[llvm] [InstCombine] Fold (X == 0 ? Y : 0) | X to X == 0 ? Y : X (PR #138373)

[Nikita Popov via llvm-commits]

================
@@ -3575,6 +3575,41 @@ static Value *foldOrOfInversions(BinaryOperator &I,
   return nullptr;
 }
 
+// Optimize patterns where an OR operation combines a select-based zero check
+// with its condition value. This handles both scalar and vector types.
+//
+// Given:
+//   (X == 0 ? Y : 0) | X  -->  X == 0 ? Y : X
+//   X | (X == 0 ? Y : 0)  -->  X == 0 ? Y : X
+//
+// Also handles cases where X might be wrapped in zero/sign extensions.
+static Instruction *foldOrOfSelectZero(BinaryOperator &BO, Value *Op0,
+                                       Value *Op1) {
+  CmpPredicate Pred;
+  Value *X, *Y;
+
+  // Check both operand orders to handle commutative OR
+  for (Value *SelVal : {Op0, Op1}) {
+    // The other operand in the OR operation (potentially X or extended X)
+    Value *Other = (SelVal == Op0) ? Op1 : Op0;
+
+    // Attempt to match the select pattern:
+    // select(icmp eq X, 0), Y, 0
+    // Where X might be:
+    // - Original value
+    // - Zero extended value (zext)
+    // - Sign extended value (sext)
+    if (match(SelVal, m_Select(m_c_ICmp(Pred, m_Value(X), m_Zero()), m_Value(Y),
----------------
nikic wrote:

```suggestion
    if (match(SelVal, m_Select(m_ICmp(Pred, m_Value(X), m_Zero()), m_Value(Y),
```
icmp constant is always on RHS.

https://github.com/llvm/llvm-project/pull/138373