[llvm] [ConstantFold][RFC] Refactor getBinOpAbsorber function (PR #109736)

[Nikita Popov via llvm-commits]

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@@ -729,13 +729,15 @@ Constant *llvm::ConstantFoldBinaryInstruction(unsigned Opcode, Constant *C1,
   // Neither constant should be UndefValue, unless these are vector constants.
   assert((!HasScalarUndefOrScalableVectorUndef) && "Unexpected UndefValue");
 
+  Constant *Absorber = ConstantExpr::getBinOpAbsorber(
+      Opcode, C1->getType(), /*AllowLHSAbsorber*/ true);
+
   // Handle simplifications when the RHS is a constant int.
   if (ConstantInt *CI2 = dyn_cast<ConstantInt>(C2)) {
+    if (C2 == Absorber)
+      return C2;
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nikic wrote:

This looks incorrect. You are using the Absorber value with AllowLHSAbsorber=true and then using it to check the RHS. This means that for example `X << 0` could get incorrectly folded to `0`, no? I think the only reason this works out here is by accident, because this case is already handled by the identity logic earlier.

https://github.com/llvm/llvm-project/pull/109736