[PATCH] D92270: [ConstantFold] Fold more operations to poison

[Juneyoung Lee via Phabricator via cfe-commits]

aqjune added a comment.

Hi,

It seems it is related to two optimizations:
(1) `select i1 x, true, y` -> `or i1 x, y`
(2) `or i1 x, poison` -> `poison`

Semantically, the first one is broken. It needs to freeze y. But, it will introduce a lot of freeze instructions. The clang patches that introduce noundef to function arguments is still ongoing.
Another workaround is to weaken (2) to `or i1 x, poison` -> `x`. This fixes the miscompilation; I'll push a commit that does this.

  --- a/llvm/lib/IR/ConstantFold.cpp
  +++ b/llvm/lib/IR/ConstantFold.cpp
  @@ -1105,7 +1105,10 @@ Constant *llvm::ConstantFoldBinaryInstruction(unsigned Opcode, Constant *C1,
     }
   
     // Binary operations propagate poison.
  -  if (isa<PoisonValue>(C1) || isa<PoisonValue>(C2))
  +  bool PoisonFold = !C1->getType()->isIntegerTy(1) || 
  +      (Opcode != Instruction::Or && Opcode != Instruction::And &&
  +       Opcode != Instruction::Xor);
  +  if (PoisonFold && (isa<PoisonValue>(C1) || isa<PoisonValue>(C2)))
       return PoisonValue::get(C1->getType());
   
     // Handle scalar UndefValue and scalable vector UndefValue. Fixed-length


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  https://reviews.llvm.org/D92270/new/

https://reviews.llvm.org/D92270