[llvm] [InstCombine] Fold adds + shifts with nsw and nuw flags (PR #88193)

[Matt Arsenault via llvm-commits]

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@@ -1450,9 +1460,24 @@ Instruction *InstCombinerImpl::visitLShr(BinaryOperator &I) {
           NewMul->setHasNoSignedWrap(true);
           return NewMul;
         }
+
+        // Special case: lshr nuw (mul (X, 3), 1) -> add nuw nsw (X, lshr(X, 1)
+        if (ShAmtC == 1 && MulC->getZExtValue() == 3) {
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arsenm wrote:

Is there a more general form for this than these specific constants? Why is this the one special case? 

https://github.com/llvm/llvm-project/pull/88193