//===- TGLexer.cpp - Lexer for TableGen -----------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// Implement the Lexer for TableGen.
//
//===----------------------------------------------------------------------===//

#include "TGLexer.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/StringExtras.h"
#include "llvm/ADT/StringSwitch.h"
#include "llvm/ADT/Twine.h"
#include "llvm/Config/config.h" // for strtoull()/strtoll() define
#include "llvm/Support/Compiler.h"
#include "llvm/Support/MemoryBuffer.h"
#include "llvm/Support/SourceMgr.h"
#include "llvm/TableGen/Error.h"
#include <cerrno>
#include <cstdio>
#include <cstdlib>
#include <cstring>

using namespace llvm;

namespace {
// A list of supported preprocessing directives with their
// internal token kinds and names.
struct PreprocessorDir {
  tgtok::TokKind Kind;
  StringRef Word;
};
} // end anonymous namespace

/// Returns true if `C` is a valid character in an identifier. If `First` is
/// true, returns true if `C` is a valid first character of an identifier,
/// else returns true if `C` is a valid non-first character of an identifier.
/// Identifiers match the following regular expression:
///   [a-zA-Z_][0-9a-zA-Z_]*
static bool isValidIDChar(char C, bool First) {
  if (C == '_' || isAlpha(C))
    return true;
  return !First && isDigit(C);
}

constexpr PreprocessorDir PreprocessorDirs[] = {{tgtok::Ifdef, "ifdef"},
                                                {tgtok::Ifndef, "ifndef"},
                                                {tgtok::Else, "else"},
                                                {tgtok::Endif, "endif"},
                                                {tgtok::Define, "define"}};

// Returns a pointer past the end of a valid macro name at the start of `Str`.
// Valid macro names match the regular expression [a-zA-Z_][0-9a-zA-Z_]*.
static const char *lexMacroName(StringRef Str) {
  assert(!Str.empty());

  // Macro names start with [a-zA-Z_].
  const char *Next = Str.begin();
  if (!isValidIDChar(*Next, /*First=*/true))
    return Next;
  // Eat the first character of the name.
  ++Next;

  // Match the rest of the identifier regex: [0-9a-zA-Z_]*
  const char *End = Str.end();
  while (Next != End && isValidIDChar(*Next, /*First=*/false))
    ++Next;
  return Next;
}

TGLexer::TGLexer(SourceMgr &SM, ArrayRef<std::string> Macros) : SrcMgr(SM) {
  CurBuffer = SrcMgr.getMainFileID();
  CurBuf = SrcMgr.getMemoryBuffer(CurBuffer)->getBuffer();
  CurPtr = CurBuf.begin();
  TokStart = nullptr;

  // Pretend that we enter the "top-level" include file.
  PrepIncludeStack.emplace_back();

  // Add all macros defined on the command line to the DefinedMacros set.
  // Check invalid macro names and print fatal error if we find one.
  for (StringRef MacroName : Macros) {
    const char *End = lexMacroName(MacroName);
    if (End != MacroName.end())
      PrintFatalError("invalid macro name `" + MacroName +
                      "` specified on command line");

    DefinedMacros.insert(MacroName);
  }
}

SMLoc TGLexer::getLoc() const { return SMLoc::getFromPointer(TokStart); }

SMRange TGLexer::getLocRange() const {
  return {getLoc(), SMLoc::getFromPointer(CurPtr)};
}

/// ReturnError - Set the error to the specified string at the specified
/// location. This is defined to always return tgtok::Error.
tgtok::TokKind TGLexer::ReturnError(SMLoc Loc, const Twine &Msg) {
  PrintError(Loc, Msg);
  return tgtok::Error;
}

tgtok::TokKind TGLexer::ReturnError(const char *Loc, const Twine &Msg) {
  return ReturnError(SMLoc::getFromPointer(Loc), Msg);
}

bool TGLexer::processEOF() {
  SMLoc ParentIncludeLoc = SrcMgr.getParentIncludeLoc(CurBuffer);
  if (ParentIncludeLoc != SMLoc()) {
    // If prepExitInclude() detects a problem with the preprocessing
    // control stack, it will return false. Pretend that we reached
    // the final EOF and stop lexing more tokens by returning false
    // to LexToken().
    if (!prepExitInclude(false))
      return false;

    CurBuffer = SrcMgr.FindBufferContainingLoc(ParentIncludeLoc);
    CurBuf = SrcMgr.getMemoryBuffer(CurBuffer)->getBuffer();
    CurPtr = ParentIncludeLoc.getPointer();
    // Make sure TokStart points into the parent file's buffer.
    // LexToken() assigns to it before calling getNextChar(),
    // so it is pointing into the included file now.
    TokStart = CurPtr;
    return true;
  }

  // Pretend that we exit the "top-level" include file.
  // Note that in case of an error (e.g. control stack imbalance)
  // the routine will issue a fatal error.
  prepExitInclude(true);
  return false;
}

int TGLexer::getNextChar() {
  char CurChar = *CurPtr++;
  switch (CurChar) {
  default:
    return (unsigned char)CurChar;

  case 0: {
    // A NUL character in the stream is either the end of the current buffer or
    // a spurious NUL in the file. Disambiguate that here.
    if (CurPtr - 1 == CurBuf.end()) {
      --CurPtr; // Arrange for another call to return EOF again.
      return EOF;
    }
    PrintError(getLoc(),
               "NUL character is invalid in source; treated as space");
    return ' ';
  }

  case '\n':
  case '\r':
    // Handle the newline character by ignoring it and incrementing the line
    // count. However, be careful about 'dos style' files with \n\r in them.
    // Only treat a \n\r or \r\n as a single line.
    if ((*CurPtr == '\n' || (*CurPtr == '\r')) && *CurPtr != CurChar)
      ++CurPtr; // Eat the two char newline sequence.
    return '\n';
  }
}

int TGLexer::peekNextChar(int Index) const { return *(CurPtr + Index); }

tgtok::TokKind TGLexer::LexToken(bool FileOrLineStart) {
  while (true) {
    TokStart = CurPtr;
    // This always consumes at least one character.
    int CurChar = getNextChar();

    switch (CurChar) {
    default:
      // Handle letters: [a-zA-Z_]
      if (isValidIDChar(CurChar, /*First=*/true))
        return LexIdentifier();

      // Unknown character, emit an error.
      return ReturnError(TokStart, "unexpected character");
    case EOF:
      // Lex next token, if we just left an include file.
      if (processEOF()) {
        // Leaving an include file means that the next symbol is located at the
        // end of the 'include "..."' construct.
        FileOrLineStart = false;
        break;
      }

      // Return EOF denoting the end of lexing.
      return tgtok::Eof;

    case ':':
      return tgtok::colon;
    case ';':
      return tgtok::semi;
    case ',':
      return tgtok::comma;
    case '<':
      return tgtok::less;
    case '>':
      return tgtok::greater;
    case ']':
      return tgtok::r_square;
    case '{':
      return tgtok::l_brace;
    case '}':
      return tgtok::r_brace;
    case '(':
      return tgtok::l_paren;
    case ')':
      return tgtok::r_paren;
    case '=':
      return tgtok::equal;
    case '?':
      return tgtok::question;
    case '#':
      if (FileOrLineStart) {
        tgtok::TokKind Kind = prepIsDirective();
        if (Kind != tgtok::Error)
          return lexPreprocessor(Kind);
      }

      return tgtok::paste;

      // The period is a separate case so we can recognize the "..."
      // range punctuator.
    case '.':
      if (peekNextChar(0) == '.') {
        ++CurPtr; // Eat second dot.
        if (peekNextChar(0) == '.') {
          ++CurPtr; // Eat third dot.
          return tgtok::dotdotdot;
        }
        return ReturnError(TokStart, "invalid '..' punctuation");
      }
      return tgtok::dot;

    case '\r':
      llvm_unreachable("getNextChar() must never return '\r'");

    case ' ':
    case '\t':
      // Ignore whitespace.
      break;
    case '\n':
      // Ignore whitespace, and identify the new line.
      FileOrLineStart = true;
      break;
    case '/':
      // If this is the start of a // comment, skip until the end of the line or
      // the end of the buffer.
      if (*CurPtr == '/')
        SkipBCPLComment();
      else if (*CurPtr == '*') {
        if (SkipCComment())
          return tgtok::Error;
      } else // Otherwise, this is an error.
        return ReturnError(TokStart, "unexpected character");
      break;
    case '-':
    case '+':
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9': {
      int NextChar = 0;
      if (isDigit(CurChar)) {
        // Allow identifiers to start with a number if it is followed by
        // an identifier.  This can happen with paste operations like
        // foo#8i.
        int i = 0;
        do {
          NextChar = peekNextChar(i++);
        } while (isDigit(NextChar));

        if (NextChar == 'x' || NextChar == 'b') {
          // If this is [0-9]b[01] or [0-9]x[0-9A-fa-f] this is most
          // likely a number.
          int NextNextChar = peekNextChar(i);
          switch (NextNextChar) {
          default:
            break;
          case '0':
          case '1':
            if (NextChar == 'b')
              return LexNumber();
            [[fallthrough]];
          case '2':
          case '3':
          case '4':
          case '5':
          case '6':
          case '7':
          case '8':
          case '9':
          case 'a':
          case 'b':
          case 'c':
          case 'd':
          case 'e':
          case 'f':
          case 'A':
          case 'B':
          case 'C':
          case 'D':
          case 'E':
          case 'F':
            if (NextChar == 'x')
              return LexNumber();
            break;
          }
        }
      }

      if (isValidIDChar(NextChar, /*First=*/true))
        return LexIdentifier();

      return LexNumber();
    }
    case '"':
      return LexString();
    case '$':
      return LexVarName();
    case '[':
      return LexBracket();
    case '!':
      return LexExclaim();
    }
  }
}

/// LexString - Lex "[^"]*"
tgtok::TokKind TGLexer::LexString() {
  const char *StrStart = CurPtr;

  CurStrVal = "";

  while (*CurPtr != '"') {
    // If we hit the end of the buffer, report an error.
    if (*CurPtr == 0 && CurPtr == CurBuf.end())
      return ReturnError(StrStart, "end of file in string literal");

    if (*CurPtr == '\n' || *CurPtr == '\r')
      return ReturnError(StrStart, "end of line in string literal");

    if (*CurPtr != '\\') {
      CurStrVal += *CurPtr++;
      continue;
    }

    ++CurPtr;

    switch (*CurPtr) {
    case '\\':
    case '\'':
    case '"':
      // These turn into their literal character.
      CurStrVal += *CurPtr++;
      break;
    case 't':
      CurStrVal += '\t';
      ++CurPtr;
      break;
    case 'n':
      CurStrVal += '\n';
      ++CurPtr;
      break;

    case '\n':
    case '\r':
      return ReturnError(CurPtr, "escaped newlines not supported in tblgen");

    // If we hit the end of the buffer, report an error.
    case '\0':
      if (CurPtr == CurBuf.end())
        return ReturnError(StrStart, "end of file in string literal");
      [[fallthrough]];
    default:
      return ReturnError(CurPtr, "invalid escape in string literal");
    }
  }

  ++CurPtr;
  return tgtok::StrVal;
}

tgtok::TokKind TGLexer::LexVarName() {
  if (!isValidIDChar(CurPtr[0], /*First=*/true))
    return ReturnError(TokStart, "invalid variable name");

  // Otherwise, we're ok, consume the rest of the characters.
  const char *VarNameStart = CurPtr++;

  while (isValidIDChar(*CurPtr, /*First=*/false))
    ++CurPtr;

  CurStrVal.assign(VarNameStart, CurPtr);
  return tgtok::VarName;
}

tgtok::TokKind TGLexer::LexIdentifier() {
  // The first letter is [a-zA-Z_].
  const char *IdentStart = TokStart;

  // Match the rest of the identifier regex: [0-9a-zA-Z_]*
  while (isValidIDChar(*CurPtr, /*First=*/false))
    ++CurPtr;

  // Check to see if this identifier is a reserved keyword.
  StringRef Str(IdentStart, CurPtr - IdentStart);

  tgtok::TokKind Kind = StringSwitch<tgtok::TokKind>(Str)
                            .Case("int", tgtok::Int)
                            .Case("bit", tgtok::Bit)
                            .Case("bits", tgtok::Bits)
                            .Case("string", tgtok::String)
                            .Case("list", tgtok::List)
                            .Case("code", tgtok::Code)
                            .Case("dag", tgtok::Dag)
                            .Case("class", tgtok::Class)
                            .Case("def", tgtok::Def)
                            .Case("true", tgtok::TrueVal)
                            .Case("false", tgtok::FalseVal)
                            .Case("foreach", tgtok::Foreach)
                            .Case("defm", tgtok::Defm)
                            .Case("defset", tgtok::Defset)
                            .Case("deftype", tgtok::Deftype)
                            .Case("multiclass", tgtok::MultiClass)
                            .Case("field", tgtok::Field)
                            .Case("let", tgtok::Let)
                            .Case("in", tgtok::In)
                            .Case("defvar", tgtok::Defvar)
                            .Case("include", tgtok::Include)
                            .Case("if", tgtok::If)
                            .Case("then", tgtok::Then)
                            .Case("else", tgtok::ElseKW)
                            .Case("assert", tgtok::Assert)
                            .Case("dump", tgtok::Dump)
                            .Default(tgtok::Id);

  // A couple of tokens require special processing.
  switch (Kind) {
  case tgtok::Include:
    if (LexInclude())
      return tgtok::Error;
    return Lex();
  case tgtok::Id:
    CurStrVal.assign(Str.begin(), Str.end());
    break;
  default:
    break;
  }

  return Kind;
}

/// LexInclude - We just read the "include" token. Get the string token that
/// comes next and enter the include.
bool TGLexer::LexInclude() {
  // The token after the include must be a string.
  tgtok::TokKind Tok = LexToken();
  if (Tok == tgtok::Error)
    return true;
  if (Tok != tgtok::StrVal) {
    PrintError(getLoc(), "expected filename after include");
    return true;
  }

  // Get the string.
  std::string Filename = CurStrVal;
  std::string IncludedFile;

  CurBuffer = SrcMgr.AddIncludeFile(Filename, SMLoc::getFromPointer(CurPtr),
                                    IncludedFile);
  if (!CurBuffer) {
    PrintError(getLoc(), "could not find include file '" + Filename + "'");
    return true;
  }

  Dependencies.insert(IncludedFile);
  // Save the line number and lex buffer of the includer.
  CurBuf = SrcMgr.getMemoryBuffer(CurBuffer)->getBuffer();
  CurPtr = CurBuf.begin();

  PrepIncludeStack.emplace_back();
  return false;
}

/// SkipBCPLComment - Skip over the comment by finding the next CR or LF.
/// Or we may end up at the end of the buffer.
void TGLexer::SkipBCPLComment() {
  ++CurPtr; // Skip the second slash.
  auto EOLPos = CurBuf.find_first_of("\r\n", CurPtr - CurBuf.data());
  CurPtr = (EOLPos == StringRef::npos) ? CurBuf.end() : CurBuf.data() + EOLPos;
}

/// SkipCComment - This skips C-style /**/ comments. The only difference from C
/// is that we allow nesting.
bool TGLexer::SkipCComment() {
  ++CurPtr; // Skip the star.
  unsigned CommentDepth = 1;

  while (true) {
    int CurChar = getNextChar();
    switch (CurChar) {
    case EOF:
      PrintError(TokStart, "unterminated comment");
      return true;
    case '*':
      // End of the comment?
      if (CurPtr[0] != '/')
        break;

      ++CurPtr; // End the */.
      if (--CommentDepth == 0)
        return false;
      break;
    case '/':
      // Start of a nested comment?
      if (CurPtr[0] != '*')
        break;
      ++CurPtr;
      ++CommentDepth;
      break;
    }
  }
}

/// LexNumber - Lex:
///    [-+]?[0-9]+
///    0x[0-9a-fA-F]+
///    0b[01]+
tgtok::TokKind TGLexer::LexNumber() {
  unsigned Base = 0;
  const char *NumStart;

  // Check if it's a hex or a binary value.
  if (CurPtr[-1] == '0') {
    NumStart = CurPtr + 1;
    if (CurPtr[0] == 'x') {
      Base = 16;
      do
        ++CurPtr;
      while (isHexDigit(CurPtr[0]));
    } else if (CurPtr[0] == 'b') {
      Base = 2;
      do
        ++CurPtr;
      while (CurPtr[0] == '0' || CurPtr[0] == '1');
    }
  }

  // For a hex or binary value, we always convert it to an unsigned value.
  bool IsMinus = false;

  // Check if it's a decimal value.
  if (Base == 0) {
    // Check for a sign without a digit.
    if (!isDigit(CurPtr[0])) {
      if (CurPtr[-1] == '-')
        return tgtok::minus;
      else if (CurPtr[-1] == '+')
        return tgtok::plus;
    }

    Base = 10;
    NumStart = TokStart;
    IsMinus = CurPtr[-1] == '-';

    while (isDigit(CurPtr[0]))
      ++CurPtr;
  }

  // Requires at least one digit.
  if (CurPtr == NumStart)
    return ReturnError(TokStart, "invalid number");

  errno = 0;
  if (IsMinus)
    CurIntVal = strtoll(NumStart, nullptr, Base);
  else
    CurIntVal = strtoull(NumStart, nullptr, Base);

  if (errno == EINVAL)
    return ReturnError(TokStart, "invalid number");
  if (errno == ERANGE)
    return ReturnError(TokStart, "number out of range");

  return Base == 2 ? tgtok::BinaryIntVal : tgtok::IntVal;
}

/// LexBracket - We just read '['. If this is a code block, return it,
/// otherwise return the bracket. Match: '[' and '[{ ( [^}]+ | }[^]] )* }]'
tgtok::TokKind TGLexer::LexBracket() {
  if (CurPtr[0] != '{')
    return tgtok::l_square;
  ++CurPtr;
  const char *CodeStart = CurPtr;
  while (true) {
    int Char = getNextChar();
    if (Char == EOF)
      break;

    if (Char != '}')
      continue;

    Char = getNextChar();
    if (Char == EOF)
      break;
    if (Char == ']') {
      CurStrVal.assign(CodeStart, CurPtr - 2);
      return tgtok::CodeFragment;
    }
  }

  return ReturnError(CodeStart - 2, "unterminated code block");
}

/// LexExclaim - Lex '!' and '![a-zA-Z]+'.
tgtok::TokKind TGLexer::LexExclaim() {
  if (!isAlpha(*CurPtr))
    return ReturnError(CurPtr - 1, "invalid \"!operator\"");

  const char *Start = CurPtr++;
  while (isAlpha(*CurPtr))
    ++CurPtr;

  // Check to see which operator this is.
  tgtok::TokKind Kind =
      StringSwitch<tgtok::TokKind>(StringRef(Start, CurPtr - Start))
          .Case("eq", tgtok::XEq)
          .Case("ne", tgtok::XNe)
          .Case("le", tgtok::XLe)
          .Case("lt", tgtok::XLt)
          .Case("ge", tgtok::XGe)
          .Case("gt", tgtok::XGt)
          .Case("if", tgtok::XIf)
          .Case("cond", tgtok::XCond)
          .Case("switch", tgtok::XSwitch)
          .Case("isa", tgtok::XIsA)
          .Case("head", tgtok::XHead)
          .Case("tail", tgtok::XTail)
          .Case("size", tgtok::XSize)
          .Case("con", tgtok::XConcat)
          .Case("dag", tgtok::XDag)
          .Case("add", tgtok::XADD)
          .Case("sub", tgtok::XSUB)
          .Case("mul", tgtok::XMUL)
          .Case("div", tgtok::XDIV)
          .Case("not", tgtok::XNOT)
          .Case("logtwo", tgtok::XLOG2)
          .Case("and", tgtok::XAND)
          .Case("or", tgtok::XOR)
          .Case("xor", tgtok::XXOR)
          .Case("shl", tgtok::XSHL)
          .Case("sra", tgtok::XSRA)
          .Case("srl", tgtok::XSRL)
          .Case("cast", tgtok::XCast)
          .Case("empty", tgtok::XEmpty)
          .Case("subst", tgtok::XSubst)
          .Case("foldl", tgtok::XFoldl)
          .Case("foreach", tgtok::XForEach)
          .Case("filter", tgtok::XFilter)
          .Case("listconcat", tgtok::XListConcat)
          .Case("listflatten", tgtok::XListFlatten)
          .Case("listsplat", tgtok::XListSplat)
          .Case("listremove", tgtok::XListRemove)
          .Case("range", tgtok::XRange)
          .Case("sort", tgtok::XSort)
          .Case("strconcat", tgtok::XStrConcat)
          .Case("initialized", tgtok::XInitialized)
          .Case("interleave", tgtok::XInterleave)
          .Case("instances", tgtok::XInstances)
          .Case("substr", tgtok::XSubstr)
          .Case("find", tgtok::XFind)
          .Case("setdagop", tgtok::XSetDagOp)
          .Case("getdagop", tgtok::XGetDagOp)
          .Case("setdagopname", tgtok::XSetDagOpName)
          .Case("getdagopname", tgtok::XGetDagOpName)
          .Case("getdagarg", tgtok::XGetDagArg)
          .Case("getdagname", tgtok::XGetDagName)
          .Case("setdagarg", tgtok::XSetDagArg)
          .Case("setdagname", tgtok::XSetDagName)
          .Case("exists", tgtok::XExists)
          .Case("tolower", tgtok::XToLower)
          .Case("toupper", tgtok::XToUpper)
          .Case("repr", tgtok::XRepr)
          .Case("match", tgtok::XMatch)
          .Default(tgtok::Error);

  return Kind != tgtok::Error ? Kind
                              : ReturnError(Start - 1, "unknown operator");
}

bool TGLexer::prepExitInclude(bool IncludeStackMustBeEmpty) {
  // Report an error, if preprocessor control stack for the current
  // file is not empty.
  if (!PrepIncludeStack.back().empty()) {
    prepReportPreprocessorStackError();

    return false;
  }

  // Pop the preprocessing controls from the include stack.
  PrepIncludeStack.pop_back();

  if (IncludeStackMustBeEmpty) {
    assert(PrepIncludeStack.empty() &&
           "preprocessor include stack is not empty");
  } else {
    assert(!PrepIncludeStack.empty() && "preprocessor include stack is empty");
  }

  return true;
}

tgtok::TokKind TGLexer::prepIsDirective() const {
  for (const auto [Kind, Word] : PreprocessorDirs) {
    if (StringRef(CurPtr, Word.size()) != Word)
      continue;
    int NextChar = peekNextChar(Word.size());

    // Check for whitespace after the directive. If there is no whitespace,
    // then we do not recognize it as a preprocessing directive.

    // New line and EOF may follow only #else/#endif. It will be reported
    // as an error for #ifdef/#define after the call to prepLexMacroName().
    if (NextChar == ' ' || NextChar == '\t' || NextChar == EOF ||
        NextChar == '\n' ||
        // It looks like TableGen does not support '\r' as the actual
        // carriage return, e.g. getNextChar() treats a single '\r'
        // as '\n'. So we do the same here.
        NextChar == '\r')
      return Kind;

    // Allow comments after some directives, e.g.:
    //     #else// OR #else/**/
    //     #endif// OR #endif/**/
    //
    // Note that we do allow comments after #ifdef/#define here, e.g.
    //     #ifdef/**/ AND #ifdef//
    //     #define/**/ AND #define//
    //
    // These cases will be reported as incorrect after calling
    // prepLexMacroName(). We could have supported C-style comments
    // after #ifdef/#define, but this would complicate the code
    // for little benefit.
    if (NextChar == '/') {
      NextChar = peekNextChar(Word.size() + 1);

      if (NextChar == '*' || NextChar == '/')
        return Kind;

      // Pretend that we do not recognize the directive.
    }
  }

  return tgtok::Error;
}

void TGLexer::prepEatPreprocessorDirective(tgtok::TokKind Kind) {
  TokStart = CurPtr;

  for (const auto [PKind, PWord] : PreprocessorDirs) {
    if (PKind == Kind) {
      // Advance CurPtr to the end of the preprocessing word.
      CurPtr += PWord.size();
      return;
    }
  }

  llvm_unreachable(
      "unsupported preprocessing token in prepEatPreprocessorDirective()");
}

tgtok::TokKind TGLexer::lexPreprocessor(tgtok::TokKind Kind,
                                        bool ReturnNextLiveToken) {
  // We must be looking at a preprocessing directive. Eat it!
  prepEatPreprocessorDirective(Kind);

  if (Kind == tgtok::Ifdef || Kind == tgtok::Ifndef) {
    StringRef MacroName = prepLexMacroName();
    StringRef IfTokName = Kind == tgtok::Ifdef ? "#ifdef" : "#ifndef";
    if (MacroName.empty())
      return ReturnError(TokStart, "expected macro name after " + IfTokName);

    bool MacroIsDefined = DefinedMacros.count(MacroName) != 0;

    // Canonicalize ifndef's MacroIsDefined to its ifdef equivalent.
    if (Kind == tgtok::Ifndef)
      MacroIsDefined = !MacroIsDefined;

    // Regardless of whether we are processing tokens or not,
    // we put the #ifdef control on stack.
    // Note that MacroIsDefined has been canonicalized against ifdef.
    PrepIncludeStack.back().push_back(
        {tgtok::Ifdef, MacroIsDefined, SMLoc::getFromPointer(TokStart)});

    if (!prepSkipDirectiveEnd())
      return ReturnError(CurPtr, "only comments are supported after " +
                                     IfTokName + " NAME");

    // If we were not processing tokens before this #ifdef,
    // then just return back to the lines skipping code.
    if (!ReturnNextLiveToken)
      return Kind;

    // If we were processing tokens before this #ifdef,
    // and the macro is defined, then just return the next token.
    if (MacroIsDefined)
      return LexToken();

    // We were processing tokens before this #ifdef, and the macro
    // is not defined, so we have to start skipping the lines.
    // If the skipping is successful, it will return the token following
    // either #else or #endif corresponding to this #ifdef.
    if (prepSkipRegion(ReturnNextLiveToken))
      return LexToken();

    return tgtok::Error;
  } else if (Kind == tgtok::Else) {
    // Check if this #else is correct before calling prepSkipDirectiveEnd(),
    // which will move CurPtr away from the beginning of #else.
    if (PrepIncludeStack.back().empty())
      return ReturnError(TokStart, "#else without #ifdef or #ifndef");

    PreprocessorControlDesc IfdefEntry = PrepIncludeStack.back().back();

    if (IfdefEntry.Kind != tgtok::Ifdef) {
      PrintError(TokStart, "double #else");
      return ReturnError(IfdefEntry.SrcPos, "previous #else is here");
    }

    // Replace the corresponding #ifdef's control with its negation
    // on the control stack.
    PrepIncludeStack.back().back() = {Kind, !IfdefEntry.IsDefined,
                                      SMLoc::getFromPointer(TokStart)};

    if (!prepSkipDirectiveEnd())
      return ReturnError(CurPtr, "only comments are supported after #else");

    // If we were processing tokens before this #else,
    // we have to start skipping lines until the matching #endif.
    if (ReturnNextLiveToken) {
      if (prepSkipRegion(ReturnNextLiveToken))
        return LexToken();

      return tgtok::Error;
    }

    // Return to the lines skipping code.
    return Kind;
  } else if (Kind == tgtok::Endif) {
    // Check if this #endif is correct before calling prepSkipDirectiveEnd(),
    // which will move CurPtr away from the beginning of #endif.
    if (PrepIncludeStack.back().empty())
      return ReturnError(TokStart, "#endif without #ifdef");

    [[maybe_unused]] auto &IfdefOrElseEntry = PrepIncludeStack.back().back();

    assert((IfdefOrElseEntry.Kind == tgtok::Ifdef ||
            IfdefOrElseEntry.Kind == tgtok::Else) &&
           "invalid preprocessor control on the stack");

    if (!prepSkipDirectiveEnd())
      return ReturnError(CurPtr, "only comments are supported after #endif");

    PrepIncludeStack.back().pop_back();

    // If we were processing tokens before this #endif, then
    // we should continue it.
    if (ReturnNextLiveToken) {
      return LexToken();
    }

    // Return to the lines skipping code.
    return Kind;
  } else if (Kind == tgtok::Define) {
    StringRef MacroName = prepLexMacroName();
    if (MacroName.empty())
      return ReturnError(TokStart, "expected macro name after #define");

    if (!DefinedMacros.insert(MacroName).second)
      PrintWarning(getLoc(),
                   "duplicate definition of macro: " + Twine(MacroName));

    if (!prepSkipDirectiveEnd())
      return ReturnError(CurPtr,
                         "only comments are supported after #define NAME");

    assert(ReturnNextLiveToken &&
           "#define must be ignored during the lines skipping");

    return LexToken();
  }

  llvm_unreachable("preprocessing directive is not supported");
}

bool TGLexer::prepSkipRegion(bool MustNeverBeFalse) {
  assert(MustNeverBeFalse && "invalid recursion.");

  do {
    // Skip all symbols to the line end.
    while (*CurPtr != '\n')
      ++CurPtr;

    // Find the first non-whitespace symbol in the next line(s).
    if (!prepSkipLineBegin())
      return false;

    // If the first non-blank/comment symbol on the line is '#',
    // it may be a start of preprocessing directive.
    //
    // If it is not '#' just go to the next line.
    if (*CurPtr == '#')
      ++CurPtr;
    else
      continue;

    tgtok::TokKind Kind = prepIsDirective();

    // If we did not find a preprocessing directive or it is #define,
    // then just skip to the next line. We do not have to do anything
    // for #define in the line-skipping mode.
    if (Kind == tgtok::Error || Kind == tgtok::Define)
      continue;

    tgtok::TokKind ProcessedKind = lexPreprocessor(Kind, false);

    // If lexPreprocessor() encountered an error during lexing this
    // preprocessor idiom, then return false to the calling lexPreprocessor().
    // This will force tgtok::Error to be returned to the tokens processing.
    if (ProcessedKind == tgtok::Error)
      return false;

    assert(Kind == ProcessedKind && "prepIsDirective() and lexPreprocessor() "
                                    "returned different token kinds");

    // If this preprocessing directive enables tokens processing,
    // then return to the lexPreprocessor() and get to the next token.
    // We can move from line-skipping mode to processing tokens only
    // due to #else or #endif.
    if (prepIsProcessingEnabled()) {
      assert((Kind == tgtok::Else || Kind == tgtok::Endif) &&
             "tokens processing was enabled by an unexpected preprocessing "
             "directive");

      return true;
    }
  } while (CurPtr != CurBuf.end());

  // We have reached the end of the file, but never left the lines-skipping
  // mode. This means there is no matching #endif.
  prepReportPreprocessorStackError();
  return false;
}

StringRef TGLexer::prepLexMacroName() {
  // Skip whitespaces between the preprocessing directive and the macro name.
  while (*CurPtr == ' ' || *CurPtr == '\t')
    ++CurPtr;

  TokStart = CurPtr;
  CurPtr = lexMacroName(StringRef(CurPtr, CurBuf.end() - CurPtr));
  return StringRef(TokStart, CurPtr - TokStart);
}

bool TGLexer::prepSkipLineBegin() {
  while (CurPtr != CurBuf.end()) {
    switch (*CurPtr) {
    case ' ':
    case '\t':
    case '\n':
    case '\r':
      break;

    case '/': {
      int NextChar = peekNextChar(1);
      if (NextChar == '*') {
        // Skip C-style comment.
        // Note that we do not care about skipping the C++-style comments.
        // If the line contains "//", it may not contain any processable
        // preprocessing directive. Just return CurPtr pointing to
        // the first '/' in this case. We also do not care about
        // incorrect symbols after the first '/' - we are in lines-skipping
        // mode, so incorrect code is allowed to some extent.

        // Set TokStart to the beginning of the comment to enable proper
        // diagnostic printing in case of error in SkipCComment().
        TokStart = CurPtr;

        // CurPtr must point to '*' before call to SkipCComment().
        ++CurPtr;
        if (SkipCComment())
          return false;
      } else {
        // CurPtr points to the non-whitespace '/'.
        return true;
      }

      // We must not increment CurPtr after the comment was lexed.
      continue;
    }

    default:
      return true;
    }

    ++CurPtr;
  }

  // We have reached the end of the file. Return to the lines skipping
  // code, and allow it to handle the EOF as needed.
  return true;
}

bool TGLexer::prepSkipDirectiveEnd() {
  while (CurPtr != CurBuf.end()) {
    switch (*CurPtr) {
    case ' ':
    case '\t':
      break;

    case '\n':
    case '\r':
      return true;

    case '/': {
      int NextChar = peekNextChar(1);
      if (NextChar == '/') {
        // Skip C++-style comment.
        // We may just return true now, but let's skip to the line/buffer end
        // to simplify the method specification.
        ++CurPtr;
        SkipBCPLComment();
      } else if (NextChar == '*') {
        // When we are skipping C-style comment at the end of a preprocessing
        // directive, we can skip several lines. If any meaningful TD token
        // follows the end of the C-style comment on the same line, it will
        // be considered as an invalid usage of TD token.
        // For example, we want to forbid usages like this one:
        //     #define MACRO class Class {}
        // But with C-style comments we also disallow the following:
        //     #define MACRO /* This macro is used
        //                      to ... */ class Class {}
        // One can argue that this should be allowed, but it does not seem
        // to be worth of the complication. Moreover, this matches
        // the C preprocessor behavior.

        // Set TokStart to the beginning of the comment to enable proper
        // diagnostic printer in case of error in SkipCComment().
        TokStart = CurPtr;
        ++CurPtr;
        if (SkipCComment())
          return false;
      } else {
        TokStart = CurPtr;
        PrintError(CurPtr, "unexpected character");
        return false;
      }

      // We must not increment CurPtr after the comment was lexed.
      continue;
    }

    default:
      // Do not allow any non-whitespaces after the directive.
      TokStart = CurPtr;
      return false;
    }

    ++CurPtr;
  }

  return true;
}

bool TGLexer::prepIsProcessingEnabled() {
  return all_of(PrepIncludeStack.back(),
                [](const PreprocessorControlDesc &I) { return I.IsDefined; });
}

void TGLexer::prepReportPreprocessorStackError() {
  auto &PrepControl = PrepIncludeStack.back().back();
  PrintError(CurBuf.end(), "reached EOF without matching #endif");
  PrintError(PrepControl.SrcPos, "the latest preprocessor control is here");

  TokStart = CurPtr;
}