Studying note of GCC-3.4.6 source (87)

5.11. Parse Translation-unit - entry

The grammar of the language can be shown as a tree (of course we need treat the recursion of parent as child node, most of which we highlight in red in the figures). The root and starting point of the tree is translation-unit. From below figures, we can see building a parser for C++ is not an easy task. What’s more, the grammar below provided by standard is left recursive in nature. To be able to build parser with parser builder, it needs be transimitted into right recursive ones to accommodate the bottom up LR parser. Even with the right recursive version, building LR(1) parser is still very difficult. So GCC adopts hand written tentative LL(n) parser – it supports retrospect. And no doubt looking through the parser is a long journey. Now let’s go!

Figure 43: syntax tree for declarations

Figure 44: syntax tree for class

Figure 45: syntax tree for statements

The major body of c_parser_file is cp_parser_translation_unit. According to [3], the abbreviate syntax tree for translation-unit is:

translation-unit

 Ⅼ declaration-seq [opt] —— declaration-seq declaration

     Ⅼ declaration

In this simple view, a translation-uint just contains a sequence of declaration. In C++, translation-unit usually includes a source file and several header files. From this translation-unit, the compiler will generate the object file and linker will join these object files together to form the target file.

 

2319 static bool

2320 cp_parser_translation_unit (cp_parser* parser)                                             in parser.c

2321 {

2322   while (true)

2323   {

2324     cp_parser_declaration_seq_opt (parser);

2325

2326     /* If there are no tokens left then all went well.  */

2327     if (cp_lexer_next_token_is (parser->lexer, CPP_EOF))

2328       break;

2329      

2330     /* Otherwise, issue an error message.  */

2331     cp_parser_error (parser, "expected declaration");

2332     return false;

2333   }

2334

2335  /* Consume the EOF token.  */

2336   cp_parser_require (parser, CPP_EOF, "end-of-file");

2337  

2338   /* Finish up.  */

2339   finish_translation_unit ();

2340

2341   /* All went well.  */

2342   return true;

2343 }

 

Note WHILE above, the successful parse jumps to line 2336 from line 2328, and by this way, it can avoid using “goto”.