[all-commits] [llvm/llvm-project] 702517: [RISCV] Optimize "dominant element" BUILD_VECTORs

[Fraser Cormack via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 70251759a29830a2c0aca146d91bbe65e0f1df0c
      https://github.com/llvm/llvm-project/commit/70251759a29830a2c0aca146d91bbe65e0f1df0c
  Author: Fraser Cormack <fraser at codeplay.com>
  Date:   2021-03-17 (Wed, 17 Mar 2021)

  Changed paths:
    M llvm/lib/Target/RISCV/RISCVISelLowering.cpp
    M llvm/test/CodeGen/RISCV/rvv/fixed-vectors-bswap.ll
    M llvm/test/CodeGen/RISCV/rvv/fixed-vectors-ctlz.ll
    M llvm/test/CodeGen/RISCV/rvv/fixed-vectors-cttz.ll
    M llvm/test/CodeGen/RISCV/rvv/fixed-vectors-fp-buildvec.ll
    M llvm/test/CodeGen/RISCV/rvv/fixed-vectors-insert.ll
    M llvm/test/CodeGen/RISCV/rvv/fixed-vectors-int-buildvec.ll
    M llvm/test/CodeGen/RISCV/rvv/fixed-vectors-int.ll

  Log Message:
  -----------
  [RISCV] Optimize "dominant element" BUILD_VECTORs

This patch adds an optimization path for BUILD_VECTOR nodes where the
majority of the elements are identical. These can be splatted, with the
remaining elements patched up with INSERT_VECTOR_ELTs. The threshold can
be tweaked as required - it is currently conservative. Undef elements
are disregarded when judging the dominance of a particular element. This
allows them to be covered by the splat value.

In addition, vectors of 2 elements are always optimized to a splat (for
the upper element) and an insert at element zero.

This optimization is disabled when optimizing for size.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D98700