[llvm] [InstCombine] Fold shift+cttz with power of 2 operands (PR #127055)

[Matthew Devereau via llvm-commits]

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@@ -1613,6 +1613,22 @@ Instruction *InstCombinerImpl::visitLShr(BinaryOperator &I) {
   if (Instruction *Overflow = foldLShrOverflowBit(I))
     return Overflow;
 
+  // Transform ((pow2 << x) >> cttz(pow2 << y)) -> ((1 << x) >> y)
+  Value *Shl0_Op0, *Shl0_Op1, *Shl1_Op0, *Shl1_Op1;
+  BinaryOperator *Shl1;
+  if (match(Op0, m_Shl(m_Value(Shl0_Op0), m_Value(Shl0_Op1))) &&
+      match(Op1, m_Intrinsic<Intrinsic::cttz>(m_BinOp(Shl1))) &&
+      match(Shl1, m_Shl(m_Value(Shl1_Op0), m_Value(Shl1_Op1))) &&
+      isKnownToBeAPowerOfTwo(Shl1, false, 0, SQ.getWithInstruction(&I).CxtI) &&
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MDevereau wrote:

Done

https://github.com/llvm/llvm-project/pull/127055