gcc/ipa-comdats.c

   1 /* Localize comdats.
   2    Copyright (C) 2014 Free Software Foundation, Inc.
   3
   4 This file is part of GCC.
   5
   6 GCC is free software; you can redistribute it and/or modify it under
   7 the terms of the GNU General Public License as published by the Free
   8 Software Foundation; either version 3, or (at your option) any later
   9 version.
  10
  11 GCC is distributed in the hope that it will be useful, but WITHOUT ANY
  12 WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13 FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14 for more details.
  15
  16 You should have received a copy of the GNU General Public License
  17 along with GCC; see the file COPYING3.  If not see
  18 <http://www.gnu.org/licenses/>.  */
  19
  20 /* This is very simple pass that looks for static symbols that are used
  21    exlusively by symbol within one comdat group.  In this case it makes
  22    sense to bring the symbol itself into the group to avoid dead code
  23    that would arrise when the comdat group from current unit is replaced
  24    by a different copy.  Consider for example:
  25
  26     static int q(void)
  27     {
  28       ....
  29     }
  30     inline int t(void)
  31     {
  32       return q();
  33     }
  34
  35    if Q is used only by T, it makes sense to put Q into T's comdat group.
  36
  37    The pass solve simple dataflow across the callgraph trying to prove what
  38    symbols are used exclusively from a given comdat group.
  39
  40    The implementation maintains a queue linked by AUX pointer terminated by
  41    pointer value 1. Lattice values are NULL for TOP, actual comdat group, or
  42    ERROR_MARK_NODE for bottom.
  43
  44    TODO: When symbol is used only by comdat symbols, but from different groups,
  45    it would make sense to produce a new comdat group for it with anonymous name.
  46
  47    TODO2: We can't mix variables and functions within one group.  Currently
  48    we just give up on references of symbols of different types.  We also should
  49    handle this by anonymous comdat group section.  */
  50
  51 #include "config.h"
  52 #include "system.h"
  53 #include "coretypes.h"
  54 #include "tm.h"
  55 #include "tree.h"
  56 #include "cgraph.h"
  57 #include "tree-pass.h"
  58 #include "hash-map.h"
  59
  60 /* Main dataflow loop propagating comdat groups across
  61    the symbol table.  All references to SYMBOL are examined
  62    and NEWGROUP is updated accordingly. MAP holds current lattice
  63    values for individual symbols.  */
  64
  65 tree
  66 propagate_comdat_group (struct symtab_node *symbol,
  67                         tree newgroup, hash_map<symtab_node *, tree> &map)
  68 {
  69   int i;
  70   struct ipa_ref *ref;
  71
  72   /* Walk all references to SYMBOL, recursively dive into aliases.  */
  73
  74   for (i = 0;
  75        symbol->iterate_referring (i, ref)
  76        && newgroup != error_mark_node; i++)
  77     {
  78       struct symtab_node *symbol2 = ref->referring;
  79
  80       if (ref->use == IPA_REF_ALIAS)
  81         {
  82           newgroup = propagate_comdat_group (symbol2, newgroup, map);
  83           continue;
  84         }
  85
  86       /* One COMDAT group can not hold both variables and functions at
  87          a same time.  For now we just go to BOTTOM, in future we may
  88          invent special comdat groups for this case.  */
  89
  90       if (symbol->type != symbol2->type)
  91         {
  92           newgroup = error_mark_node;
  93           break;
  94         }
  95
  96       /* If we see inline clone, its comdat group actually
  97          corresponds to the comdat group of the function it is inlined
  98          to.  */
  99
 100       if (cgraph_node * cn = dyn_cast <cgraph_node *> (symbol2))
 101         {
 102           if (cn->global.inlined_to)
 103             symbol2 = cn->global.inlined_to;
 104         }
 105
 106       /* The actual merge operation.  */
 107
 108       tree *val2 = map.get (symbol2);
 109
 110       if (val2 && *val2 != newgroup)
 111         {
 112           if (!newgroup)
 113             newgroup = *val2;
 114           else
 115             newgroup = error_mark_node;
 116         }
 117     }
 118
 119   /* If we analyze function, walk also callers.  */
 120
 121   cgraph_node *cnode = dyn_cast <cgraph_node *> (symbol);
 122
 123   if (cnode)
 124     for (struct cgraph_edge * edge = cnode->callers;
 125          edge && newgroup != error_mark_node; edge = edge->next_caller)
 126       {
 127         struct symtab_node *symbol2 = edge->caller;
 128
 129         /* If we see inline clone, its comdat group actually
 130            corresponds to the comdat group of the function it is inlined
 131            to.  */
 132
 133         if (cgraph_node * cn = dyn_cast <cgraph_node *> (symbol2))
 134           {
 135             if (cn->global.inlined_to)
 136               symbol2 = cn->global.inlined_to;
 137           }
 138
 139         /* The actual merge operation.  */
 140
 141         tree *val2 = map.get (symbol2);
 142
 143         if (val2 && *val2 != newgroup)
 144           {
 145             if (!newgroup)
 146               newgroup = *val2;
 147             else
 148               newgroup = error_mark_node;
 149           }
 150       }
 151   return newgroup;
 152 }
 153
 154
 155 /* Add all references of SYMBOL that are defined into queue started by FIRST
 156    and linked by AUX pointer (unless they are already enqueued).
 157    Walk recursively inlined functions.  */
 158
 159 void
 160 enqueue_references (symtab_node **first,
 161                     symtab_node *symbol)
 162 {
 163   int i;
 164   struct ipa_ref *ref = NULL;
 165
 166   for (i = 0; symbol->iterate_reference (i, ref); i++)
 167     {
 168       symtab_node *node = ref->referred->ultimate_alias_target ();
 169       if (!node->aux && node->definition)
 170         {
 171            node->aux = *first;
 172            *first = node;
 173         }
 174     }
 175
 176   if (cgraph_node *cnode = dyn_cast <cgraph_node *> (symbol))
 177     {
 178       struct cgraph_edge *edge;
 179
 180       for (edge = cnode->callees; edge; edge = edge->next_callee)
 181         if (!edge->inline_failed)
 182           enqueue_references (first, edge->callee);
 183         else
 184           {
 185             symtab_node *node = edge->callee->ultimate_alias_target ();
 186             if (!node->aux && node->definition)
 187               {
 188                  node->aux = *first;
 189                  *first = node;
 190               }
 191           }
 192     }
 193 }
 194
 195 /* Set comdat group of SYMBOL to GROUP.
 196    Callback for symtab_for_node_and_aliases.  */
 197
 198 bool
 199 set_comdat_group (symtab_node *symbol,
 200                   void *head_p)
 201 {
 202   symtab_node *head = (symtab_node *)head_p;
 203
 204   gcc_assert (!symbol->get_comdat_group ());
 205   symbol->set_comdat_group (head->get_comdat_group ());
 206   symbol->add_to_same_comdat_group (head);
 207   return false;
 208 }
 209
 210 /* The actual pass with the main dataflow loop.  */
 211
 212 static unsigned int
 213 ipa_comdats (void)
 214 {
 215   hash_map<symtab_node *, tree> map (251);
 216   hash_map<tree, symtab_node *> comdat_head_map (251);
 217   symtab_node *symbol;
 218   bool comdat_group_seen = false;
 219   symtab_node *first = (symtab_node *) (void *) 1;
 220   tree group;
 221
 222   /* Start the dataflow by assigning comdat group to symbols that are in comdat
 223      groups already.  All other externally visible symbols must stay, we use
 224      ERROR_MARK_NODE as bottom for the propagation.  */
 225
 226   FOR_EACH_DEFINED_SYMBOL (symbol)
 227     if (!symbol->real_symbol_p ())
 228       ;
 229     else if ((group = symbol->get_comdat_group ()) != NULL)
 230       {
 231         map.put (symbol, group);
 232         comdat_head_map.put (group, symbol);
 233         comdat_group_seen = true;
 234
 235         /* Mark the symbol so we won't waste time visiting it for dataflow.  */
 236         symbol->aux = (symtab_node *) (void *) 1;
 237       }
 238     /* See symbols that can not be privatized to comdats; that is externally
 239        visible symbols or otherwise used ones.  We also do not want to mangle
 240        user section names.  */
 241     else if (symbol->externally_visible
 242              || symbol->force_output
 243              || symbol->used_from_other_partition
 244              || TREE_THIS_VOLATILE (symbol->decl)
 245              || symbol->get_section ()
 246              || (TREE_CODE (symbol->decl) == FUNCTION_DECL
 247                  && (DECL_STATIC_CONSTRUCTOR (symbol->decl)
 248                      || DECL_STATIC_DESTRUCTOR (symbol->decl))))
 249       {
 250         map.put (symbol->ultimate_alias_target (), error_mark_node);
 251
 252         /* Mark the symbol so we won't waste time visiting it for dataflow.  */
 253         symbol->aux = (symtab_node *) (void *) 1;
 254       }
 255     else
 256       {
 257         /* Enqueue symbol for dataflow.  */
 258         symbol->aux = first;
 259         first = symbol;
 260       }
 261
 262   if (!comdat_group_seen)
 263     {
 264       FOR_EACH_DEFINED_SYMBOL (symbol)
 265         symbol->aux = NULL;
 266       return 0;
 267     }
 268
 269   /* The actual dataflow.  */
 270
 271   while (first != (void *) 1)
 272     {
 273       tree group = NULL;
 274       tree newgroup, *val;
 275
 276       symbol = first;
 277       first = (symtab_node *)first->aux;
 278
 279       /* Get current lattice value of SYMBOL.  */
 280       val = map.get (symbol);
 281       if (val)
 282         group = *val;
 283
 284       /* If it is bottom, there is nothing to do; do not clear AUX
 285          so we won't re-queue the symbol.  */
 286       if (group == error_mark_node)
 287         continue;
 288
 289       newgroup = propagate_comdat_group (symbol, group, map);
 290
 291       /* If nothing changed, proceed to next symbol.  */
 292       if (newgroup == group)
 293         {
 294           symbol->aux = NULL;
 295           continue;
 296         }
 297
 298       /* Update lattice value and enqueue all references for re-visiting.  */
 299       gcc_assert (newgroup);
 300       if (val)
 301         *val = newgroup;
 302       else
 303         map.put (symbol, newgroup);
 304       enqueue_references (&first, symbol);
 305
 306       /* We may need to revisit the symbol unless it is BOTTOM.  */
 307       if (newgroup != error_mark_node)
 308         symbol->aux = NULL;
 309     }
 310
 311   /* Finally assign symbols to the sections.  */
 312
 313   FOR_EACH_DEFINED_SYMBOL (symbol)
 314     {
 315       symbol->aux = NULL;
 316       if (!symbol->get_comdat_group ()
 317           && !symbol->alias
 318           && symbol->real_symbol_p ())
 319         {
 320           tree group = *map.get (symbol);
 321
 322           if (group == error_mark_node)
 323             continue;
 324           if (dump_file)
 325             {
 326               fprintf (dump_file, "Localizing symbol\n");
 327               symbol->dump (dump_file);
 328               fprintf (dump_file, "To group: %s\n", IDENTIFIER_POINTER (group));
 329             }
 330           symbol->call_for_symbol_and_aliases (set_comdat_group,
 331                                              *comdat_head_map.get (group),
 332                                              true);
 333         }
 334     }
 335   return 0;
 336 }
 337
 338 namespace {
 339
 340 const pass_data pass_data_ipa_comdats =
 341 {
 342   IPA_PASS, /* type */
 343   "comdats", /* name */
 344   OPTGROUP_NONE, /* optinfo_flags */
 345   TV_IPA_COMDATS, /* tv_id */
 346   0, /* properties_required */
 347   0, /* properties_provided */
 348   0, /* properties_destroyed */
 349   0, /* todo_flags_start */
 350   0, /* todo_flags_finish */
 351 };
 352
 353 class pass_ipa_comdats : public ipa_opt_pass_d
 354 {
 355 public:
 356   pass_ipa_comdats (gcc::context *ctxt)
 357     : ipa_opt_pass_d (pass_data_ipa_comdats, ctxt,
 358                       NULL, /* generate_summary */
 359                       NULL, /* write_summary */
 360                       NULL, /* read_summary */
 361                       NULL, /* write_optimization_summary */
 362                       NULL, /* read_optimization_summary */
 363                       NULL, /* stmt_fixup */
 364                       0, /* function_transform_todo_flags_start */
 365                       NULL, /* function_transform */
 366                       NULL) /* variable_transform */
 367   {}
 368
 369   /* opt_pass methods: */
 370   virtual bool gate (function *);
 371   virtual unsigned int execute (function *) { return ipa_comdats (); }
 372
 373 }; // class pass_ipa_comdats
 374
 375 bool
 376 pass_ipa_comdats::gate (function *)
 377 {
 378   return optimize;
 379 }
 380
 381 } // anon namespace
 382
 383 ipa_opt_pass_d *
 384 make_pass_ipa_comdats (gcc::context *ctxt)
 385 {
 386   return new pass_ipa_comdats (ctxt);
 387 }