Add a few comments

Author: s-barannikov

llvm/utils/TableGen/DecoderEmitter.cpp

@@ -1109,6 +1109,9 @@ void DecoderTableBuilder::emitBinaryParser(raw_ostream &OS, indent Indent,
  if (IgnoreNonDecodableOperands)
  return;
  assert(!OpInfo.Decoder.empty());
+ // The operand has no encoding, so the corresponding argument is omitted.
+ // This avoids confusion and allows the function to be overloaded if the
+ // operand does have an encoding in other instructions.
  OS << Indent << "if (!Check(S, " << OpInfo.Decoder << "(MI, Decoder)))\n"
  << Indent << " return MCDisassembler::Fail;\n";
  return;
@@ -1117,6 +1120,8 @@ void DecoderTableBuilder::emitBinaryParser(raw_ostream &OS, indent Indent,
  if (OpInfo.Fields.empty() && OpInfo.InitValue && IgnoreFullyDefinedOperands)
  return;
 
+ // We need to construct the encoding of the operand from pieces if it is not
+ // encoded sequentially or has a non-zero constant part in the encoding.
  bool UseInsertBits = OpInfo.numFields() > 1 || OpInfo.InitValue.value_or(0);
 
  if (UseInsertBits) {
@@ -1989,6 +1994,19 @@ static void addOneOperandFields(const Record *EncodingDef, const BitsInit &Bits,
  }
  }
 
+ // Find out where the variable bits of the operand are encoded.
+ // The bits don't have to be consecutive or in ascending order.
+ // For example, an operand could be encoded as follows:
+ //
+ // 7 6 5 4 3 2 1 0
+ // {1, op{5}, op{2}, op{1}, 0, op{4}, op{3}, ?}
+ //
+ // In this example the operand is encoded in three segments:
+ //
+ // Base Width Offset
+ // op{2...1} 4 2 1
+ // op{4...3} 1 2 3
+ // op{5} 6 1 5
  for (unsigned I = 0, J = 0; I != Bits.getNumBits(); I = J) {
  const VarInit *Var;
  unsigned Offset = 0;