[AVR] Don't assert on an undefined operand

[aykevl]

Summary

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 an error instead until the necessary support has been implemented.

[dylanmckay]

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

103

Add a code comment above this, your commit message should do nicely

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 an error instead until the necessary support has been implemented.

138

Recommend duplicating the code comment here; the extra verbosity is worth the extra clarity when we come to inevitably remove it.

[aykevl]

Added comments, as requested.

I agree, this makes it clearer why this check is there in the first place.

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

if (MRI.getNumSubRegIndices() > 0) {
     RegNum = (RegLoNum != AVR::NoRegister) ? RegLoNum : RegNum;
   }
 
   return getRegisterName(RegNum);
 }
 
 void AVRInstPrinter::printOperand(const MCInst *MI, unsigned OpNo,
                                   raw_ostream &O) {
+  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);
   const MCOperandInfo &MOI = this->MII.get(MI->getOpcode()).OpInfo[OpNo];
 
   if (Op.isReg()) {
     bool isPtrReg = (MOI.RegClass == AVR::PTRREGSRegClassID) ||
                     (MOI.RegClass == AVR::PTRDISPREGSRegClassID) ||
                     (MOI.RegClass == AVR::ZREGRegClassID);
 
if (Op.isReg()) {
     O << *Op.getExpr();
   }
 }
 
 /// This is used to print an immediate value that ends up
 /// being encoded as a pc-relative value.
 void AVRInstPrinter::printPCRelImm(const MCInst *MI, unsigned OpNo,
                                    raw_ostream &O) {
+  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.isImm()) {
     int64_t Imm = Op.getImm();
     O << '.';
 
     // Print a position sign if needed.
     // Negative values have their sign printed automatically.