[llvm] [InstCombine] Fold mul (shr exact (X, N)), 2^N + 1 -> add (X , shr exact (X, N)) (PR #112407)

[Yingwei Zheng via llvm-commits]

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@@ -261,6 +261,37 @@ Instruction *InstCombinerImpl::visitMul(BinaryOperator &I) {
     }
   }
 
+  // mul (shr exact X, N), (2^N + 1) -> add (X, shr exact (X, N))
+  {
+    Value *NewOp;
+    const APInt *ShiftC;
+    const APInt *MulAP;
+    if (match(&I, m_Mul(m_Exact(m_Shr(m_Value(NewOp), m_APInt(ShiftC))),
+                        m_APInt(MulAP)))) {
+      if (BitWidth > 2 && (*MulAP - 1).isPowerOf2() &&
+          *ShiftC == MulAP->logBase2()) {
+        Value *BinOp = Op0;
+        BinaryOperator *OpBO = cast<BinaryOperator>(Op0);
+        if (!isGuaranteedNotToBeUndef(NewOp, &AC, &I, &DT))
+          NewOp = Builder.CreateFreeze(NewOp, NewOp->getName() + ".fr");
+
+        // mul (ashr nuw exact X, N) -> add (X, lshr nuw exact (X, N))
+        if (HasNUW && OpBO->getOpcode() == Instruction::AShr &&
+            OpBO->hasOneUse())
+          BinOp = Builder.CreateLShr(NewOp, ConstantInt::get(Ty, *ShiftC), "",
+                                     /*isExact=*/true);
+
+        auto *NewAdd = BinaryOperator::CreateAdd(NewOp, BinOp);
+        if (HasNSW && (HasNUW || OpBO->getOpcode() == Instruction::LShr ||
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dtcxzyw wrote:

I mean `nsw` preservation: https://alive2.llvm.org/ce/z/Bbwycp

https://github.com/llvm/llvm-project/pull/112407