深刻研究Clang（七） Clang Lexer代碼閱讀筆記之Lexer

做者：史寧寧（snsn1984）

源碼位置：clang/lib/Lexer.cpp

源碼網絡地址：http://clang.llvm.org/doxygen/Lexer_8cpp_source.html

Lexer.cpp這個文件，是Clang這個前端的詞法分析器的主要文件，它的內容是對Lexer這個類的具體實現，原文件的註釋中：「This file implements the Lexer and Token interfaces.」 這麼解釋這個文件的，可是Token只有兩個簡單函數的實現，剩下的都是Lexer的實現。因此要想搞清楚Clang的詞法分析器是怎麼實現的，那麼必須對這個文件有着深刻的理解。

從Lexer的初始化函數開始入手：

void Lexer::InitLexer(const char *BufStart, const char *BufPtr,
   56                       const char *BufEnd) {
   57   BufferStart = BufStart;
   58   BufferPtr = BufPtr;
   59   BufferEnd = BufEnd;
   60 
   61   assert(BufEnd[0] == 0 &&
   62          "We assume that the input buffer has a null character at the end"
   63          " to simplify lexing!");
   64 
   65   // Check whether we have a BOM in the beginning of the buffer. If yes - act
   66   // accordingly. Right now we support only UTF-8 with and without BOM, so, just
   67   // skip the UTF-8 BOM if it's present.
   68   if (BufferStart == BufferPtr) {
   69     // Determine the size of the BOM.
   70     StringRef Buf(BufferStart, BufferEnd - BufferStart);
   71     size_t BOMLength = llvm::StringSwitch<size_t>(Buf)
   72       .StartsWith("\xEF\xBB\xBF", 3) // UTF-8 BOM
   73       .Default(0);
   74 
   75     // Skip the BOM.
   76     BufferPtr += BOMLength;
   77   }
   78 
   79   Is_PragmaLexer = false;
   80   CurrentConflictMarkerState = CMK_None;
   81 
   82   // Start of the file is a start of line.
   83   IsAtStartOfLine = true;
   84   IsAtPhysicalStartOfLine = true;
   85 
   86   HasLeadingSpace = false;
   87   HasLeadingEmptyMacro = false;
   88 
   89   // We are not after parsing a #.
   90   ParsingPreprocessorDirective = false;
   91 
   92   // We are not after parsing #include.
   93   ParsingFilename = false;
   94 
   95   // We are not in raw mode.  Raw mode disables diagnostics and interpretation
   96   // of tokens (e.g. identifiers, thus disabling macro expansion).  It is used
   97   // to quickly lex the tokens of the buffer, e.g. when handling a "#if 0" block
   98   // or otherwise skipping over tokens.
   99   LexingRawMode = false;
  100 
  101   // Default to not keeping comments.
  102   ExtendedTokenMode = 0;
  103 }

這個初始化函數，是在Lexer類的兩個構造函數裏被調用的，具體代碼以下：

  104 
  105 /// Lexer constructor - Create a new lexer object for the specified buffer
  106 /// with the specified preprocessor managing the lexing process.  This lexer
  107 /// assumes that the associated file buffer and Preprocessor objects will
  108 /// outlive it, so it doesn't take ownership of either of them.
  109 Lexer::Lexer(FileID FID, const llvm::MemoryBuffer *InputFile, Preprocessor &PP)
  110   : PreprocessorLexer(&PP, FID),
  111     FileLoc(PP.getSourceManager().getLocForStartOfFile(FID)),
  112     LangOpts(PP.getLangOpts()) {
  113 
  114   InitLexer(InputFile->getBufferStart(), InputFile->getBufferStart(),
  115             InputFile->getBufferEnd());
  116 
  117   resetExtendedTokenMode();
  118 }
  119 
  120 void Lexer::resetExtendedTokenMode() {
  121   assert(PP && "Cannot reset token mode without a preprocessor");
  122   if (LangOpts.TraditionalCPP)
  123     SetKeepWhitespaceMode(true);
  124   else
  125     SetCommentRetentionState(PP->getCommentRetentionState());
  126 }
  127 
  128 /// Lexer constructor - Create a new raw lexer object.  This object is only
  129 /// suitable for calls to 'LexFromRawLexer'.  This lexer assumes that the text
  130 /// range will outlive it, so it doesn't take ownership of it.
  131 Lexer::Lexer(SourceLocation fileloc, const LangOptions &langOpts,
  132              const char *BufStart, const char *BufPtr, const char *BufEnd)
  133   : FileLoc(fileloc), LangOpts(langOpts) {
  134 
  135   InitLexer(BufStart, BufPtr, BufEnd);
  136 
  137   // We *are* in raw mode.
  138   LexingRawMode = true;
  139 }

這兩個構造函數各有不一樣，從輸入參數上就能夠看出。也有相同的地方，就是對一些參數只是引用的關係，並無獲取這些參數的全部權。

Lexer的構造函數，在本身的類內部，分別被如下的函數所調用：

Create_PragmaLexer: Lexer constructor - Create a new lexer object for _Pragma expansion.
http://clang.llvm.org/doxygen/classclang_1_1Lexer.html#ac7f3b1ce4f2eeaec8d787d22bf197cd0

getSpelling - This method is used to get the spelling of a token into a preallocated buffer, instead of as an std::string.

http://clang.llvm.org/doxygen/classclang_1_1Lexer.html#a94f2c5710332ae19d7955c609ac37adb

getRawToken

http://clang.llvm.org/doxygen/classclang_1_1Lexer.html#adac8b8cf001621ec3b109d82a7074f05

 getBeginningOfFileToken

http://clang.llvm.org/doxygen/Lexer_8cpp.html#a4845396d18432c436e605303b057dbb4

findLocationAfterToken

http://clang.llvm.org/doxygen/classclang_1_1Lexer.html#a099b99b2d19ef5cdd8fcb80d8cf4064e