[all-commits] [llvm/llvm-project] 9fc5af: [RISCV] Use vslide1down lowering for two element n...

[Philip Reames via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 9fc5af1b8466606c0f11f7035451fad8126c0dd6
      https://github.com/llvm/llvm-project/commit/9fc5af1b8466606c0f11f7035451fad8126c0dd6
  Author: Philip Reames <preames at rivosinc.com>
  Date:   2023-05-03 (Wed, 03 May 2023)

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

  Log Message:
  -----------
  [RISCV] Use vslide1down lowering for two element non-constant build_vectors

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).

Differential Revision: https://reviews.llvm.org/D149667