[PATCH] D154791: [InstCombine] Transform bitwise (A >> C - 1, zext(icmp)) -> zext (bitwise(A < 0, icmp)) fold.

[Hongyu Chen via Phabricator via llvm-commits]

XChy added a comment.

In D154791#4497148 <https://reviews.llvm.org/D154791#4497148>, @XChy wrote:

> I update the diff to ensure that we fold exactly what can be folded by **foldXorOfICmps** and **foldAndOrOfICmps**.
>
> However, I'm not sure what unexpected result the code below may bring.
>
>   // remove the deferred 2 instructions : 
>   // icmp slt A, 0
>   // bitwise (A < 0, icmp) 
>   // otherwise there will be infinite loops of combining
>   Worklist.popDeferred()->eraseFromParent();
>   Worklist.popDeferred()->eraseFromParent();

Actually, I copy and edit the code to avoid infinite loops from:

  Instruction *eraseInstFromFunction(Instruction &I) override {
    LLVM_DEBUG(dbgs() << "IC: ERASE " << I << '\n');
    assert(I.use_empty() && "Cannot erase instruction that is used!");
    salvageDebugInfo(I);
  
    // Make sure that we reprocess all operands now that we reduced their
    // use counts.
    SmallVector<Value *> Ops(I.operands());
    Worklist.remove(&I);
    I.eraseFromParent();
    for (Value *Op : Ops)
      Worklist.handleUseCountDecrement(Op);
    MadeIRChange = true;
    return nullptr; // Don't do anything with FI
  }

I omit `MadeIRChange = true;` to avoid the infinite loops, which are caused by `MadeIRChange` with the same instructions **deferred and erased**(Actually, IR do not change).


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  https://reviews.llvm.org/D154791/new/

https://reviews.llvm.org/D154791