[llvm] [InstCombine] Fold (X << Y) / (X << Z) -> 1 << Y >> Z (PR #68863)

[via llvm-commits]

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@@ -980,6 +980,28 @@ static Instruction *foldIDivShl(BinaryOperator &I,
       Ret = BinaryOperator::CreateSDiv(X, Y);
   }
 
+  // If X << Y and X << Z does not overflow, then:
+  // (X << Y) / (X << Z) -> (1 << Y) / (1 << Z) -> 1 << Y >> Z
+  if (match(Op0, m_Shl(m_Value(X), m_Value(Y))) &&
+      match(Op1, m_Shl(m_Specific(X), m_Value(Z)))) {
+    auto *Shl0 = cast<OverflowingBinaryOperator>(Op0);
+    auto *Shl1 = cast<OverflowingBinaryOperator>(Op1);
+
+    if ((!IsSigned && Shl0->hasNoUnsignedWrap() && Shl1->hasNoUnsignedWrap()) ||
+        (IsSigned && Shl0->hasNoSignedWrap() && Shl1->hasNoSignedWrap())) {
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goldsteinn wrote:

nit: Would prefer ternary here i.e `isSigned? (Shl0->Nsw && Shl1->Nsw) : (...Nuw)`

https://github.com/llvm/llvm-project/pull/68863