[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()));
+      }
+    }
+
----------------
AtariDreams wrote:

> if (match(Op0, m_NUWMul(m_Value(X), m_APInt(MulC))) && BitWidth > 2 &&
>     (*MulC - 1).isPowerOf2() && MulC->logBase2() == ShAmtC) {
>   auto *BO0 = cast<OverflowingBinaryOperator>(Op0);
>   // Look for a "splat" mul pattern - it replicates bits across each half
>   // of a value, so a right shift is just a mask of the low bits:
>   // lshr i[2N] (mul nuw X, (2^N)+1), N --> and iN X, (2^N)-1
>   if (ShAmtC * 2 == BitWidth && BO0->hasNoUnsignedWrap())
>     return BinaryOperator::CreateAnd(X, ConstantInt::get(Ty, *MulC - 2));
> 
>   // lshr (mul nuw (X, 2^N + 1)), N -> add nuw (X, lshr(X, N))
>   if (Op0->hasOneUse() &&
>       (BO0->hasNoUnsignedWrap() || &&BO0->hasNoSignedWrap())) {
>     auto *NewAdd = BinaryOperator::CreateAdd(
>         X,
>         Builder.CreateLShr(X, ConstantInt::get(Ty, ShAmtC), "", I.isExact()));
>     NewAdd->setHasNoSignedWrap(BO0->hasNoSignedWrap());
>     NewAdd->setHasNoUnsignedWrap(BO0->hasNoSignedWrap());
>     return NewAdd;
>   }
> }

this eliminates the MulC >> ShiftC optimization

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