[llvm] [InstCombine] Optimize (select %x, op(%x), 0) to op(%x) for operations where op(0) == 0 (PR #147605)

[Yingwei Zheng via llvm-commits]

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@@ -913,11 +915,28 @@ static Instruction *foldSelectZeroOrMul(SelectInst &SI, InstCombinerImpl &IC) {
   if (!match(MergedC, m_Zero()) && !match(MergedC, m_Undef()))
     return nullptr;
 
-  auto *FalseValI = cast<Instruction>(FalseVal);
-  auto *FrY = IC.InsertNewInstBefore(new FreezeInst(Y, Y->getName() + ".fr"),
-                                     FalseValI->getIterator());
-  IC.replaceOperand(*FalseValI, FalseValI->getOperand(0) == Y ? 0 : 1, FrY);
-  return IC.replaceInstUsesWith(SI, FalseValI);
+  if (match(FalseVal, m_c_Mul(m_Specific(X), m_Value(Y))) ||
+      match(FalseVal, m_c_And(m_Specific(X), m_Value(Y))) ||
+      match(FalseVal, m_Shl(m_Specific(X), m_Value(Y))) ||
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dtcxzyw wrote:

I think we can start with constant (splat) shamts (i.e., use `m_APInt(C)` and `C->ult(BW)`).

> A separate note, if we could get static bool shiftAmountKnownInRange(const Value *ShiftAmount) access to information about Value's ranges (i.e. range(i64 0 10)) it would be effective in more situations.

If you want to handle variable shamts, use `computeKnownBits().getMaxValue().ult(BW)`.


https://github.com/llvm/llvm-project/pull/147605