[llvm] [DAGCombine] Count leading ones: refine post DAG/Type Legalisation if promotion (PR #102877)

[via llvm-commits]

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@@ -3755,6 +3755,51 @@ SDValue DAGCombiner::foldSubToUSubSat(EVT DstVT, SDNode *N, const SDLoc &DL) {
   return SDValue();
 }
 
+template <class MatchContextClass>
+static SDValue foldSubCtlzNot(SDNode *N, SelectionDAG &DAG) {
+  const SDLoc DL(N);
+  SDValue N0 = N->getOperand(0);
+  EVT VT = N0.getValueType();
+  unsigned BitWidth = VT.getScalarSizeInBits();
+
+  MatchContextClass Matcher(DAG, DAG.getTargetLoweringInfo(), N);
+
+  APInt AndMask;
+  APInt XorMask;
+  APInt BitWidthDiff;
+
+  SDValue CtlzOp;
+  SDValue Src;
+
+  if (!sd_context_match(
+          N, Matcher,
+          m_Sub(m_Node(ISD::CTLZ, m_Value(CtlzOp)), m_ConstInt(BitWidthDiff))))
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v01dXYZ wrote:

The commit is not complete. I am implenting tests 

https://github.com/llvm/llvm-project/pull/102877