[llvm] [AMDGPU] Use LSH for lowering ctlz_zero_undef.i8/i16 (PR #88512)

[Matt Arsenault via llvm-commits]

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@@ -3081,20 +3081,30 @@ static bool isCttzOpc(unsigned Opc) {
 SDValue AMDGPUTargetLowering::lowerCTLZResults(SDValue Op,
                                                SelectionDAG &DAG) const {
   auto SL = SDLoc(Op);
+  auto Opc = Op.getOpcode();
   auto Arg = Op.getOperand(0u);
   auto ResultVT = Op.getValueType();
 
   if (ResultVT != MVT::i8 && ResultVT != MVT::i16)
     return {};
 
-  assert(isCtlzOpc(Op.getOpcode()));
+  assert(isCtlzOpc(Opc));
   assert(ResultVT == Arg.getValueType());
 
-  auto const LeadingZeroes = 32u - ResultVT.getFixedSizeInBits();
-  auto SubVal = DAG.getConstant(LeadingZeroes, SL, MVT::i32);
-  auto NewOp = DAG.getNode(ISD::ZERO_EXTEND, SL, MVT::i32, Arg);
-  NewOp = DAG.getNode(Op.getOpcode(), SL, MVT::i32, NewOp);
-  NewOp = DAG.getNode(ISD::SUB, SL, MVT::i32, NewOp, SubVal);
+  auto const NumBits = ResultVT.getFixedSizeInBits();
+  auto NumExtBits = DAG.getConstant(32u - NumBits, SL, MVT::i32);
+  auto NewOp = SDValue();
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arsenm wrote:

No auto and don't need explicit constructor 

https://github.com/llvm/llvm-project/pull/88512