[PATCH] D80489: [TargetLowering] Improve expandFunnelShift shift amount masking

[Simon Pilgrim via Phabricator via llvm-commits]

RKSimon created this revision.
RKSimon added reviewers: foad, craig.topper, spatel, lebedev.ri.
Herald added a subscriber: hiraditya.
Herald added a project: LLVM.

For the 'inverse shift', we currently always perform a subtraction of the original (masked) shift amount.

But for the case where we are handling power-of-2 type widths, we can replace:

  (sub bw-1, (and amt, bw-1) ) -> (and (xor amt, bw-1), bw-1) -> (and ~amt, bw-1)

This allows x86 shifts to fold away the and-mask.

Followup to D77301 <https://reviews.llvm.org/D77301> + D80466 <https://reviews.llvm.org/D80466>.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D80489

Files:
  llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
  llvm/test/CodeGen/X86/fshl.ll
  llvm/test/CodeGen/X86/fshr.ll

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