[all-commits] [llvm/llvm-project] 280aa3: [DAG] Add a generic expansion for SHIFT_PARTS opco...

[Simon Pilgrim via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 280aa3415e408cacc520274fdb948ec9fc63865a
      https://github.com/llvm/llvm-project/commit/280aa3415e408cacc520274fdb948ec9fc63865a
  Author: Simon Pilgrim <llvm-dev at redking.me.uk>
  Date:   2021-05-07 (Fri, 07 May 2021)

  Changed paths:
    M llvm/include/llvm/CodeGen/TargetLowering.h
    M llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
    M llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
    M llvm/lib/Target/AArch64/AArch64ISelLowering.h
    M llvm/lib/Target/AMDGPU/R600ISelLowering.cpp
    M llvm/lib/Target/AMDGPU/R600ISelLowering.h
    M llvm/lib/Target/X86/X86ISelLowering.cpp
    M llvm/test/CodeGen/AArch64/arm64-long-shift.ll
    M llvm/test/CodeGen/AMDGPU/fp_to_sint.ll
    M llvm/test/CodeGen/AMDGPU/fp_to_uint.ll
    M llvm/test/CodeGen/AMDGPU/shl.ll
    M llvm/test/CodeGen/AMDGPU/sra.ll
    M llvm/test/CodeGen/AMDGPU/srl.ll

  Log Message:
  -----------
  [DAG] Add a generic expansion for SHIFT_PARTS opcodes using funnel shifts

Based off a discussion on D89281 - where the AARCH64 implementations were being replaced to use funnel shifts.

Any target that has efficient funnel shift lowering can handle the shift parts expansion using the same expansion, avoiding a lot of duplication.

I've generalized the X86 implementation and moved it to TargetLowering - so far I've found that AARCH64 and AMDGPU benefit, but many other targets (ARM, PowerPC + RISCV in particular) could easily use this with a few minor improvements to their funnel shift lowering (or the folding of their target ops that funnel shifts lower to).

NOTE: I'm trying to avoid adding full SHIFT_PARTS legalizer handling as I think it might actually be possible to remove these opcodes in the medium-term and use funnel shift / libcall expansion directly.

Differential Revision: https://reviews.llvm.org/D101987