[llvm] [InstCombine] Fold `(X * (Y << K)) u>> K -> X * Y` when highbits are not demanded (PR #111151)

[Nikita Popov via llvm-commits]

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@@ -770,6 +770,15 @@ Value *InstCombinerImpl::SimplifyDemandedUseBits(Instruction *I,
             return InsertNewInstWith(Shl, I->getIterator());
           }
         }
+
+        const APInt *Factor;
+        if (match(I->getOperand(0),
+                  m_OneUse(m_Mul(m_Value(X), m_APInt(Factor)))) &&
+            Factor->countr_zero() >= ShiftAmt) {
+          BinaryOperator *Mul = BinaryOperator::CreateMul(
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nikic wrote:

I don't think the two patterns would combine well. In one case we can optimize because there is no overflow. In the other case we can optimize because the bits affected by overflow are not demanded.

https://github.com/llvm/llvm-project/pull/111151