[llvm] [DAGCombine] Transform `shl X, cttz(Y)` to `mul (Y & -Y), X` if cttz is unsupported (PR #85066)

[Matt Arsenault via llvm-commits]

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@@ -9962,6 +9962,18 @@ SDValue DAGCombiner::visitSHL(SDNode *N) {
     if (SDValue NewSHL = visitShiftByConstant(N))
       return NewSHL;
 
+  // fold (shl X, cttz(Y)) -> (mul (Y & -Y), X) if cttz is unsupported on the
+  // target.
+  if ((N1.getOpcode() == ISD::CTTZ || N1.getOpcode() == ISD::CTTZ_ZERO_UNDEF) &&
+      N1.hasOneUse() && !TLI.isOperationLegalOrCustom(ISD::CTTZ, VT) &&
+      TLI.isOperationLegalOrCustom(ISD::MUL, VT)) {
+    SDValue Y = N1.getOperand(0);
+    SDLoc DL(N);
+    SDValue NegY = DAG.getNode(ISD::SUB, DL, VT, DAG.getConstant(0, DL, VT), Y);
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arsenm wrote:

DAG.getNegative 

https://github.com/llvm/llvm-project/pull/85066