Studying note of GCC-3.4.6 source (48)

4.2.8. Data for prologue, epilogue, sibcall epilogue in function invocation

In backend_init, following invokes init_function_once.

 

8140 void

8141 init_function_once (void)                                                                           in function.c

8142 {

8143   VARRAY_INT_INIT (prologue, 0, "prologue");

8144   VARRAY_INT_INIT (epilogue, 0, "epilogue");

8145   VARRAY_INT_INIT (sibcall_epilogue, 0, "sibcall_epilogue");

8146 }

 

Above, prologue records the INSN_UIDs of the prologue instructions, epilogue records the INSN_UIDs of the epilogue instructions, and sibcall_epilogue records INSN_UIDs for each sibcall epilogue in the function. INSN_UID is a unique number for each instruction which is created by compiler to tell about instructions in the source. They are not necessarily sequentially increasing. And for INSN rtx objects, INSN_UID is their first child. prologue, epilogue, and sibcall_epilogue are of varray_type type, which is the virtual array can hold multiple type objects. The definition of varray_type is quite similar with that of rtx. It is an interesting data structure.

 

132    struct varray_head_tag GTY(()) {                                                              in varray.h

133      size_t       num_elements;  /* Maximum element number allocated.  */

134      size_t    elements_used;  /* The number of elements used, if

135                                           using VARRAY_PUSH/VARRAY_POP.  */

136      enum varray_data_enum type;  /* The kind of elements in the varray.  */

137      const char   *name;               /* name of the varray for reporting errors */

138      varray_data  GTY ((desc ("%0.type"))) data;  /* The data elements follow,

139                                                                 must be last.  */

140    };

141    typedef struct varray_head_tag *varray_type;

 

Note that varray_head_tag is not just a header to define its size, element’s type, it also contains the body – varray_data.

 

88      typedef union varray_data_tag GTY (()) {                                                  in varray.h

89        char          GTY ((length ("%0.num_elements"),

90                         tag ("VARRAY_DATA_C"))) c[1];

91        unsigned char   GTY ((length ("%0.num_elements"),

92                         tag ("VARRAY_DATA_UC"))) uc[1];

93        short        GTY ((length ("%0.num_elements"),

94                         tag ("VARRAY_DATA_S")))       s[1];

95        unsigned short  GTY ((length ("%0.num_elements"),

96                         tag ("VARRAY_DATA_US"))) us[1];

97        int                  GTY ((length ("%0.num_elements"),

98                         tag ("VARRAY_DATA_I"))) i[1];

99        unsigned int     GTY ((length ("%0.num_elements"),

100                       tag ("VARRAY_DATA_U"))) u[1];

101      long                GTY ((length ("%0.num_elements"),

102                       tag ("VARRAY_DATA_L"))) l[1];

103      unsigned long  GTY ((length ("%0.num_elements"),

104                       tag ("VARRAY_DATA_UL"))) ul[1];

105      HOST_WIDE_INT  GTY ((length ("%0.num_elements"),

106                       tag ("VARRAY_DATA_HINT"))) hint[1];

107      unsigned HOST_WIDE_INT GTY ((length ("%0.num_elements"),

108                       tag ("VARRAY_DATA_UHINT"))) uhint[1];

109      PTR                GTY ((length ("%0.num_elements"), use_param (""),

110                        tag ("VARRAY_DATA_GENERIC"))) generic[1];

111       char                *GTY ((length ("%0.num_elements"),

112                        tag ("VARRAY_DATA_CPTR"))) cptr[1];

113       rtx                  GTY ((length ("%0.num_elements"),

114                        tag ("VARRAY_DATA_RTX"))) rtx[1];

115       rtvec               GTY ((length ("%0.num_elements"),

116                        tag ("VARRAY_DATA_RTVEC"))) rtvec[1];

117       tree                 GTY ((length ("%0.num_elements"),

118                        tag ("VARRAY_DATA_TREE"))) tree[1];

119       struct bitmap_head_def *GTY ((length ("%0.num_elements"),

120                       tag ("VARRAY_DATA_BITMAP"))) bitmap[1];

121      struct reg_info_def  *GTY ((length ("%0.num_elements"), skip (""),

122                       tag ("VARRAY_DATA_REG"))) reg[1];

123      struct const_equiv_data GTY ((length ("%0.num_elements"),

124                       tag ("VARRAY_DATA_CONST_EQUIV"))) const_equiv[1];

125      struct basic_block_def *GTY ((length ("%0.num_elements"), skip (""),

126                       tag ("VARRAY_DATA_BB"))) bb[1];

127      struct elt_list    *GTY ((length ("%0.num_elements"),

128                       tag ("VARRAY_DATA_TE"))) te[1];

129    } varray_data;

 

VARRAY_INT_INIT initializes these varray with 0 element of int type – it’s empty now.

 

159    #define VARRAY_INT_INIT(va, num, name) /                                           in varray.h

160      va = varray_init (num, VARRAY_DATA_I, name)

 

115     varray_type

116     varray_init (size_t num_elements, enum varray_data_enum element_kind,    in varray.c

117               const char *name)

118     {

119      size_t data_size = num_elements * element[element_kind].size;

120      varray_type ptr;

121    #ifdef GATHER_STATISTICS

122      struct varray_descriptor *desc = varray_descriptor (name);

123   

124      desc->created++;

125      desc->allocated += data_size + VARRAY_HDR_SIZE;

126    #endif

127      if (element[element_kind].uses_ggc)

128        ptr = ggc_alloc_cleared (VARRAY_HDR_SIZE + data_size);

129      else

130        ptr = xcalloc (VARRAY_HDR_SIZE + data_size, 1);

131   

132      ptr->num_elements = num_elements;

133      ptr->elements_used = 0;

134      ptr->type = element_kind;

135      ptr->name = name;

136      return ptr;

137    }

 

To decide the size and the type of the element holds, some information needed. This information is saved in a constant static array named element, which is shown below.

 

87      static const struct {                                                                                   in varray.c

88        unsigned char size;

89        bool uses_ggc;

90      } element[NUM_VARRAY_DATA] = {

91        { sizeof (char), 1 },

92        { sizeof (unsigned char), 1 },

93        { sizeof (short), 1 },

94        { sizeof (unsigned short), 1 },

95        { sizeof (int), 1 },

96        { sizeof (unsigned int), 1 },

97        { sizeof (long), 1 },

98        { sizeof (unsigned long), 1 },

99        { sizeof (HOST_WIDE_INT), 1 },

100      { sizeof (unsigned HOST_WIDE_INT), 1 },

101      { sizeof (void *), 1 },

102      { sizeof (char *), 1 },

103      { sizeof (struct rtx_def *), 1 },

104      { sizeof (struct rtvec_def *), 1 },

105      { sizeof (union tree_node *), 1 },

106      { sizeof (struct bitmap_head_def *), 1 },

107      { sizeof (struct reg_info_def *), 0 },

108      { sizeof (struct const_equiv_data), 0 },

109      { sizeof (struct basic_block_def *), 0 },

110       { sizeof (struct elt_list *), 1 },

111     };

4.2.9. Prepare for assemble generation

Back in backend_init, the following function is init_varasm_once.

 

4584 void

4585 init_varasm_once (void)                                                                             in varasm.c

4586 {

4587   in_named_htab = htab_create_ggc (31, in_named_entry_hash,

4588                        in_named_entry_eq, NULL);

4589   const_desc_htab = htab_create_ggc (1009, const_desc_hash,

4590                          const_desc_eq, NULL);

4591  

4592     const_alias_set = new_alias_set ();

4593   }

 

Above in_named_htab is the hash table of flags that have been used for a particular named section. const_desc_htab is the hash table of all constants rtx objects during assemble emission. const_alias_set describes the alias set of these constans (constant’s alias set only contains itself).

4.2.10. Evaluate cost for arithmatic operations

4.2.10.1.              Create dummy function context

Back to backend_init, following is init_dummy_function_start. It initializes the rtl expansion mechanism so that we can do simple things like generate sequences. This is used to provide a context during global initialization of some passes.

 

6518 void

6519 init_dummy_function_start (void)                                                                      in function.c

6520 {

6521   prepare_function_start (NULL);

6522 }

 

Here it invokes prepare_function_start to create a dummy function context cfun as following initialization functions need to generate rtl (in general code compilation, only after code parsing and optimizing supported by the front-end, the representation in rtl form would be generated and passed to the back-end).

 

6482 static void

6483 prepare_function_start (tree fndecl)                                                            in function.c

6484 {

6485   if (fndecl && DECL_SAVED_INSNS (fndecl))

6486     cfun = DECL_SAVED_INSNS (fndecl);

6487   else

6488     allocate_struct_function (fndecl);

6489   init_emit ();

6490   init_varasm_status (cfun);

6491   init_expr ();

6492

6493   cse_not_expected = ! optimize;

6494

6495   /* Caller save not needed yet.  */

6496   caller_save_needed = 0;

6497

6498   /* We haven't done register allocation yet.  */

6499   reg_renumber = 0;

6500

6501   /* Indicate that we need to distinguish between the return value of the

6502     present function and the return value of a function being called.  */

6503   rtx_equal_function_value_matters = 1;

6504

6505   /* Indicate that we have not instantiated virtual registers yet.  */

6506   virtuals_instantiated = 0;

6507

6508   /* Indicate that we want CONCATs now.  */

6509   generating_concat_p = 1;

6510

6511   /* Indicate we have no need of a frame pointer yet.  */

6512   frame_pointer_needed = 0;

6513 }

 

Struct function saves all the important global and static variables describing the status of the current function under compilation.

 

176    struct function GTY(())                                                                            in function.h

177    {

178      struct eh_status *eh;

179      struct stmt_status *stmt;

180      struct expr_status *expr;

181      struct emit_status *emit;

182      struct varasm_status *varasm;

183   

184      /* For function.c.  */

185   

186      /* Points to the FUNCTION_DECL of this function.  */

187      tree decl;

188   

189      /* Function containing this function, if any.  */

190      struct function *outer;

191   

192     /* Number of bytes of args popped by function being compiled on its return.

193        Zero if no bytes are to be popped.

194        May affect compilation of return insn or of function epilogue.  */

195      int pops_args;

196   

197      /* If function's args have a fixed size, this is that size, in bytes.

198        Otherwise, it is -1.

199        May affect compilation of return insn or of function epilogue.  */

200      int args_size;

201   

202      /* # bytes the prologue should push and pretend that the caller pushed them.

203         The prologue must do this, but only if parms can be passed in

204         registers.  */

205      int pretend_args_size;

206   

207      /* # of bytes of outgoing arguments. If ACCUMULATE_OUTGOING_ARGS is

208         defined, the needed space is pushed by the prologue.  */

209      int outgoing_args_size;

210   

211       /* This is the offset from the arg pointer to the place where the first

212        anonymous arg can be found, if there is one.  */

213      rtx arg_offset_rtx;

214   

215      /* Quantities of various kinds of registers

216        used for the current function's args.  */

217      CUMULATIVE_ARGS args_info;

218   

219      /* If nonzero, an RTL expression for the location at which the current

220        function returns its result. If the current function returns its

221        result in a register, current_function_return_rtx will always be

222        the hard register containing the result.  */

223      rtx return_rtx;

224   

225     /* The arg pointer hard register, or the pseudo into which it was copied.  */

226      rtx internal_arg_pointer;

227   

228      /* Language-specific reason why the current function cannot be made

229        inline.  */

230      const char *cannot_inline;

231   

232      /* Opaque pointer used by get_hard_reg_initial_val and

233        has_hard_reg_initial_val (see integrate.[hc]).  */

234      struct initial_value_struct *hard_reg_initial_vals;

235   

236      /* Number of function calls seen so far in current function.  */

237      int x_function_call_count;

238   

239     /* List (chain of TREE_LIST) of LABEL_DECLs for all nonlocal labels

240        (labels to which there can be nonlocal gotos from nested functions)

241        in this function.  */

242      tree x_nonlocal_labels;

243   

244      /* List (chain of EXPR_LIST) of stack slots that hold the current handlers

245        for nonlocal gotos. There is one for every nonlocal label in the

246        function; this list matches the one in nonlocal_labels.

247        Zero when function does not have nonlocal labels.  */

248      rtx x_nonlocal_goto_handler_slots;

249   

250      /* List (chain of EXPR_LIST) of labels heading the current handlers for

251        nonlocal gotos.  */

252      rtx x_nonlocal_goto_handler_labels;

253   

254      /* RTX for stack slot that holds the stack pointer value to restore

255        for a nonlocal goto.

256        Zero when function does not have nonlocal labels.  */

257      rtx x_nonlocal_goto_stack_level;

258   

259      /* Label that will go on parm cleanup code, if any.

260        Jumping to this label runs cleanup code for parameters, if

261        such code must be run. Following this code is the logical return

262        label.  */

263      rtx x_cleanup_label;

264   

265      /* Label that will go on function epilogue.

266        Jumping to this label serves as a "return" instruction

267        on machines which require execution of the epilogue on all returns.  */

268      rtx x_return_label;

269   

270      /* Label that will go on the end of function epilogue.

271        Jumping to this label serves as a "naked return" instruction

272        on machines which require execution of the epilogue on all returns.  */

273      rtx x_naked_return_label;

274   

275      /* Label and register for unswitching computed gotos.  */

276      rtx computed_goto_common_label;

277      rtx computed_goto_common_reg;

278   

279      /* List (chain of EXPR_LISTs) of pseudo-regs of SAVE_EXPRs.

280        So we can mark them all live at the end of the function, if nonopt.  */

281      rtx x_save_expr_regs;

282   

283      /* List (chain of EXPR_LISTs) of all stack slots in this function.

284        Made for the sake of unshare_all_rtl.  */

285      rtx x_stack_slot_list;

286   

287      /* Chain of all RTL_EXPRs that have insns in them.  */

288      tree x_rtl_expr_chain;

289   

290      /* Label to jump back to for tail recursion, or 0 if we have

291        not yet needed one for this function.  */

292      rtx x_tail_recursion_label;

293   

294      /* Place after which to insert the tail_recursion_label if we need one.  */

295      rtx x_tail_recursion_reentry;

296   

297      /* Location at which to save the argument pointer if it will need to be

298        referenced. There are two cases where this is done: if nonlocal gotos

299        exist, or if vars stored at an offset from the argument pointer will be

300        needed by inner routines.  */

301      rtx x_arg_pointer_save_area;

302   

303      /* If the function returns non-void, we will emit a clobber of the

304        return registers just in case the user fell off the end without

305        returning a proper value. This is that insn.  */

306      rtx x_clobber_return_insn;

307   

308      /* Offset to end of allocated area of stack frame.

309        If stack grows down, this is the address of the last stack slot allocated.

310        If stack grows up, this is the address for the next slot.  */

311       HOST_WIDE_INT x_frame_offset;

312   

313      /* List (chain of TREE_LISTs) of static chains for containing functions.

314        Each link has a FUNCTION_DECL in the TREE_PURPOSE and a reg rtx

315        in an RTL_EXPR in the TREE_VALUE.  */

316      tree x_context_display;

317   

318      /* List (chain of TREE_LISTs) of trampolines for nested functions.

319        The trampoline sets up the static chain and jumps to the function.

320        We supply the trampoline's address when the function's address is

321        requested.

322   

323        Each link has a FUNCTION_DECL in the TREE_PURPOSE and a reg rtx

324        in an RTL_EXPR in the TREE_VALUE.  */

325      tree x_trampoline_list;

326   

327      /* Insn after which register parms and SAVE_EXPRs are born, if nonopt.  */

328      rtx x_parm_birth_insn;

329   

330      /* Last insn of those whose job was to put parms into their nominal

331        homes.  */

332      rtx x_last_parm_insn;

333   

334      /* 1 + last pseudo register number possibly used for loading a copy

335        of a parameter of this function.  */

336      unsigned int x_max_parm_reg;

337   

338      /* Vector indexed by REGNO, containing location on stack in which

339        to put the parm which is nominally in pseudo register REGNO,

340        if we discover that that parm must go in the stack. The highest

341        element in this vector is one less than MAX_PARM_REG, above.  */

342      rtx * GTY ((length ("%h.x_max_parm_reg"))) x_parm_reg_stack_loc;

343   

344      /* List of all temporaries allocated, both available and in use.  */

345      struct temp_slot *x_temp_slots;

346   

347     /* Current nesting level for temporaries.  */

348      int x_temp_slot_level;

349   

350     /* Current nesting level for variables in a block.  */

351      int x_var_temp_slot_level;

352   

353      /* When temporaries are created by TARGET_EXPRs, they are created at

354        this level of temp_slot_level, so that they can remain allocated

355        until no longer needed. CLEANUP_POINT_EXPRs define the lifetime

356        of TARGET_EXPRs.  */

357      int x_target_temp_slot_level;

358   

359     /* This slot is initialized as 0 and is added to

360        during the nested function.  */

361      struct var_refs_queue *fixup_var_refs_queue;

362   

363      /* For integrate.c.  */

364      int inlinable;

365      int no_debugging_symbols;

366      rtvec original_arg_vector;

367      tree original_decl_initial;

368      /* Last insn of those whose job was to put parms into their nominal

369        homes.  */

370      rtx inl_last_parm_insn;

371      /* Highest label number in current function.  */

372      int inl_max_label_num;

373   

374      /* Function sequence number for profiling, debugging, etc.  */

375      int funcdef_no;

376   

377     /* For md files.  */

378   

379      /* tm.h can use this to store whatever it likes.  */

380      struct machine_function * GTY ((maybe_undef (""))) machine;

381      /* The largest alignment of slot allocated on the stack.  */

382      int stack_alignment_needed;

383      /* Preferred alignment of the end of stack frame.  */

384      int preferred_stack_boundary;

385      /* Set when the call to function itself has been emit.  */

386      bool recursive_call_emit;

387   

388     /* Language-specific code can use this to store whatever it likes.  */

389      struct language_function * language;

390   

391     /* For reorg.  */

392   

393      /* If some insns can be deferred to the delay slots of the epilogue, the

394        delay list for them is recorded here.  */

395      rtx epilogue_delay_list;

396   

397      /* How commonly executed the function is. Initialized during branch

398        probabilities pass.  */

399      enum function_frequency {

400        /* This function most likely won't be executed at all.

401          (set only when profile feedback is available).  */

402        FUNCTION_FREQUENCY_UNLIKELY_EXECUTED,

403        /* The default value.  */

404        FUNCTION_FREQUENCY_NORMAL,

405        /* Optimize this function hard

406          (set only when profile feedback is available).  */

407        FUNCTION_FREQUENCY_HOT

408      } function_frequency;

409   

410     /* Maximal number of entities in the single jumptable. Used to estimate

411         final flowgraph size.  */

412      int max_jumptable_ents;

413   

414      /* Collected bit flags.  */

415   

416      /* Nonzero if function being compiled needs to be given an address

417        where the value should be stored.  */

418      unsigned int returns_struct : 1;

419   

420      /* Nonzero if function being compiled needs to

421        return the address of where it has put a structure value.  */

422      unsigned int returns_pcc_struct : 1;

423   

424      /* Nonzero if the current function returns a pointer type.  */

425      unsigned int returns_pointer : 1;

426   

427     /* Nonzero if function being compiled needs to be passed a static chain.  */

428      unsigned int needs_context : 1;

429   

430      /* Nonzero if function being compiled can call setjmp.  */

431      unsigned int calls_setjmp : 1;

432   

433      /* Nonzero if function being compiled can call longjmp.  */

434      unsigned int calls_longjmp : 1;

435   

436      /* Nonzero if function being compiled can call alloca,

437        either as a subroutine or builtin.  */

438      unsigned int calls_alloca : 1;

439   

440      /* Nonzero if the function calls __builtin_eh_return.  */

441      unsigned int calls_eh_return : 1;

442   

443      /* Nonzero if the function calls __builtin_constant_p.  */

444      unsigned int calls_constant_p : 1;

445   

446      /* Nonzero if function being compiled receives nonlocal gotos

447        from nested functions.  */

448      unsigned int has_nonlocal_label : 1;

449   

450      /* Nonzero if function being compiled has nonlocal gotos to parent

451        function.  */

452      unsigned int has_nonlocal_goto : 1;

453   

454     /* Nonzero if function being compiled contains nested functions.  */

455      unsigned int contains_functions : 1;

456   

457      /* Nonzero if the function being compiled issues a computed jump.  */

458      unsigned int has_computed_jump : 1;

459   

460      /* Nonzero if the current function is a thunk, i.e., a lightweight

461        function implemented by the output_mi_thunk hook) that just

462        adjusts one of its arguments and forwards to another

463        function.  */

464      unsigned int is_thunk : 1;

465   

466      /* This bit is used by the exception handling logic. It is set if all

467        calls (if any) are sibling calls. Such functions do not have to

468        have EH tables generated, as they cannot throw. A call to such a

469        function, however, should be treated as throwing if any of its callees

470        can throw.  */

471      unsigned int all_throwers_are_sibcalls : 1;

472   

473      /* Nonzero if instrumentation calls for function entry and exit should be

474        generated.  */

475      unsigned int instrument_entry_exit : 1;

476   

477      /* Nonzero if profiling code should be generated.  */

478      unsigned int profile : 1;

479   

480      /* Nonzero if stack limit checking should be enabled in the current

481        function.  */

482      unsigned int limit_stack : 1;

483   

484      /* Nonzero if current function uses stdarg.h or equivalent.  */

485      unsigned int stdarg : 1;

486   

487      /* Nonzero if this function is being processed in function-at-a-time

488        mode. In other words, if all tree structure for this function,

489        including the BLOCK tree, is created before RTL generation

490        commences.  */

491      unsigned int x_whole_function_mode_p : 1;

492   

493      /* Nonzero if the back-end should not keep track of expressions that

494        determine the size of variable-sized objects. Normally, such

495        expressions are saved away, and then expanded when the next

496        function is started. For example, if a parameter has a

497        variable-sized type, then the size of the parameter is computed

498        when the function body is entered. However, some front-ends do

499        not desire this behavior.  */

500      unsigned int x_dont_save_pending_sizes_p : 1;

501   

502      /* Nonzero if the current function uses the constant pool.  */

503      unsigned int uses_const_pool : 1;

504   

505      /* Nonzero if the current function uses pic_offset_table_rtx.  */

506      unsigned int uses_pic_offset_table : 1;

507   

508      /* Nonzero if the current function needs an lsda for exception handling.  */

509      unsigned int uses_eh_lsda : 1;

510   

511       /* Nonzero if code to initialize arg_pointer_save_area has been emitted.  */

512      unsigned int arg_pointer_save_area_init : 1;

513   

514      /* Flag for use by ther rtl inliner, to tell if the function has been

515        processed at least once.  */

516      unsigned int rtl_inline_init : 1;

517   

518      /* Nonzero if the rtl inliner has saved the function for inlining.  */

519      unsigned int saved_for_inline : 1;

520    };

 

The most important parts are its first six members. Among them, eh_status controls the exception handling in the function; stmt_status records the accumulated effect of statements upon stack, data flow etc, and which will affect the translation of the current statement; expr_status records the detail about the invoke and exit behavior of the function; emit_status controls the emission of corresponding rtx instruction; varasm_status offers the way to record constants of the function.

 

6434 void

6435 allocate_struct_function (tree fndecl)                                                          in function.c

6436 {

6437   tree result;

6438

6439   cfun = ggc_alloc_cleared (sizeof (struct function));

6440

6441   max_parm_reg = LAST_VIRTUAL_REGISTER + 1;

6442

6443   cfun->stack_alignment_needed = STACK_BOUNDARY;

6444   cfun->preferred_stack_boundary = STACK_BOUNDARY;

6445

6446   current_function_funcdef_no = funcdef_no++;

6447

6448   cfun->function_frequency = FUNCTION_FREQUENCY_NORMAL;

6449

6450   init_stmt_for_function ();

6451   init_eh_for_function ();

6452

6453   (*lang_hooks.function.init) (cfun);

6454   if (init_machine_status)

6455     cfun->machine = (*init_machine_status) ();

6456

6457   if (fndecl == NULL)

6458     return;

  …

6477 }

 

Above, current_function_funcdef_no, current_function_returns_pcc_struct, current_function_returns_struct, current_function_returns_pointer, max_parm_reg, current_function_needs_context are all macros used to select certain member of the function sturcture. For example:

 

558    #define current_function_funcdef_no (cfun->funcdef_no)                                   in function.h

567    #define max_parm_reg (cfun->x_max_parm_reg)

 

At line 6441, for x86 machine, LAST_VIRTUAL_REGISTER is defined as FIRST_PSEUDO_REGISTER +4. The value is 57, it defines the maxium number of registers can be used in rtx form.

At line 6443 and 6444, STACK_BOUNDARY defines the alignment requirement for the stack. For x86 it is 32 (4 bytes).

At line 6446, funcdef_no assigns unique numbers to labels generated for profiling, debugging purpose etc. And the corresponding field in the function structure records this value.

Following, at line 6450 and 6451, functions init_stmt_for_function, init_eh_for_function just allocate eh and stmt field of the structure, which are used for exception handling and statement respectively.

At line 6453, lang_hooks.function.init for C++ language is cxx_push_function_context. For current context, it just allocate instance of language_function for the function object. Structure language_function saves and restores the variables that keep track of the progress of compilation of the current function. It is used for nested functions.

At line 6454, init_machine_status points to ix86_init_machine_status for x86 machine. It creates instance of machine_function for the function object, which saves machine dependent information relate with function handling.

At line 6457, in this time invocation, fndecl is null, the function returns at here.

Back to prepare_function_start, it then invokes init_emit initializes the data structures and variables related with RTL language generation. The data sturtcure related is emit_status, the emit object in function structure.

 

57    struct emit_status GTY(())                                                                          in function.h

58    {

59     /* This is reset to LAST_VIRTUAL_REGISTER + 1 at the start of each function.

60        After rtl generation, it is 1 plus the largest register number used.  */

61      int x_reg_rtx_no;

62   

63     /* Lowest label number in current function.  */

64      int x_first_label_num;

65   

66      /* The ends of the doubly-linked chain of rtl for the current function.

67        Both are reset to null at the start of rtl generation for the function.

68   

69        start_sequence saves both of these on `sequence_stack' along with

70        `sequence_rtl_expr' and then starts a new, nested sequence of insns.  */

71      rtx x_first_insn;

72      rtx x_last_insn;

73   

74      /* RTL_EXPR within which the current sequence will be placed. Use to

75        prevent reuse of any temporaries within the sequence until after the

76        RTL_EXPR is emitted.  */

77      tree sequence_rtl_expr;

78   

79      /* Stack of pending (incomplete) sequences saved by `start_sequence'.

80        Each element describes one pending sequence.

81        The main insn-chain is saved in the last element of the chain,

82        unless the chain is empty.  */

83      struct sequence_stack *sequence_stack;

84   

85      /* INSN_UID for next insn emitted.

86        Reset to 1 for each function compiled.  */

87      int x_cur_insn_uid;

88   

89      /* Location the last line-number NOTE emitted.

90        This is used to avoid generating duplicates.  */

91      location_t x_last_location;

92   

93     /* The length of the regno_pointer_align, regno_decl, and x_regno_reg_rtx

94        vectors. Since these vectors are needed during the expansion phase when

95        the total number of registers in the function is not yet known, the

96        vectors are copied and made bigger when necessary.  */

97      int regno_pointer_align_length;

98   

99      /* Indexed by pseudo register number, if nonzero gives the known alignment

100      for that pseudo (if REG_POINTER is set in x_regno_reg_rtx).

101      Allocated in parallel with x_regno_reg_rtx.  */

102    unsigned char * GTY ((length ("%h.x_reg_rtx_no")))

103    regno_pointer_align;

104 

105    /* Indexed by pseudo register number, gives the rtx for that pseudo.

106      Allocated in parallel with regno_pointer_align.

107 

108      Note MEM expressions can appear in this array due to the actions

109      of put_var_into_stack.  */

110     rtx * GTY ((length ("%h.x_reg_rtx_no"))) x_regno_reg_rtx;

111   };

 

This structure controls the emition of rtl expression for the function under compiled.

 

5333 void

5334 init_emit (void)                                                                                         in emit-rtl.c

5335 {

5336   struct function *f = cfun;

5337

5338   f->emit = ggc_alloc (sizeof (struct emit_status));

5339   first_insn = NULL;

5340   last_insn = NULL;

5341   seq_rtl_expr = NULL;

5342   cur_insn_uid = 1;

5343   reg_rtx_no = LAST_VIRTUAL_REGISTER + 1;

5344   last_location.line = 0;

5345   last_location.file = 0;

5346   first_label_num = label_num;

5347   last_label_num = 0;

5348   seq_stack = NULL;

5349

5350   /* Init the tables that describe all the pseudo regs.  */

5351

5352   f->emit->regno_pointer_align_length = LAST_VIRTUAL_REGISTER + 101;

5353

5354   f->emit->regno_pointer_align

5355     = ggc_alloc_cleared (f->emit->regno_pointer_align_length

5356       * sizeof (unsigned char));

5357

5358   regno_reg_rtx

5359     = ggc_alloc (f->emit->regno_pointer_align_length * sizeof (rtx));

5360

5361   /* Put copies of all the hard registers into regno_reg_rtx.  */

5362   memcpy (regno_reg_rtx,

5363          static_regno_reg_rtx,

5364          FIRST_PSEUDO_REGISTER * sizeof (rtx));

5365

5366   /* Put copies of all the virtual register rtx into regno_reg_rtx.  */

5367   init_virtual_regs (f->emit);

5368

5369   /* Indicate that the virtual registers and stack locations are

5370     all pointers.  */

5371   REG_POINTER (stack_pointer_rtx) = 1;

5372   REG_POINTER (frame_pointer_rtx) = 1;

5373   REG_POINTER (hard_frame_pointer_rtx) = 1;

5374   REG_POINTER (arg_pointer_rtx) = 1;

5375

5376   REG_POINTER (virtual_incoming_args_rtx) = 1;

5377   REG_POINTER (virtual_stack_vars_rtx) = 1;

5378   REG_POINTER (virtual_stack_dynamic_rtx) = 1;

5379   REG_POINTER (virtual_outgoing_args_rtx) = 1;

5380   REG_POINTER (virtual_cfa_rtx) = 1;

5381

5382 #ifdef STACK_BOUNDARY

5383   REGNO_POINTER_ALIGN (STACK_POINTER_REGNUM) = STACK_BOUNDARY;

5384   REGNO_POINTER_ALIGN (FRAME_POINTER_REGNUM) = STACK_BOUNDARY;

5385   REGNO_POINTER_ALIGN (HARD_FRAME_POINTER_REGNUM) = STACK_BOUNDARY;

5386   REGNO_POINTER_ALIGN (ARG_POINTER_REGNUM) = STACK_BOUNDARY;

5387

5388   REGNO_POINTER_ALIGN (VIRTUAL_INCOMING_ARGS_REGNUM) = STACK_BOUNDARY;

5389   REGNO_POINTER_ALIGN (VIRTUAL_STACK_VARS_REGNUM) = STACK_BOUNDARY;

5390   REGNO_POINTER_ALIGN (VIRTUAL_STACK_DYNAMIC_REGNUM) = STACK_BOUNDARY;

5391   REGNO_POINTER_ALIGN (VIRTUAL_OUTGOING_ARGS_REGNUM) = STACK_BOUNDARY;

5392   REGNO_POINTER_ALIGN (VIRTUAL_CFA_REGNUM) = BITS_PER_WORD;

5393 #endif

5394

5395 #ifdef INIT_EXPANDERS

5396   INIT_EXPANDERS;

5397 #endif

5398 }

 

In source, to facilitate data member accession, macros are defined for all members in structure function, following are those used in above function.

 

114   #define reg_rtx_no (cfun->emit->x_reg_rtx_no)                                           in function.h

115   #define seq_rtl_expr (cfun->emit->sequence_rtl_expr)

116   #define regno_reg_rtx (cfun->emit->x_regno_reg_rtx)

117   #define seq_stack (cfun->emit->sequence_stack)

 

173  #define first_insn (cfun->emit->x_first_insn)                                                in emit-rtl.h

174  #define last_insn (cfun->emit->x_last_insn)

175  #define cur_insn_uid (cfun->emit->x_cur_insn_uid)

176  #define last_location (cfun->emit->x_last_location)

177  #define first_label_num (cfun->emit->x_first_label_num)

 

At line 5362, remember that static_regno_reg_rtx is created in init_emit_once which stands for hard registers in rtl language. It can be regarded as a constant set, for every time before compiling a function, the whole static_regno_reg_rtx is copied into x_reg_rtx_no of emit_status as here. Then init_virtual_regs updates special virtual registers as following.

 

5147 void

5148 init_virtual_regs (struct emit_status *es)                                                      in emit-rtl.c

5149 {

5150   rtx *ptr = es->x_regno_reg_rtx;

5151   ptr[VIRTUAL_INCOMING_ARGS_REGNUM] = virtual_incoming_args_rtx;

5152   ptr[VIRTUAL_STACK_VARS_REGNUM] = virtual_stack_vars_rtx;

5153   ptr[VIRTUAL_STACK_DYNAMIC_REGNUM] = virtual_stack_dynamic_rtx;

5154   ptr[VIRTUAL_OUTGOING_ARGS_REGNUM] = virtual_outgoing_args_rtx;

5155   ptr[VIRTUAL_CFA_REGNUM] = virtual_cfa_rtx;

5156 }

 

Above, on the right hand side of the assignments, are all macros selecting certain items of global_rtx, which are also created in init_emit_once, and note that they all of Pmode. Pmode is target dependent and not considered during static_regno_reg_rtx creation.

Macro REG_POINTER checks if rtx object is a register that holds a pointer value.

 

1032 #define REG_POINTER(RTX)                                       /                           in rtl.h

1033   (RTL_FLAG_CHECK1("REG_POINTER", (RTX), REG)->frame_related)

 

405  #define RTL_FLAG_CHECK1(NAME, RTX, C1) __extension__      /             in rtl.h

406  ({ rtx const _rtx = (RTX);                                      /

407     if (GET_CODE(_rtx) != C1)                                     /

408       rtl_check_failed_flag  (NAME, _rtx, __FILE__, __LINE__,         /

409                     __FUNCTION__);                            /

410     _rtx; })

 

We can see that all these needed special treated registers are frame related, as the assignment tells. Macro REGNO_POINTER_ALIGN selects the alignment requirement for the certain registers in emit_status instance. STACK_BOUNDARY in 32 bits ABI x86 machine is word size (ie. 4 bytes).

 

119   #define REGNO_POINTER_ALIGN(REGNO) (cfun->emit->regno_pointer_align[REGNO])

 

x86 machine doesn’t define INIT_EXPANDERS at line 5395.

In prepare_function_start, the next function is init_varasm_status, it initializes constant hashing pool for the new function object. Constant hashing pool is contained in structure varasm_status.

 

75    #define MAX_RTX_HASH_TABLE 61

76   

77    struct varasm_status GTY(())                                                                             in varasm.c

78    {

79      /* Hash facility for making memory-constants

80        from constant rtl-expressions. It is used on RISC machines

81        where immediate integer arguments and constant addresses are restricted

82        so that such constants must be stored in memory.

83   

84        This pool of constants is reinitialized for each function

85        so each function gets its own constants-pool that comes right before

86        it.  */

87      struct constant_descriptor_rtx ** GTY ((length ("MAX_RTX_HASH_TABLE")))

88        x_const_rtx_hash_table;

89      struct pool_constant ** GTY ((length ("MAX_RTX_HASH_TABLE")))

90        x_const_rtx_sym_hash_table;

91   

92     /* Pointers to first and last constant in pool.  */

93      struct pool_constant *x_first_pool;

94      struct pool_constant *x_last_pool;

95   

96      /* Current offset in constant pool (does not include any machine-specific

97        header).  */

98      HOST_WIDE_INT x_pool_offset;

99   

100    /* Number of tree-constants deferred during the expansion of this

101      function.  */

102    unsigned int deferred_constants;

103  };

 

At line 89, 93, and 94 above, pool_constant records sufficient information about a constant. In it, the key part is the rtx object constant, at line 2642 below.

 

2636 struct pool_constant GTY(())                                                                             in varasm.c

2637 {

2638   struct constant_descriptor_rtx *desc;

2639   struct pool_constant *next;

2640   struct pool_constant *next_sym;

2641   rtx constant;

2642   enum machine_mode mode;

2643   int labelno;

2644   unsigned int align;

2645   HOST_WIDE_INT offset;

2646   int mark;

2647 };

 

2657 void

2658 init_varasm_status (struct function *f)                                                         in varasm.c

2659 {

2660   struct varasm_status *p;

2661   p = ggc_alloc (sizeof (struct varasm_status));

2662   f->varasm = p;

2663   p->x_const_rtx_hash_table

2664     = ggc_alloc_cleared (MAX_RTX_HASH_TABLE

2665                        * sizeof (struct constant_descriptor_rtx *));

2666   p->x_const_rtx_sym_hash_table

2667     = ggc_alloc_cleared (MAX_RTX_HASH_TABLE

2668                        * sizeof (struct pool_constant *));

2669

2670   p->x_first_pool = p->x_last_pool = 0;

2671   p->x_pool_offset = 0;

2672   p->deferred_constants = 0;

2673 }

 

Back to prepare_function_start, it then invokes init_expr to create an instance of expr_status. It is used to control the handling of every expression in the function.

 

121  struct expr_status GTY(())                                                                          in function.h

122  {

123    /* Number of units that we should eventually pop off the stack.

124      These are the arguments to function calls that have already returned.  */

125    int x_pending_stack_adjust;

126 

127    /* Under some ABIs, it is the caller's responsibility to pop arguments

128      pushed for function calls. A naive implementation would simply pop

129      the arguments immediately after each call. However, if several

130      function calls are made in a row, it is typically cheaper to pop

131      all the arguments after all of the calls are complete since a

132      single pop instruction can be used. Therefore, GCC attempts to

133      defer popping the arguments until absolutely necessary.  (For

134      example, at the end of a conditional, the arguments must be popped,

135      since code outside the conditional won't know whether or not the

136      arguments need to be popped.)

137 

138      When INHIBIT_DEFER_POP is nonzero, however, the compiler does not

139      attempt to defer pops. Instead, the stack is popped immediately

140      after each call. Rather then setting this variable directly, use

141      NO_DEFER_POP and OK_DEFER_POP.  */

142    int x_inhibit_defer_pop;

143 

144   /* If PREFERRED_STACK_BOUNDARY and PUSH_ROUNDING are defined, the

145      stack boundary can be momentarily unaligned while pushing the arguments.

146      Record the delta since last aligned boundary here in order to get

147      stack alignment in the nested function calls working right.  */

148    int x_stack_pointer_delta;

149 

150   /* Nonzero means __builtin_saveregs has already been done in this function.

151      The value is the pseudoreg containing the value __builtin_saveregs

152      returned.  */

153    rtx x_saveregs_value;

154 

155    /* Similarly for __builtin_apply_args.  */

156    rtx x_apply_args_value;

157 

158   /* List of labels that must never be deleted.  */

159    rtx x_forced_labels;

160 

161    /* Postincrements that still need to be expanded.  */

162    rtx x_pending_chain;

163  };

 

At last, ggc_alloc_cleared creates an object filled with zero.

 

325  void

326  init_expr (void)                                                                                         in expr.c

327  {

328    cfun->expr = ggc_alloc_cleared (sizeof (struct expr_status));

329  }