[llvm] [SDAG] Use shifts if ISD::MUL is illegal when lowering ISD::CTPOP (PR #86505)

[Matt Arsenault via llvm-commits]

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@@ -8710,11 +8710,20 @@ SDValue TargetLowering::expandCTPOP(SDNode *Node, SelectionDAG &DAG) const {
   }
 
   // v = (v * 0x01010101...) >> (Len - 8)
-  SDValue Mask01 =
-      DAG.getConstant(APInt::getSplat(Len, APInt(8, 0x01)), dl, VT);
-  return DAG.getNode(ISD::SRL, dl, VT,
-                     DAG.getNode(ISD::MUL, dl, VT, Op, Mask01),
-                     DAG.getConstant(Len - 8, dl, ShVT));
+  SDValue V;
+  if (isOperationLegalOrCustomOrPromote(
+          ISD::MUL, getTypeToTransformTo(*DAG.getContext(), VT))) {
+    SDValue Mask01 =
+        DAG.getConstant(APInt::getSplat(Len, APInt(8, 0x01)), dl, VT);
+    V = DAG.getNode(ISD::MUL, dl, VT, Op, Mask01);
+  } else {
+    V = Op;
+    for (unsigned Shift = 8; Shift < Len; Shift *= 2)
+      V = DAG.getNode(ISD::ADD, dl, VT, V,
+                      DAG.getNode(ISD::SHL, dl, VT, V,
+                                  DAG.getShiftAmountConstant(Shift, VT, dl)));
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arsenm wrote:

Ugly line wrapping, use some intermediate variables? 

https://github.com/llvm/llvm-project/pull/86505