[llvm] [InstCombine] lshr (mul (X, 2^N + 1)), N -> add (X, lshr(X, N)) (PR #90295)

[via llvm-commits]

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@@ -1483,6 +1494,16 @@ Instruction *InstCombinerImpl::visitLShr(BinaryOperator &I) {
       }
     }
 
+    // lshr (mul nsw (X, 2^N + 1)), N -> add nsw (X, lshr(X, N))
+    if (match(Op0, m_OneUse(m_NSWMul(m_Value(X), m_APInt(MulC))))) {
+      if (BitWidth > 2 && (*MulC - 1).isPowerOf2() &&
+          MulC->logBase2() == ShAmtC) {
+        return BinaryOperator::CreateNSWAdd(
+            X, Builder.CreateLShr(X, ConstantInt::get(Ty, ShAmtC), "",
+                                  I.isExact()));
+      }
+    }
+
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goldsteinn wrote:

That wasn't exact code, just the idea of putting all the `m_Mul` and `(*MulC - 1).isPowerOf2() && MulC->logBase2() == ShAmtC` together to simplify the code.

https://github.com/llvm/llvm-project/pull/90295