[all-commits] [llvm/llvm-project] d9507a: [DAGCombine] Fix miscompilation caused by PR94008 ...

[Yingwei Zheng via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: d9507a3e10d1750d88dd518c14b9a9a62b9eefcd
      https://github.com/llvm/llvm-project/commit/d9507a3e10d1750d88dd518c14b9a9a62b9eefcd
  Author: Yingwei Zheng <dtcxzyw2333 at gmail.com>
  Date:   2024-06-08 (Sat, 08 Jun 2024)

  Changed paths:
    M llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
    A llvm/test/CodeGen/X86/pr94824.ll

  Log Message:
  -----------
  [DAGCombine] Fix miscompilation caused by PR94008 (#94850)

The pr description in #94008 mismatches with the code.
> + When VT is smaller than ShiftVT, it is safe to use trunc.
> + When VT is larger than ShiftVT, it is safe to use zext iff
`is_zero_poison` is true (i.e., `opcode == ISD::CTTZ_ZERO_UNDEF`). See
also the counterexample `src_shl_cttz2 -> tgt_shl_cttz2` in the alive2
    proofs.

Closes #94824.



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