV] Use vslide1down lowering for two element non-constant build_vectors

[Philip Reames via Phabricator via llvm-commits]

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When the values are in GPRs, the vslide1down lowering is always better.  We need to greatly improve the splat-and-mask cost model to handle constants in a meaningful way, so for now, limit this to non-constant vectors.

This does send the "partially constant" case down the vslide1down path.  This could cause some regressions, though I don't see any in practice.

The cost modeling for the general case is annoyingly tricky.  We have a great amount of inconsistency around immediate operands, and as a result, the exact constant and exact lowering choice matters a lot.  I'm hoping that we get a "good enough" result without modeling this exactly, but we may need to do something analogous to getIntMatCost (i.e. a search w/costing).


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D149667

Files:
  llvm/lib/Target/RISCV/RISCVISelLowering.cpp
  llvm/test/CodeGen/RISCV/rvv/fixed-vector-shuffle-reverse.ll
  llvm/test/CodeGen/RISCV/rvv/fixed-vectors-bitcast.ll
  llvm/test/CodeGen/RISCV/rvv/fixed-vectors-fp2i-sat.ll
  llvm/test/CodeGen/RISCV/rvv/fixed-vectors-load.ll
  llvm/test/CodeGen/RISCV/rvv/fixed-vectors-mask-buildvec.ll
  llvm/test/CodeGen/RISCV/rvv/fixed-vectors-masked-scatter.ll
  llvm/test/CodeGen/RISCV/rvv/fixed-vectors-store.ll
  llvm/test/CodeGen/RISCV/rvv/fold-vector-cmp.ll

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