[RISCV] Replace NoX0 SDNodeXForm with a ComplexPattern to do the selection of the VL operand.

I think this is a more standard way of doing this.

Reviewed By: rogfer01

Differential Revision: https://reviews.llvm.org/D95833

Author: topperc

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp

@@ -1011,6 +1011,23 @@ bool RISCVDAGToDAGISel::MatchSLLIUW(SDNode *N) const {
   return (VC1 >> VC2) == UINT64_C(0xFFFFFFFF);
 }
 
+// X0 has special meaning for vsetvl/vsetvli.
+//  rd | rs1 |   AVL value | Effect on vl
+//--------------------------------------------------------------
+// !X0 |  X0 |       VLMAX | Set vl to VLMAX
+//  X0 |  X0 | Value in vl | Keep current vl, just change vtype.
+bool RISCVDAGToDAGISel::selectVLOp(SDValue N, SDValue &VL) {
+  // If the VL value is a constant 0, manually select it to an ADDI with 0
+  // immediate to prevent the default selection path from matching it to X0.
+  auto *C = dyn_cast<ConstantSDNode>(N);
+  if (C && C->isNullValue())
+    VL = SDValue(selectImm(CurDAG, SDLoc(N), 0, Subtarget->getXLenVT()), 0);
+  else
+    VL = N;
+
+  return true;
+}
+
 bool RISCVDAGToDAGISel::selectVSplat(SDValue N, SDValue &SplatVal) {
   if (N.getOpcode() != ISD::SPLAT_VECTOR &&
       N.getOpcode() != RISCVISD::SPLAT_VECTOR_I64)

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.h

@@ -53,6 +53,8 @@ class RISCVDAGToDAGISel : public SelectionDAGISel {
   bool MatchSROIW(SDNode *N) const;
   bool MatchSLLIUW(SDNode *N) const;
 
+  bool selectVLOp(SDValue N, SDValue &VL);
+
   bool selectVSplat(SDValue N, SDValue &SplatVal);
   bool selectVSplatSimm5(SDValue N, SDValue &SplatVal);
   bool selectVSplatUimm5(SDValue N, SDValue &SplatVal);