[AVR] Add assembly parser

[dylanmckay]

Summary

This patch adds the AVRAsmParser library.

[dylanmckay]

Ping

[dylanmckay]

Ping

[dylanmckay]

Ping

[simoncook]

lib/Target/AVR/AsmParser/AVRAsmParser.cpp

11

It's not clear why this line is here, I couldn't see any use of std::stringstream, std::stringbuf, etc.?

46

I don't see this being generated. I think you're missing the -gen-asm-matcher line from your CMakeLists.txt.

Likewise, I think you're missing the has_asmparser = 1 part of LLVMBuild.txt

252

You might want to make these two generated by TableGen comments consistent.

404

It's not clear why you're starting a block here?

[dylanmckay]

Code fixups from Simon Cook

[dylanmckay]

lib/Target/AVR/AsmParser/AVRAsmParser.cpp

12

Good catch. It'll be a leftover from when I started copying from the MIPS backend. Fixed.

47

You're right - updated that now.

[kparzysz]

lib/Target/AVR/AsmParser/AVRAsmParser.cpp

31

Local includes usually go first and are sorted alphabetically, if there are no reasons against it.

450

Avoid else after return. Also, the condition could be inverted and made into an early "return true".

564

Avoid else after continue.

[dylanmckay]

Update code review points from Krzysztof

[kparzysz]

LGTM with one remaining inline comment.

lib/Target/AVR/AsmParser/AVRAsmParser.cpp

564

The recommendation from the coding standards is to avoid "else" if the "then" part of an if statement cannot fall through into the following code. So, if there is "if (...) { return; } else { ... }" then the else is not necessary because the else part can only execute is the condition was false regardless of whether the else is there or not. The same applies to continue/break/goto/etc.

Seeing that you switched the code around and indeed avoided else after continue, I realize that the intent of the comment wasn't clear.

[dylanmckay]

Reworked early returns after else

lib/Target/AVR/AsmParser/AVRAsmParser.cpp

564

Have fixed that up now, thanks for the clarification :)

lib/Target/AVR/AsmParser/AVRAsmParser.cpp

+//===---- AVRAsmParser.cpp - Parse AVR assembly to MCInst instructions ----===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "AVR.h"
+#include "AVRRegisterInfo.h"
+#include "MCTargetDesc/AVRMCExpr.h"
+#include "MCTargetDesc/AVRMCTargetDesc.h"
+
+#include "llvm/ADT/APInt.h"
+#include "llvm/ADT/StringSwitch.h"
+#include "llvm/MC/MCContext.h"
+#include "llvm/MC/MCExpr.h"
+#include "llvm/MC/MCInst.h"
+#include "llvm/MC/MCInstBuilder.h"
+#include "llvm/MC/MCStreamer.h"
+#include "llvm/MC/MCSubtargetInfo.h"
+#include "llvm/MC/MCSymbol.h"
+#include "llvm/MC/MCParser/MCAsmLexer.h"
+#include "llvm/MC/MCParser/MCParsedAsmOperand.h"
+#include "llvm/MC/MCParser/MCTargetAsmParser.h"
+#include "llvm/MC/MCValue.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/MathExtras.h"
+#include "llvm/Support/TargetRegistry.h"
+
+#define DEBUG_TYPE "avr-asm-parser"
+
+namespace llvm {
+
+/// Parses AVR assembly from a stream.
+class AVRAsmParser : public MCTargetAsmParser {
+  const MCSubtargetInfo &STI;
+  MCAsmParser &Parser;
+  const MCRegisterInfo *MRI;
+
+#define GET_ASSEMBLER_HEADER
+#include "AVRGenAsmMatcher.inc"
+
+  bool MatchAndEmitInstruction(SMLoc IDLoc, unsigned &Opcode,
+                               OperandVector &Operands, MCStreamer &Out,
+                               uint64_t &ErrorInfo,
+                               bool MatchingInlineAsm) override;
+
+  bool ParseRegister(unsigned &RegNo, SMLoc &StartLoc, SMLoc &EndLoc) override;
+
+  bool ParseInstruction(ParseInstructionInfo &Info, StringRef Name,
+                        SMLoc NameLoc, OperandVector &Operands) override;
+
+  bool ParseDirective(AsmToken directiveID) override;
+
+  OperandMatchResultTy parseMemriOperand(OperandVector &Operands);
+
+  bool parseOperand(OperandVector &Operands);
+  int parseRegisterName(unsigned (*matchFn)(StringRef));
+  int parseRegisterName();
+  int parseRegister();
+  bool tryParseRegisterOperand(OperandVector &Operands);
+  bool tryParseExpression(OperandVector &Operands);
+  bool tryParseRelocExpression(OperandVector &Operands);
+  void eatComma();
+
+  unsigned validateTargetOperandClass(MCParsedAsmOperand &Op,
+                                      unsigned Kind) override;
+
+  unsigned toDREG(unsigned Reg, unsigned From = AVR::sub_lo) {
+    MCRegisterClass const *Class = &AVRMCRegisterClasses[AVR::DREGSRegClassID];
+    return MRI->getMatchingSuperReg(Reg, From, Class);
+  }
+
+  bool emit(MCInst &Instruction, SMLoc const &Loc, MCStreamer &Out) const;
+  bool invalidOperand(SMLoc const &Loc, OperandVector const &Operands,
+                      uint64_t const &ErrorInfo);
+  bool missingFeature(SMLoc const &Loc, uint64_t const &ErrorInfo);
+
+public:
+  AVRAsmParser(const MCSubtargetInfo &STI, MCAsmParser &Parser,
+               const MCInstrInfo &MII, const MCTargetOptions &Options)
+      : MCTargetAsmParser(Options, STI), STI(STI), Parser(Parser) {
+    MCAsmParserExtension::Initialize(Parser);
+    MRI = getContext().getRegisterInfo();
+
+    setAvailableFeatures(ComputeAvailableFeatures(STI.getFeatureBits()));
+  }
+
+  MCAsmParser &getParser() const { return Parser; }
+  MCAsmLexer &getLexer() const { return Parser.getLexer(); }
+};
+
+/// An parsed AVR assembly operand.
+class AVROperand : public MCParsedAsmOperand {
+  typedef MCParsedAsmOperand Base;
+  enum KindTy { k_Immediate, k_Register, k_Token, k_Memri } Kind;
+
+public:
+  AVROperand(StringRef Tok, SMLoc const &S)
+      : Base(), Kind(k_Token), Tok(Tok), Start(S), End(S) {}
+  AVROperand(unsigned Reg, SMLoc const &S, SMLoc const &E)
+      : Base(), Kind(k_Register), RegImm({Reg, nullptr}), Start(S), End(E) {}
+  AVROperand(MCExpr const *Imm, SMLoc const &S, SMLoc const &E)
+      : Base(), Kind(k_Immediate), RegImm({0, Imm}), Start(S), End(E) {}
+  AVROperand(unsigned Reg, MCExpr const *Imm, SMLoc const &S, SMLoc const &E)
+      : Base(), Kind(k_Memri), RegImm({Reg, Imm}), Start(S), End(E) {}
+
+  struct RegisterImmediate {
+    unsigned Reg;
+    MCExpr const *Imm;
+  };
+  union {
+    StringRef Tok;
+    RegisterImmediate RegImm;
+  };
+
+  SMLoc Start, End;
+
+public:
+  void addRegOperands(MCInst &Inst, unsigned N) const {
+    assert(Kind == k_Register && "Unexpected operand kind");
+    assert(N == 1 && "Invalid number of operands!");
+
+    Inst.addOperand(MCOperand::createReg(getReg()));
+  }
+
+  void addExpr(MCInst &Inst, const MCExpr *Expr) const {
+    // Add as immediate when possible
+    if (!Expr)
+      Inst.addOperand(MCOperand::createImm(0));
+    else if (const MCConstantExpr *CE = dyn_cast<MCConstantExpr>(Expr))
+      Inst.addOperand(MCOperand::createImm(CE->getValue()));
+    else
+      Inst.addOperand(MCOperand::createExpr(Expr));
+  }
+
+  void addImmOperands(MCInst &Inst, unsigned N) const {
+    assert(Kind == k_Immediate && "Unexpected operand kind");
+    assert(N == 1 && "Invalid number of operands!");
+
+    const MCExpr *Expr = getImm();
+    addExpr(Inst, Expr);
+  }
+
+  /// Adds the contained reg+imm operand to an instruction.
+  void addMemriOperands(MCInst &Inst, unsigned N) const {
+    assert(Kind == k_Memri && "Unexpected operand kind");
+    assert(N == 2 && "Invalid number of operands");
+
+    Inst.addOperand(MCOperand::createReg(getReg()));
+    addExpr(Inst, getImm());
+  }
+
+  bool isReg() const { return Kind == k_Register; }
+  bool isImm() const { return Kind == k_Immediate; }
+  bool isToken() const { return Kind == k_Token; }
+  bool isMem() const { return Kind == k_Memri; }
+  bool isMemri() const { return Kind == k_Memri; }
+
+  StringRef getToken() const {
+    assert(Kind == k_Token && "Invalid access!");
+    return Tok;
+  }
+
+  unsigned getReg() const {
+    assert((Kind == k_Register || Kind == k_Memri) && "Invalid access!");
+
+    return RegImm.Reg;
+  }
+
+  const MCExpr *getImm() const {
+    assert((Kind == k_Immediate || Kind == k_Memri) && "Invalid access!");
+    return RegImm.Imm;
+  }
+
+  static std::unique_ptr<AVROperand> CreateToken(StringRef Str, SMLoc S) {
+    return make_unique<AVROperand>(Str, S);
+  }
+
+  static std::unique_ptr<AVROperand> CreateReg(unsigned RegNum, SMLoc S,
+                                               SMLoc E) {
+    return make_unique<AVROperand>(RegNum, S, E);
+  }
+
+  static std::unique_ptr<AVROperand> CreateImm(const MCExpr *Val, SMLoc S,
+                                               SMLoc E) {
+    return make_unique<AVROperand>(Val, S, E);
+  }
+
+  static std::unique_ptr<AVROperand>
+  CreateMemri(unsigned RegNum, const MCExpr *Val, SMLoc S, SMLoc E) {
+    return make_unique<AVROperand>(RegNum, Val, S, E);
+  }
+
+  void makeToken(StringRef Token) {
+    Kind = k_Token;
+    Tok = Token;
+  }
+
+  void makeReg(unsigned RegNo) {
+    Kind = k_Register;
+    RegImm = {RegNo, nullptr};
+  }
+
+  void makeImm(MCExpr const *Ex) {
+    Kind = k_Immediate;
+    RegImm = {0, Ex};
+  }
+
+  void makeMemri(unsigned RegNo, MCExpr const *Imm) {
+    Kind = k_Memri;
+    RegImm = {RegNo, Imm};
+  }
+
+  SMLoc getStartLoc() const { return Start; }
+  SMLoc getEndLoc() const { return End; }
+
+  virtual void print(raw_ostream &O) const {
+    switch (Kind) {
+    case k_Token:
+      O << "Token: \"" << getToken() << "\"";
+      break;
+    case k_Register:
+      O << "Register: " << getReg();
+      break;
+    case k_Immediate:
+      O << "Immediate: \"" << *getImm() << "\"";
+      break;
+    case k_Memri: {
+      // only manually print the size for non-negative values,
+      // as the sign is inserted automatically.
+      O << "Memri: \"" << getReg() << '+' << *getImm() << "\"";
+      break;
+    }
+    }
+    O << "\n";
+  }
+};
+
+// Auto-generated Match Functions
+
+/// Maps from the set of all register names to a register number.
+/// \note Generated by TableGen.
+static unsigned MatchRegisterName(StringRef Name);
+
+/// Maps from the set of all alternative registernames to a register number.
+/// \note Generated by TableGen.
+static unsigned MatchRegisterAltName(StringRef Name);
+
+bool AVRAsmParser::invalidOperand(SMLoc const &Loc,
+                                  OperandVector const &Operands,
+                                  uint64_t const &ErrorInfo) {
+  SMLoc ErrorLoc = Loc;
+  char const *Diag = 0;
+
+  if (ErrorInfo != ~0U) {
+    if (ErrorInfo >= Operands.size()) {
+      Diag = "too few operands for instruction.";
+    } else {
+      AVROperand const &Op = (AVROperand const &)*Operands[ErrorInfo];
+
+      // TODO: See if we can do a better error than just "invalid ...".
+      if (Op.getStartLoc() != SMLoc()) {
+        ErrorLoc = Op.getStartLoc();
+      }
+    }
+  }
+
+  if (!Diag) {
+    Diag = "invalid operand for instruction";
+  }
+
+  return Error(ErrorLoc, Diag);
+}
+
+bool AVRAsmParser::missingFeature(llvm::SMLoc const &Loc,
+                                  uint64_t const &ErrorInfo) {
+  return Error(Loc, "instruction requires a CPU feature not currently enabled");
+}
+
+bool AVRAsmParser::emit(MCInst &Inst, SMLoc const &Loc, MCStreamer &Out) const {
+  Inst.setLoc(Loc);
+  Out.EmitInstruction(Inst, STI);
+
+  return false;
+}
+
+bool AVRAsmParser::MatchAndEmitInstruction(SMLoc Loc, unsigned &Opcode,
+                                           OperandVector &Operands,
+                                           MCStreamer &Out, uint64_t &ErrorInfo,
+                                           bool MatchingInlineAsm) {
+  MCInst Inst;
+  unsigned MatchResult =
+      MatchInstructionImpl(Operands, Inst, ErrorInfo, MatchingInlineAsm);
+
+  switch (MatchResult) {
+  case Match_Success:        return emit(Inst, Loc, Out);
+  case Match_MissingFeature: return missingFeature(Loc, ErrorInfo);
+  case Match_InvalidOperand: return invalidOperand(Loc, Operands, ErrorInfo);
+  case Match_MnemonicFail:   return Error(Loc, "invalid instruction");
+  default:                   return true;
+  }
+}
+
+/// Parses a register name using a given matching function.
+/// Checks for lowercase or uppercase if necessary.
+int AVRAsmParser::parseRegisterName(unsigned (*matchFn)(StringRef)) {
+  StringRef Name = Parser.getTok().getString();
+
+  int RegNum = matchFn(Name);
+
+  // GCC supports case insensitive register names. Some of the AVR registers
+  // are all lower case, some are all upper case but non are mixed. We prefer
+  // to use the original names in the register definitions. That is why we
+  // have to test both upper and lower case here.
+  if (RegNum == AVR::NoRegister) {
+    RegNum = matchFn(Name.lower());
+  }
+  if (RegNum == AVR::NoRegister) {
+    RegNum = matchFn(Name.upper());
+  }
+
+  return RegNum;
+}
+
+int AVRAsmParser::parseRegisterName() {
+  int RegNum = parseRegisterName(&MatchRegisterName);
+
+  if (RegNum == AVR::NoRegister)
+    RegNum = parseRegisterName(&MatchRegisterAltName);
+
+  return RegNum;
+}
+
+int AVRAsmParser::parseRegister() {
+  int RegNum = AVR::NoRegister;
+
+  if (Parser.getTok().is(AsmToken::Identifier)) {
+    // Check for register pair syntax
+    if (Parser.getLexer().peekTok().is(AsmToken::Colon)) {
+      Parser.Lex();
+      Parser.Lex(); // Eat high (odd) register and colon
+
+      if (Parser.getTok().is(AsmToken::Identifier)) {
+        // Convert lower (even) register to DREG
+        RegNum = toDREG(parseRegisterName());
+      }
+    } else {
+      RegNum = parseRegisterName();
+    }
+  }
+  return RegNum;
+}
+
+bool AVRAsmParser::tryParseRegisterOperand(OperandVector &Operands) {
+  int RegNo = parseRegister();
+
+  if (RegNo == AVR::NoRegister)
+    return true;
+
+  AsmToken const &T = Parser.getTok();
+  Operands.push_back(AVROperand::CreateReg(RegNo, T.getLoc(), T.getEndLoc()));
+  Parser.Lex(); // Eat register token.
+
+  return false;
+}
+
+bool AVRAsmParser::tryParseExpression(OperandVector &Operands) {
+  SMLoc S = Parser.getTok().getLoc();
+
+  if (!tryParseRelocExpression(Operands))
+    return false;
+
+  if ((Parser.getTok().getKind() == AsmToken::Plus ||
+       Parser.getTok().getKind() == AsmToken::Minus) &&
+      Parser.getLexer().peekTok().getKind() == AsmToken::Identifier) {
+    // Don't handle this case - it should be split into two
+    // separate tokens.
+    return true;
+  }
+
+  // Parse (potentially inner) expression
+  MCExpr const *Expression;
+  if (getParser().parseExpression(Expression))
+    return true;
+
+  SMLoc E = SMLoc::getFromPointer(Parser.getTok().getLoc().getPointer() - 1);
+  Operands.push_back(AVROperand::CreateImm(Expression, S, E));
+  return false;
+}
+
+bool AVRAsmParser::tryParseRelocExpression(OperandVector &Operands) {
+  bool isNegated = false;
+  AVRMCExpr::VariantKind ModifierKind = AVRMCExpr::VK_AVR_None;
+
+  SMLoc S = Parser.getTok().getLoc();
+
+  // Check for sign
+  AsmToken tokens[2];
+  size_t ReadCount = Parser.getLexer().peekTokens(tokens);
+
+  if (ReadCount == 2) {
+    if (tokens[0].getKind() == AsmToken::Identifier &&
+        tokens[1].getKind() == AsmToken::LParen) {
+
+      AsmToken::TokenKind CurTok = Parser.getLexer().getKind();
+      if (CurTok == AsmToken::Minus) {
+        isNegated = true;
+      } else {
+        assert(CurTok == AsmToken::Plus);
+        isNegated = false;
+      }
+
+      // Eat the sign
+      Parser.Lex();
+    }
+  }
+
+  // Check if we have a target specific modifier (lo8, hi8, &c)
+  if (Parser.getTok().getKind() != AsmToken::Identifier ||
+      Parser.getLexer().peekTok().getKind() != AsmToken::LParen) {
+    // Not a reloc expr
+    return true;
+  } else {
+    StringRef ModifierName = Parser.getTok().getString();
+    ModifierKind = AVRMCExpr::getKindByName(ModifierName.str().c_str());
+
+    if (ModifierKind != AVRMCExpr::VK_AVR_None) {
+      Parser.Lex();
+      Parser.Lex(); // Eat modifier name and parenthesis
+    } else {
+      return Error(Parser.getTok().getLoc(), "unknown modifier");
+    }
+
+    MCExpr const *InnerExpression;
+    if (getParser().parseExpression(InnerExpression))
+      return true;
+
+    // If we have a modifier wrap the inner expression
+    assert(Parser.getTok().getKind() == AsmToken::RParen);
+    Parser.Lex(); // Eat closing parenthesis
+
+    MCExpr const *Expression = AVRMCExpr::create(ModifierKind, InnerExpression,
+                                                 isNegated, getContext());
+
+    SMLoc E = SMLoc::getFromPointer(Parser.getTok().getLoc().getPointer() - 1);
+    Operands.push_back(AVROperand::CreateImm(Expression, S, E));
+
+    return false;
+  }
+}
+
+bool AVRAsmParser::parseOperand(OperandVector &Operands) {
+  DEBUG(dbgs() << "parseOperand\n");
+
+  switch (getLexer().getKind()) {
+  default:
+    return Error(Parser.getTok().getLoc(), "unexpected token in operand");
+
+  case AsmToken::Identifier:
+    // Try to parse a register, if it fails,
+    // fall through to the next case.
+    if (!tryParseRegisterOperand(Operands)) {
+      return false;
+    }
+  case AsmToken::LParen:
+  case AsmToken::Integer:
+  case AsmToken::Dot:
+    return tryParseExpression(Operands);
+  case AsmToken::Plus:
+  case AsmToken::Minus: {
+    // If the sign preceeds a number, parse the number,
+    // otherwise treat the sign a an independent token.
+    switch (getLexer().peekTok().getKind()) {
+    case AsmToken::Integer:
+    case AsmToken::BigNum:
+    case AsmToken::Identifier:
+    case AsmToken::Real:
+      if (!tryParseExpression(Operands))
+        return false;
+    default:
+      break;
+    }
+    // Treat the token as an independent token.
+    Operands.push_back(AVROperand::CreateToken(Parser.getTok().getString(),
+                                               Parser.getTok().getLoc()));
+    Parser.Lex(); // Eat the token.
+    return false;
+  }
+  }
+
+  // Could not parse operand
+  return true;
+}
+
+AVRAsmParser::OperandMatchResultTy
+AVRAsmParser::parseMemriOperand(OperandVector &Operands) {
+  DEBUG(dbgs() << "parseMemriOperand()\n");
+
+  SMLoc E, S;
+  MCExpr const *Expression;
+  int RegNo;
+
+  // Parse register.
+  {
+    RegNo = parseRegister();
+
+    if (RegNo == AVR::NoRegister)
+      return MatchOperand_ParseFail;
+
+    S = SMLoc::getFromPointer(Parser.getTok().getLoc().getPointer() - 1);
+    Parser.Lex(); // Eat register token.
+  }
+
+  // Parse immediate;
+  {
+    if (getParser().parseExpression(Expression))
+      return MatchOperand_ParseFail;
+
+    E = SMLoc::getFromPointer(Parser.getTok().getLoc().getPointer() - 1);
+  }
+
+  Operands.push_back(AVROperand::CreateMemri(RegNo, Expression, S, E));
+
+  return MatchOperand_Success;
+}
+
+bool AVRAsmParser::ParseRegister(unsigned &RegNo, SMLoc &StartLoc,
+                                 SMLoc &EndLoc) {
+  StartLoc = Parser.getTok().getLoc();
+  RegNo = parseRegister();
+  EndLoc = Parser.getTok().getLoc();
+
+  return (RegNo == AVR::NoRegister);
+}
+
+void AVRAsmParser::eatComma() {
+  if (getLexer().is(AsmToken::Comma)) {
+    Parser.Lex();
+  } else {
+    // GCC allows commas to be omitted.
+  }
+}
+
+bool AVRAsmParser::ParseInstruction(ParseInstructionInfo &Info,
+                                    StringRef Mnemonic, SMLoc NameLoc,
+                                    OperandVector &Operands) {
+  Operands.push_back(AVROperand::CreateToken(Mnemonic, NameLoc));
+
+  bool first = true;
+  while (getLexer().isNot(AsmToken::EndOfStatement)) {
+    if (!first) eatComma();
+
+    first = false;
+
+    auto MatchResult = MatchOperandParserImpl(Operands, Mnemonic);
+
+    if (MatchResult == MatchOperand_ParseFail) {
+      SMLoc Loc = getLexer().getLoc();
+      Parser.eatToEndOfStatement();
+
+      return Error(Loc, "failed to parse register and immediate pair");
+    } else if (MatchResult == MatchOperand_Success) {
+      continue;
+    }
+
+    if (parseOperand(Operands)) {
+      SMLoc Loc = getLexer().getLoc();
+      Parser.eatToEndOfStatement();
+      return Error(Loc, "unexpected token in argument list");
+    }
+  }
+  Parser.Lex(); // Consume the EndOfStatement
+  return false;
+}
+
+bool AVRAsmParser::ParseDirective(llvm::AsmToken DirectiveID) { return true; }
+
+extern "C" void LLVMInitializeAVRAsmParser() {
+  RegisterMCAsmParser<AVRAsmParser> X(TheAVRTarget);
+}
+
+#define GET_REGISTER_MATCHER
+#define GET_MATCHER_IMPLEMENTATION
+#include "AVRGenAsmMatcher.inc"
+
+// Uses enums defined in AVRGenAsmMatcher.inc
+unsigned AVRAsmParser::validateTargetOperandClass(MCParsedAsmOperand &AsmOp,
+                                                  unsigned ExpectedKind) {
+  AVROperand &Op = static_cast<AVROperand &>(AsmOp);
+  MatchClassKind Expected = static_cast<MatchClassKind>(ExpectedKind);
+
+  // If need be, GCC converts bare numbers to register names
+  // It's ugly, but GCC supports it.
+  if (Op.isImm()) {
+    if (MCConstantExpr const *Const = dyn_cast<MCConstantExpr>(Op.getImm())) {
+      int64_t RegNum = Const->getValue();
+      std::ostringstream RegName;
+      RegName << "r" << RegNum;
+      RegNum = MatchRegisterName(RegName.str().c_str());
+      if (RegNum != AVR::NoRegister) {
+        Op.makeReg(RegNum);
+        if (validateOperandClass(Op, Expected) == Match_Success) {
+          return Match_Success;
+        }
+      }
+      // Let the other quirks try their magic.
+    }
+  }
+
+  if (Op.isReg()) {
+    // If the instruction uses a register pair but we got a single, lower
+    // register we perform a "class cast".
+    if (isSubclass(Expected, MCK_DREGS)) {
+      unsigned correspondingDREG = toDREG(Op.getReg());
+
+      if (correspondingDREG != AVR::NoRegister) {
+        Op.makeReg(correspondingDREG);
+        return validateOperandClass(Op, Expected);
+      }
+    }
+  }
+  return Match_InvalidOperand;
+}
+
+} // end of namespace llvm

lib/Target/AVR/AsmParser/CMakeLists.txt

+add_llvm_library(LLVMAVRAsmParser
+  AVRAsmParser.cpp
+)

lib/Target/AVR/AsmParser/LLVMBuild.txt

+;===- ./lib/Target/AVR/AsmParser/LLVMBuild.txt -----------------*- Conf -*--===;
+;
+;                     The LLVM Compiler Infrastructure
+;
+; This file is distributed under the University of Illinois Open Source
+; License. See LICENSE.TXT for details.
+;
+;===------------------------------------------------------------------------===;
+;
+; This is an LLVMBuild description file for the components in this subdirectory.
+;
+; For more information on the LLVMBuild system, please see:
+;
+;   http://llvm.org/docs/LLVMBuild.html
+;
+;===------------------------------------------------------------------------===;
+
+[component_0]
+type = Library
+name = AVRAsmParser
+parent = AVR
+required_libraries = MC MCParser AVRDesc AVRInfo Support
+add_to_library_groups = AVR

lib/Target/AVR/CMakeLists.txt

 set(LLVM_TARGET_DEFINITIONS AVR.td)
 
+tablegen(LLVM AVRGenAsmMatcher.inc -gen-asm-matcher)
 tablegen(LLVM AVRGenRegisterInfo.inc -gen-register-info)
 tablegen(LLVM AVRGenInstrInfo.inc -gen-instr-info)
 tablegen(LLVM AVRGenCallingConv.inc -gen-callingconv)

 
 add_subdirectory(MCTargetDesc)
 add_subdirectory(TargetInfo)
+add_subdirectory(AsmParser)
 

lib/Target/AVR/LLVMBuild.txt

 name = AVR
 parent = Target
 has_asmprinter = 0
-has_asmparser = 0
+has_asmparser = 1
 
 [component_1]
 type = Library