GCC-3.4.6源代码学习笔记（18）

3.     对命令行选项的处理

3.1. 为语言准备的钩子

回到toplev_main，就像我们以前提到过的，前端最好能设计成使新语言的引入变得容易。在GCC中，前端为语言提供了一个框架，在这个框架中，所有语言相关的处理都被定义为回调（callback），而且这些回调被捆绑至以下的结构中。

 

212  struct lang_hooks                                                                                in langhooks.h

213  {

214    /* String identifying the front end. e.g. "GNU C++".  */

215    const char *name;

216 

217   /* sizeof (struct lang_identifier), so make_node () creates

218      identifier nodes long enough for the language-specific slots.  */

219    size_t identifier_size;

220 

221   /* Determines the size of any language-specific 'x' or 'c' nodes.

222      Since it is called from make_node, the only information available

223      is the tree code. Expected to abort on unrecognized codes.  */

224    size_t (*tree_size) (enum tree_code);

225 

226    /* The first callback made to the front end, for simple

227      initialization needed before any calls to handle_option. Return

228      the language mask to filter the switch array with.  */

229    unsigned int (*init_options) (unsigned int argc, const char **argv);

230 

231    /* Callback used to perform language-specific initialization for the

232      global diagnostic context structure.  */

233    void (*initialize_diagnostics) (struct diagnostic_context *);

234 

235    /* Handle the switch CODE, which has real type enum opt_code from

236      options.h. If the switch takes an argument, it is passed in ARG

237      which points to permanent storage. The handler is responsible for

238      checking whether ARG is NULL, which indicates that no argument

239      was in fact supplied. For -f and -W switches, VALUE is 1 or 0

240      for the positive and negative forms respectively.

241 

242      Return 1 if the switch is valid, 0 if invalid, and -1 if it's

243      valid and should not be treated as language-independent too.  */

244    int (*handle_option) (size_t code, const char *arg, int value);

245 

246   /* Return false to use the default complaint about a missing

247      argument, otherwise output a complaint and return true.  */

248    bool (*missing_argument) (const char *opt, size_t code);

249 

250    /* Called when all command line options have been parsed to allow

251      further processing and initialization

252 

253      Should return true to indicate that a compiler back-end is

254      not required, such as with the -E option.

255 

256      If errorcount is nonzero after this call the compiler exits

257      immediately and the finish hook is not called.  */

258    bool (*post_options) (const char **);

259 

260   /* Called after post_options to initialize the front end. Return

261      false to indicate that no further compilation be performed, in

262      which case the finish hook is called immediately.  */

263    bool (*init) (void);

264 

265   /* Called at the end of compilation, as a finalizer.  */

266    void (*finish) (void);

267 

268   /* Parses the entire file. The argument is nonzero to cause bison

269      parsers to dump debugging information during parsing.  */

270    void (*parse_file) (int);

271 

272    /* Called immediately after parsing to clear the binding stack.  */

273    void (*clear_binding_stack) (void);

274 

275    /* Called to obtain the alias set to be used for an expression or type.

276      Returns -1 if the language does nothing special for it.  */

277    HOST_WIDE_INT (*get_alias_set) (tree);

278 

279    /* Called with an expression that is to be processed as a constant.

280      Returns either the same expression or a language-independent

281      constant equivalent to its input.  */

282    tree (*expand_constant) (tree);

283 

284    /* Called by expand_expr for language-specific tree codes.

285      Fourth argument is actually an enum expand_modifier.  */

286    rtx (*expand_expr) (tree, rtx, enum machine_mode, int, rtx *);

287 

288    /* Prepare expr to be an argument of a TRUTH_NOT_EXPR or other logical

289      operation.

290 

291      This preparation consists of taking the ordinary representation

292      of an expression expr and producing a valid tree boolean

293      expression describing whether expr is nonzero. We could simply

294      always do build_binary_op (NE_EXPR, expr, integer_zero_node, 1),

295      but we optimize comparisons, &&, ||, and !.

296 

297      The result should be an expression of boolean type (if not an

298      error_mark_node).  */

299    tree (*truthvalue_conversion) (tree);

300 

301   /* Hook called by safe_from_p for language-specific tree codes. It is

302      up to the language front-end to install a hook if it has any such

303      codes that safe_from_p needs to know about. Since same_from_p will

304      recursively explore the TREE_OPERANDs of an expression, this hook

305      should not reexamine those pieces. This routine may recursively

306      call safe_from_p; it should always pass `0' as the TOP_P

307      parameter.  */

308    int (*safe_from_p) (rtx, tree);

309 

310    /* Function to finish handling an incomplete decl at the end of

311      compilation. Default hook is does nothing.  */

312    void (*finish_incomplete_decl) (tree);

313 

314    /* Function used by unsafe_for_reeval. A non-negative number is

315      returned directly from unsafe_for_reeval, a negative number falls

316      through. The default hook returns a negative number.  */

317    int (*unsafe_for_reeval) (tree);

318 

319   /* Mark EXP saying that we need to be able to take the address of

320      it; it should not be allocated in a register. Return true if

321      successful.  */

322    bool (*mark_addressable) (tree);

323 

324   /* Hook called by staticp for language-specific tree codes.  */

325    int (*staticp) (tree);

326 

327   /* Replace the DECL_LANG_SPECIFIC data, which may be NULL, of the

328      DECL_NODE with a newly GC-allocated copy.  */

329    void (*dup_lang_specific_decl) (tree);

330 

331    /* Called before its argument, an UNSAVE_EXPR, is to be

332      unsaved. Modify it in-place so that all the evaluate only once

333      things are cleared out.  */

334    tree (*unsave_expr_now) (tree);

335 

336    /* Called by expand_expr to build and return the cleanup-expression

337      for the passed TARGET_EXPR. Return NULL if there is none.  */

338    tree (*maybe_build_cleanup) (tree);

339 

340    /* Set the DECL_ASSEMBLER_NAME for a node. If it is the sort of

341      thing that the assembler should talk about, set

342      DECL_ASSEMBLER_NAME to an appropriate IDENTIFIER_NODE.

343      Otherwise, set it to the ERROR_MARK_NODE to ensure that the

344      assembler does not talk about it.  */

345    void (*set_decl_assembler_name) (tree);

346 

347    /* Return nonzero if fold-const is free to use bit-field

348      optimizations, for instance in fold_truthop().  */

349    bool (*can_use_bit_fields_p) (void);

350 

351   /*Nonzero if TYPE_READONLY and TREE_READONLY should always be honored.*/

352    bool honor_readonly;

353 

354   /* Nonzero if this front end does not generate a dummy BLOCK between

355      the outermost scope of the function and the FUNCTION_DECL. See

356      is_body_block in stmt.c, and its callers.  */

357    bool no_body_blocks;

358 

359    /* The front end can add its own statistics to -fmem-report with

360      this hook. It should output to stderr.  */

361    void (*print_statistics) (void);

362 

363    /* Called by print_tree when there is a tree of class 'x' that it

364      doesn't know how to display.  */

365    lang_print_tree_hook print_xnode;

366 

367    /* Called to print language-dependent parts of a class 'd', class

368      't', and IDENTIFIER_NODE nodes.  */

369    lang_print_tree_hook print_decl;

370    lang_print_tree_hook print_type;

371    lang_print_tree_hook print_identifier;

372 

373    /* Computes the name to use to print a declaration. DECL is the

374      non-NULL declaration in question. VERBOSITY determines what

375      information will be printed: 0: DECL_NAME, demangled as

376      necessary. 1: and scope information. 2: and any other

377      information that might be interesting, such as function parameter

378     types in C++.  */

379    const char *(*decl_printable_name) (tree decl, int verbosity);

380 

381   /* Given a CALL_EXPR, return a function decl that is its target.  */

382    tree (*lang_get_callee_fndecl) (tree);

383 

384    /* Called by report_error_function to print out function name.  */

385    void (*print_error_function) (struct diagnostic_context *, const char *);

386 

387   /* Called from expr_size to calculate the size of the value of an

388      expression in a language-dependent way. Returns a tree for the size

389      in bytes. A frontend can call lhd_expr_size to get the default

390      semantics in cases that it doesn't want to handle specially.  */

391    tree (*expr_size) (tree);

392 

393    /* Called from uninitialized_vars_warning to find out if a variable is

394      uninitialized based on DECL_INITIAL.  */

395    bool (*decl_uninit) (tree);

396 

397    /* Pointers to machine-independent attribute tables, for front ends

398      using attribs.c. If one is NULL, it is ignored. Respectively, a

399      table of attributes specific to the language, a table of

400      attributes common to two or more languages (to allow easy

401      sharing), and a table of attributes for checking formats.  */

402    const struct attribute_spec *attribute_table;

403    const struct attribute_spec *common_attribute_table;

404    const struct attribute_spec *format_attribute_table;

405 

406    /* Function-related language hooks.  */

407    struct lang_hooks_for_functions function;

408 

409    struct lang_hooks_for_tree_inlining tree_inlining;

410 

411    struct lang_hooks_for_callgraph callgraph;

412 

413    struct lang_hooks_for_tree_dump tree_dump;

414 

415    struct lang_hooks_for_decls decls;

416 

417    struct lang_hooks_for_types types;

418 

419    struct lang_hooks_for_rtl_expansion rtl_expand;

420 

421    /* Whenever you add entries here, make sure you adjust langhooks-def.h

422      and langhooks.c accordingly.  */

423  };

 

这是大而复杂又十分重要的数据结构，它有一个全局实例lang_hooks。GCC中的每个语言前端都要设置这个结构。它通过宏LANG_HOOKS_INITIALIZER初始化。前端不需要提供全部的定义，对于语言前端不提供的部分，将使用默认的行为（一般来说是无害的空操作，或者是返回false值）。后面我们结合C++前端，将会看到各部分的具体功能。

3.2. 初始化源文件阅读器

显然不同的语言有不同的编译选项，这些选项的处理是前端相关的。函数decode_options为语言处理编译选项，注意它能被所有的前端调用。

 

479  void

480  decode_options (unsigned int argc, const char **argv)                                          in opts.c

481  {

482    unsigned int i, lang_mask;

483 

484    /* Perform language-specific options initialization.  */

485    lang_mask = (*lang_hooks.init_options) (argc, argv);

486 

487    lang_hooks.initialize_diagnostics (global_dc);

 

这里， C和C++前端（C++前端必须支持C）对钩子init_options的设置是一样的，都为c_common_init_options。它的主要作用是创建源文件阅读器。

 

188  unsigned int

189  c_common_init_options (unsigned int argc, const char **argv)                       in c-opts.c

190  {

191    static const unsigned int lang_flags[] = {CL_C, CL_ObjC, CL_CXX, CL_ObjCXX};

192    unsigned int i, result;

193 

194    /* This is conditionalized only because that is the way the front

195      ends used to do it. Maybe this should be unconditional?  */

196    if (c_dialect_cxx ())

197    {

198      /* By default wrap lines at 80 characters. Is getenv

199       ("COLUMNS") preferable?  */

200      diagnostic_line_cutoff (global_dc) = 80;

201      /* By default, emit location information once for every

202       diagnostic message.  */

203      diagnostic_prefixing_rule (global_dc) = DIAGNOSTICS_SHOW_PREFIX_ONCE;

204    }

205 

206    parse_in = cpp_create_reader (c_dialect_cxx () ? CLK_GNUCXX: CLK_GNUC89,

207                       ident_hash);

 

195行的c_dialect_cxx是一个简单的宏，它测试当前是否使用C++。

 

254  #define c_dialect_cxx()         (c_language & clk_cxx)                            in c-common.h

 

c_language则是枚举类型c_language_kind的一个实例。它由前端初始化，例如，对于C++前端，它被设为clk_cxx。

 

241  typedef enum c_language_kind                                                             in c-common.h

242  {

243    clk_c          = 0,        /* C90, C94 or C99 */

244    clk_objc      = 1,        /* clk_c with ObjC features.  */

245    clk_cxx       = 2,        /* ANSI/ISO C++ */

246    clk_objcxx  = 3         /* clk_cxx with ObjC features.  */

247  }

248  c_language_kind;

249 

250  /* To test for a specific language use c_language, defined by each

251    front end.  For "ObjC features" or "not C++" use the macros.  */

252  extern c_language_kind c_language;                                                      in c-common.h