[all-commits] [llvm/llvm-project] 42b341: [AArch64] Split LSLFast into Addr and ALU parts

David Green via All-commits all-commits at lists.llvm.org
Fri Aug 18 00:59:38 PDT 2023

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 42b3419339dba270107e9323a1fbfe3b39ed39bd
  Author: David Green <david.green at arm.com>
  Date:   2023-08-18 (Fri, 18 Aug 2023)

  Changed paths:
    M llvm/lib/Target/AArch64/AArch64.td
    M llvm/lib/Target/AArch64/AArch64ISelDAGToDAG.cpp
    M llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
    M llvm/lib/Target/AArch64/GISel/AArch64InstructionSelector.cpp
    M llvm/test/CodeGen/AArch64/GlobalISel/load-addressing-modes.mir
    M llvm/test/CodeGen/AArch64/aarch64-fold-lslfast.ll
    A llvm/test/CodeGen/AArch64/lslfast.ll
    M llvm/test/CodeGen/AArch64/mul_pow2.ll

  Log Message:
  [AArch64] Split LSLFast into Addr and ALU parts

As far as I can tell FeatureLSLFast was originally added to specify that a lsl
of <= 3 was cheap when folded into an addressing operand, so should override
the one-use checks usually intended to make sure we don't perform redundant
work. At a later point it also came to also mean that add x0, x1, x2, lsl N
with N <= 4 was cheap, in that it took a single cycle not multiple cycles that
more complex adds usually take.

This patch splits those two concepts out into separate subtarget features. The
biggest change is the change to AArch64DAGToDAGISel::isWorthFoldingALU, making
ALU operations now produce a ADDWrs if the shift is <= 4.

Otherwise the patch is mostly an NFC as it tries to keep the subtarget features
the same for each cpu. I believe that the Arm OoO CPUs should eventually be
changed to a new subtarget feature that specifies that a shift of 2 or 3 with
any extend should be treated as cheap (just not shifts of 1 or 4).

Differential Revision: https://reviews.llvm.org/D157982

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