[all-commits] [llvm/llvm-project] c66340: [RISCV] Prefer vrgatherei16 for shuffles (#66291)

[Philip Reames via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: c663401f69ecb999eb0c5a2af183b920c6c50a6a
      https://github.com/llvm/llvm-project/commit/c663401f69ecb999eb0c5a2af183b920c6c50a6a
  Author: Philip Reames <preames at rivosinc.com>
  Date:   2023-09-15 (Fri, 15 Sep 2023)

  Changed paths:
    M llvm/lib/Target/RISCV/RISCVISelLowering.cpp
    M llvm/test/CodeGen/RISCV/rvv/fixed-vectors-fp-buildvec.ll
    M llvm/test/CodeGen/RISCV/rvv/fixed-vectors-fp-interleave.ll
    M llvm/test/CodeGen/RISCV/rvv/fixed-vectors-fp-shuffles.ll
    M llvm/test/CodeGen/RISCV/rvv/fixed-vectors-int-interleave.ll
    M llvm/test/CodeGen/RISCV/rvv/fixed-vectors-int-shuffles.ll
    M llvm/test/CodeGen/RISCV/rvv/fixed-vectors-interleaved-access.ll
    M llvm/test/CodeGen/RISCV/rvv/fixed-vectors-shuffle-reverse.ll
    M llvm/test/CodeGen/RISCV/rvv/fixed-vectors-shuffle-rotate.ll
    M llvm/test/CodeGen/RISCV/rvv/shuffle-reverse.ll
    M llvm/test/CodeGen/RISCV/rvv/vector-interleave-fixed.ll

  Log Message:
  -----------
  [RISCV] Prefer vrgatherei16 for shuffles (#66291)

If the data type is larger than e16, and the requires more than LMUL1 register
class, prefer the use of vrgatherei16.  This has three major benefits:
1) Less work needed to evaluate the constant for e.g. vid sequences.  Remember
that arithmetic generally scales lineary with LMUL.
2) Less register pressure.  In particular, the source and indices registers
*can* overlap so using a smaller index can significantly help at m8.
3) Smaller constants.  We've got a bunch of tricks for materializing small
constants, and if needed, can use a EEW=16 load.