[PATCH] D128769: [DAGCombiner] Fold sra (sub AddC, (shl X, N1C)), N1C --> sext (sub AddC1',(trunc X to (width - N1C)))

[Craig Topper via Phabricator via llvm-commits]

craig.topper created this revision.
craig.topper added reviewers: RKSimon, spatel.
Herald added subscribers: jsji, StephenFan, ecnelises, pengfei, hiraditya.
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We already handled this case for add with a constant RHS. A
similar pattern can occur for sub with a constant left hand side.

Test cases use add and a mul representing (neg (shl X, C)) because
that's what I saw in the wild. The mul will be decomposed and then
the new transform can kick in.

Tests have not been committed, but this patch shows the changes.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D128769

Files:
  llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  llvm/test/CodeGen/X86/shift-combine.ll

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