[AVR] Remove workarounds for instructions using Z register (NFCI) (#156361)

The generated disassembler can now correctly decode these instructions.
All we need to do is to add `bits<0> z` to their encodings and provide a
decoder method that adds Z register to the instruction.

Author: s-barannikov

llvm/lib/Target/AVR/AVRInstrFormats.td

@@ -79,6 +79,7 @@ class FRdRr<bits<4> opcode, bits<2> f, dag outs, dag ins, string asmstr,
 //===----------------------------------------------------------------------===//
 class FZRd<bits<3> t, dag outs, dag ins, string asmstr, list<dag> pattern>
     : AVRInst16<outs, ins, asmstr, pattern> {
+  bits<0> z;
   bits<5> rd;
 
   let Inst{15 - 12} = 0b1001;

llvm/lib/Target/AVR/AVRInstrInfo.td

@@ -1230,7 +1230,9 @@ let Uses = [R1, R0] in {
 
   let Defs = [R31R30] in 
   def SPMZPi : F16<0b1001010111111000, (outs), (ins ZREG:$z), "spm $z+", []>,
-               Requires<[HasSPMX]>;
+               Requires<[HasSPMX]> {
+    bits<0> z;
+  }
 }
 
 // Read data from IO location operations.

llvm/lib/Target/AVR/CMakeLists.txt

@@ -6,8 +6,7 @@ tablegen(LLVM AVRGenAsmMatcher.inc -gen-asm-matcher)
 tablegen(LLVM AVRGenAsmWriter.inc -gen-asm-writer)
 tablegen(LLVM AVRGenCallingConv.inc -gen-callingconv)
 tablegen(LLVM AVRGenDAGISel.inc -gen-dag-isel)
-tablegen(LLVM AVRGenDisassemblerTables.inc -gen-disassembler
-              -ignore-non-decodable-operands)
+tablegen(LLVM AVRGenDisassemblerTables.inc -gen-disassembler)
 tablegen(LLVM AVRGenInstrInfo.inc -gen-instr-info)
 tablegen(LLVM AVRGenMCCodeEmitter.inc -gen-emitter)
 tablegen(LLVM AVRGenRegisterInfo.inc -gen-register-info)

llvm/lib/Target/AVR/Disassembler/AVRDisassembler.cpp

@@ -91,6 +91,12 @@ static DecodeStatus DecodeLD8RegisterClass(MCInst &Inst, unsigned RegNo,
   return MCDisassembler::Success;
 }
 
+static DecodeStatus DecodeZREGRegisterClass(MCInst &Inst,
+                                            const MCDisassembler *Decoder) {
+  Inst.addOperand(MCOperand::createReg(AVR::R31R30));
+  return MCDisassembler::Success;
+}
+
 static DecodeStatus decodeFIOARr(MCInst &Inst, unsigned Insn, uint64_t Address,
                                  const MCDisassembler *Decoder) {
   unsigned addr = 0;

llvm/lib/Target/AVR/MCTargetDesc/AVRInstPrinter.cpp

@@ -101,23 +101,6 @@ const char *AVRInstPrinter::getPrettyRegisterName(MCRegister Reg,
 void AVRInstPrinter::printOperand(const MCInst *MI, unsigned OpNo,
                                   raw_ostream &O) {
   const MCOperandInfo &MOI = this->MII.get(MI->getOpcode()).operands()[OpNo];
-  if (MOI.RegClass == AVR::ZREGRegClassID) {
-    // Special case for the Z register, which sometimes doesn't have an operand
-    // in the MCInst.
-    O << "Z";
-    return;
-  }
-
-  if (OpNo >= MI->size()) {
-    // Not all operands are correctly disassembled at the moment. This means
-    // that some machine instructions won't have all the necessary operands
-    // set.
-    // To avoid asserting, print <unknown> instead until the necessary support
-    // has been implemented.
-    O << "<unknown>";
-    return;
-  }
-
   const MCOperand &Op = MI->getOperand(OpNo);
 
   if (Op.isReg()) {