[all-commits] [llvm/llvm-project] d991b7: [RISCV] Pass undef VECTOR_SHUFFLE indices on to BU...

[Fraser Cormack via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: d991b7212b4c852c29b03d6d9aec40a6e819be95
      https://github.com/llvm/llvm-project/commit/d991b7212b4c852c29b03d6d9aec40a6e819be95
  Author: Fraser Cormack <fraser at codeplay.com>
  Date:   2021-07-13 (Tue, 13 Jul 2021)

  Changed paths:
    M llvm/lib/Target/RISCV/RISCVISelLowering.cpp
    A llvm/test/CodeGen/RISCV/rvv/common-shuffle-patterns.ll
    M llvm/test/CodeGen/RISCV/rvv/fixed-vectors-fp-buildvec.ll
    M llvm/test/CodeGen/RISCV/rvv/fixed-vectors-fp-shuffles.ll
    M llvm/test/CodeGen/RISCV/rvv/fixed-vectors-int-shuffles.ll
    M llvm/test/CodeGen/RISCV/rvv/interleave-crash.ll

  Log Message:
  -----------
  [RISCV] Pass undef VECTOR_SHUFFLE indices on to BUILD_VECTOR

Often when lowering vector shuffles, we split the shuffle into two
LHS/RHS shuffles which are then blended together. To do so we split the
original indices into two, indexed into each respective vector. These
two index vectors are then separately lowered as BUILD_VECTORs.

This patch forwards on any undef indices to the BUILD_VECTOR, rather
than having the VECTOR_SHUFFLE lowering decide on an optimal concrete
index. The motiviation for ths change is so that we don't duplicate
optimization logic between the two lowering methods and let BUILD_VECTOR
do what it does best.

Propagating undef in this way allows us, for example, to generate
`vid.v` to produce the LHS indices of commonly-used interleave-type
shuffles. I have designs on further optimizing interleave-type and other
common shuffle patterns in the near future.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D104789