gcc/lto-symtab.c

   1 /* LTO symbol table.
   2    Copyright 2009, 2010 Free Software Foundation, Inc.
   3    Contributed by CodeSourcery, Inc.
   4
   5 This file is part of GCC.
   6
   7 GCC is free software; you can redistribute it and/or modify it under
   8 the terms of the GNU General Public License as published by the Free
   9 Software Foundation; either version 3, or (at your option) any later
  10 version.
  11
  12 GCC is distributed in the hope that it will be useful, but WITHOUT ANY
  13 WARRANTY; without even the implied warranty of MERCHANTABILITY or
  14 FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  15 for more details.
  16
  17 You should have received a copy of the GNU General Public License
  18 along with GCC; see the file COPYING3.  If not see
  19 <http://www.gnu.org/licenses/>.  */
  20
  21 #include "config.h"
  22 #include "system.h"
  23 #include "coretypes.h"
  24 #include "diagnostic-core.h"
  25 #include "tree.h"
  26 #include "gimple.h"
  27 #include "ggc.h"
  28 #include "hashtab.h"
  29 #include "plugin-api.h"
  30 #include "lto-streamer.h"
  31
  32 /* Vector to keep track of external variables we've seen so far.  */
  33 VEC(tree,gc) *lto_global_var_decls;
  34
  35 /* Symbol table entry.  */
  36
  37 struct GTY(()) lto_symtab_entry_def
  38 {
  39   /* The symbol table entry key, an IDENTIFIER.  */
  40   tree id;
  41   /* The symbol table entry, a DECL.  */
  42   tree decl;
  43   /* The cgraph node if decl is a function decl.  Filled in during the
  44      merging process.  */
  45   struct cgraph_node *node;
  46   /* The varpool node if decl is a variable decl.  Filled in during the
  47      merging process.  */
  48   struct varpool_node *vnode;
  49   /* LTO file-data and symbol resolution for this decl.  */
  50   struct lto_file_decl_data * GTY((skip (""))) file_data;
  51   enum ld_plugin_symbol_resolution resolution;
  52   /* True when resolution was guessed and not read from the file.  */
  53   bool guessed;
  54   /* Pointer to the next entry with the same key.  Before decl merging
  55      this links all symbols from the different TUs.  After decl merging
  56      this links merged but incompatible decls, thus all prevailing ones
  57      remaining.  */
  58   struct lto_symtab_entry_def *next;
  59 };
  60 typedef struct lto_symtab_entry_def *lto_symtab_entry_t;
  61
  62 /* A poor man's symbol table. This hashes identifier to prevailing DECL
  63    if there is one. */
  64
  65 static GTY ((if_marked ("lto_symtab_entry_marked_p"),
  66              param_is (struct lto_symtab_entry_def)))
  67   htab_t lto_symtab_identifiers;
  68
  69 /* Free symtab hashtable.  */
  70
  71 void
  72 lto_symtab_free (void)
  73 {
  74   htab_delete (lto_symtab_identifiers);
  75   lto_symtab_identifiers = NULL;
  76 }
  77
  78 /* Return the hash value of an lto_symtab_entry_t object pointed to by P.  */
  79
  80 static hashval_t
  81 lto_symtab_entry_hash (const void *p)
  82 {
  83   const struct lto_symtab_entry_def *base =
  84     (const struct lto_symtab_entry_def *) p;
  85   return IDENTIFIER_HASH_VALUE (base->id);
  86 }
  87
  88 /* Return non-zero if P1 and P2 points to lto_symtab_entry_def structs
  89    corresponding to the same symbol.  */
  90
  91 static int
  92 lto_symtab_entry_eq (const void *p1, const void *p2)
  93 {
  94   const struct lto_symtab_entry_def *base1 =
  95      (const struct lto_symtab_entry_def *) p1;
  96   const struct lto_symtab_entry_def *base2 =
  97      (const struct lto_symtab_entry_def *) p2;
  98   return (base1->id == base2->id);
  99 }
 100
 101 /* Returns non-zero if P points to an lto_symtab_entry_def struct that needs
 102    to be marked for GC.  */
 103
 104 static int
 105 lto_symtab_entry_marked_p (const void *p)
 106 {
 107   const struct lto_symtab_entry_def *base =
 108      (const struct lto_symtab_entry_def *) p;
 109
 110   /* Keep this only if the common IDENTIFIER_NODE of the symtab chain
 111      is marked which it will be if at least one of the DECLs in the
 112      chain is marked.  */
 113   return ggc_marked_p (base->id);
 114 }
 115
 116 /* Lazily initialize resolution hash tables.  */
 117
 118 static void
 119 lto_symtab_maybe_init_hash_table (void)
 120 {
 121   if (lto_symtab_identifiers)
 122     return;
 123
 124   lto_symtab_identifiers =
 125     htab_create_ggc (1021, lto_symtab_entry_hash,
 126                      lto_symtab_entry_eq, NULL);
 127 }
 128
 129 /* Registers DECL with the LTO symbol table as having resolution RESOLUTION
 130    and read from FILE_DATA. */
 131
 132 void
 133 lto_symtab_register_decl (tree decl,
 134                           ld_plugin_symbol_resolution_t resolution,
 135                           struct lto_file_decl_data *file_data)
 136 {
 137   lto_symtab_entry_t new_entry;
 138   void **slot;
 139
 140   /* Check that declarations reaching this function do not have
 141      properties inconsistent with having external linkage.  If any of
 142      these asertions fail, then the object file reader has failed to
 143      detect these cases and issue appropriate error messages.  */
 144   gcc_assert (decl
 145               && TREE_PUBLIC (decl)
 146               && (TREE_CODE (decl) == VAR_DECL
 147                   || TREE_CODE (decl) == FUNCTION_DECL)
 148               && DECL_ASSEMBLER_NAME_SET_P (decl));
 149   if (TREE_CODE (decl) == VAR_DECL
 150       && DECL_INITIAL (decl))
 151     gcc_assert (!DECL_EXTERNAL (decl)
 152                 || (TREE_STATIC (decl) && TREE_READONLY (decl)));
 153   if (TREE_CODE (decl) == FUNCTION_DECL)
 154     gcc_assert (!DECL_ABSTRACT (decl));
 155
 156   new_entry = ggc_alloc_cleared_lto_symtab_entry_def ();
 157   new_entry->id = (*targetm.asm_out.mangle_assembler_name)
 158                   (IDENTIFIER_POINTER (DECL_ASSEMBLER_NAME (decl)));
 159   new_entry->decl = decl;
 160   new_entry->resolution = resolution;
 161   new_entry->file_data = file_data;
 162
 163   lto_symtab_maybe_init_hash_table ();
 164   slot = htab_find_slot (lto_symtab_identifiers, new_entry, INSERT);
 165   new_entry->next = (lto_symtab_entry_t) *slot;
 166   *slot = new_entry;
 167 }
 168
 169 /* Get the lto_symtab_entry_def struct associated with ID
 170    if there is one.  */
 171
 172 static lto_symtab_entry_t
 173 lto_symtab_get (tree id)
 174 {
 175   struct lto_symtab_entry_def temp;
 176   void **slot;
 177
 178   lto_symtab_maybe_init_hash_table ();
 179   temp.id = id;
 180   slot = htab_find_slot (lto_symtab_identifiers, &temp, NO_INSERT);
 181   return slot ? (lto_symtab_entry_t) *slot : NULL;
 182 }
 183
 184 /* Get the linker resolution for DECL.  */
 185
 186 enum ld_plugin_symbol_resolution
 187 lto_symtab_get_resolution (tree decl)
 188 {
 189   lto_symtab_entry_t e;
 190
 191   gcc_assert (DECL_ASSEMBLER_NAME_SET_P (decl));
 192
 193   e = lto_symtab_get ((*targetm.asm_out.mangle_assembler_name)
 194                       (IDENTIFIER_POINTER (DECL_ASSEMBLER_NAME (decl))));
 195   while (e && e->decl != decl)
 196     e = e->next;
 197   if (!e)
 198     return LDPR_UNKNOWN;
 199
 200   return e->resolution;
 201 }
 202
 203
 204 /* Replace the cgraph node NODE with PREVAILING_NODE in the cgraph, merging
 205    all edges and removing the old node.  */
 206
 207 static void
 208 lto_cgraph_replace_node (struct cgraph_node *node,
 209                          struct cgraph_node *prevailing_node)
 210 {
 211   struct cgraph_edge *e, *next;
 212   bool compatible_p;
 213
 214   if (cgraph_dump_file)
 215     {
 216       fprintf (cgraph_dump_file, "Replacing cgraph node %s/%i by %s/%i"
 217                " for symbol %s\n",
 218                cgraph_node_name (node), node->uid,
 219                cgraph_node_name (prevailing_node),
 220                prevailing_node->uid,
 221                IDENTIFIER_POINTER ((*targetm.asm_out.mangle_assembler_name)
 222                  (IDENTIFIER_POINTER (DECL_ASSEMBLER_NAME (node->decl)))));
 223     }
 224
 225   /* Merge node flags.  */
 226   if (node->needed)
 227     cgraph_mark_needed_node (prevailing_node);
 228   if (node->reachable)
 229     cgraph_mark_reachable_node (prevailing_node);
 230   if (node->address_taken)
 231     {
 232       gcc_assert (!prevailing_node->global.inlined_to);
 233       cgraph_mark_address_taken_node (prevailing_node);
 234     }
 235
 236   /* Redirect all incoming edges.  */
 237   compatible_p
 238     = types_compatible_p (TREE_TYPE (TREE_TYPE (prevailing_node->decl)),
 239                           TREE_TYPE (TREE_TYPE (node->decl)));
 240   for (e = node->callers; e; e = next)
 241     {
 242       next = e->next_caller;
 243       cgraph_redirect_edge_callee (e, prevailing_node);
 244       /* If there is a mismatch between the supposed callee return type and
 245          the real one do not attempt to inline this function.
 246          ???  We really need a way to match function signatures for ABI
 247          compatibility and perform related promotions at inlining time.  */
 248       if (!compatible_p)
 249         e->call_stmt_cannot_inline_p = 1;
 250     }
 251   /* Redirect incomming references.  */
 252   ipa_clone_refering (prevailing_node, NULL, &node->ref_list);
 253
 254   /* Finally remove the replaced node.  */
 255   cgraph_remove_node (node);
 256 }
 257
 258 /* Replace the cgraph node NODE with PREVAILING_NODE in the cgraph, merging
 259    all edges and removing the old node.  */
 260
 261 static void
 262 lto_varpool_replace_node (struct varpool_node *vnode,
 263                           struct varpool_node *prevailing_node)
 264 {
 265   /* Merge node flags.  */
 266   if (vnode->needed)
 267     {
 268       gcc_assert (!vnode->analyzed || prevailing_node->analyzed);
 269       varpool_mark_needed_node (prevailing_node);
 270     }
 271   /* Relink aliases.  */
 272   if (vnode->extra_name && !vnode->alias)
 273     {
 274       struct varpool_node *alias, *last;
 275       for (alias = vnode->extra_name;
 276            alias; alias = alias->next)
 277         {
 278           last = alias;
 279           alias->extra_name = prevailing_node;
 280         }
 281
 282       if (prevailing_node->extra_name)
 283         {
 284           last->next = prevailing_node->extra_name;
 285           prevailing_node->extra_name->prev = last;
 286         }
 287       prevailing_node->extra_name = vnode->extra_name;
 288       vnode->extra_name = NULL;
 289     }
 290   gcc_assert (!vnode->finalized || prevailing_node->finalized);
 291   gcc_assert (!vnode->analyzed || prevailing_node->analyzed);
 292
 293   /* When replacing by an alias, the references goes to the original
 294      variable.  */
 295   if (prevailing_node->alias && prevailing_node->extra_name)
 296     prevailing_node = prevailing_node->extra_name;
 297   ipa_clone_refering (NULL, prevailing_node, &vnode->ref_list);
 298
 299   /* Be sure we can garbage collect the initializer.  */
 300   if (DECL_INITIAL (vnode->decl))
 301     DECL_INITIAL (vnode->decl) = error_mark_node;
 302   /* Finally remove the replaced node.  */
 303   varpool_remove_node (vnode);
 304 }
 305
 306 /* Merge two variable or function symbol table entries PREVAILING and ENTRY.
 307    Return false if the symbols are not fully compatible and a diagnostic
 308    should be emitted.  */
 309
 310 static bool
 311 lto_symtab_merge (lto_symtab_entry_t prevailing, lto_symtab_entry_t entry)
 312 {
 313   tree prevailing_decl = prevailing->decl;
 314   tree decl = entry->decl;
 315   tree prevailing_type, type;
 316
 317   /* Merge decl state in both directions, we may still end up using
 318      the new decl.  */
 319   TREE_ADDRESSABLE (prevailing_decl) |= TREE_ADDRESSABLE (decl);
 320   TREE_ADDRESSABLE (decl) |= TREE_ADDRESSABLE (prevailing_decl);
 321
 322   /* The linker may ask us to combine two incompatible symbols.
 323      Detect this case and notify the caller of required diagnostics.  */
 324
 325   if (TREE_CODE (decl) == FUNCTION_DECL)
 326     {
 327       if (!types_compatible_p (TREE_TYPE (prevailing_decl),
 328                                TREE_TYPE (decl)))
 329         /* If we don't have a merged type yet...sigh.  The linker
 330            wouldn't complain if the types were mismatched, so we
 331            probably shouldn't either.  Just use the type from
 332            whichever decl appears to be associated with the
 333            definition.  If for some odd reason neither decl is, the
 334            older one wins.  */
 335         (void) 0;
 336
 337       return true;
 338     }
 339
 340   /* Now we exclusively deal with VAR_DECLs.  */
 341
 342   /* Sharing a global symbol is a strong hint that two types are
 343      compatible.  We could use this information to complete
 344      incomplete pointed-to types more aggressively here, ignoring
 345      mismatches in both field and tag names.  It's difficult though
 346      to guarantee that this does not have side-effects on merging
 347      more compatible types from other translation units though.  */
 348
 349   /* We can tolerate differences in type qualification, the
 350      qualification of the prevailing definition will prevail.
 351      ???  In principle we might want to only warn for structurally
 352      incompatible types here, but unless we have protective measures
 353      for TBAA in place that would hide useful information.  */
 354   prevailing_type = TYPE_MAIN_VARIANT (TREE_TYPE (prevailing_decl));
 355   type = TYPE_MAIN_VARIANT (TREE_TYPE (decl));
 356
 357   if (!types_compatible_p (prevailing_type, type))
 358     {
 359       if (COMPLETE_TYPE_P (type))
 360         return false;
 361
 362       /* If type is incomplete then avoid warnings in the cases
 363          that TBAA handles just fine.  */
 364
 365       if (TREE_CODE (prevailing_type) != TREE_CODE (type))
 366         return false;
 367
 368       if (TREE_CODE (prevailing_type) == ARRAY_TYPE)
 369         {
 370           tree tem1 = TREE_TYPE (prevailing_type);
 371           tree tem2 = TREE_TYPE (type);
 372           while (TREE_CODE (tem1) == ARRAY_TYPE
 373                  && TREE_CODE (tem2) == ARRAY_TYPE)
 374             {
 375               tem1 = TREE_TYPE (tem1);
 376               tem2 = TREE_TYPE (tem2);
 377             }
 378
 379           if (TREE_CODE (tem1) != TREE_CODE (tem2))
 380             return false;
 381
 382           if (!types_compatible_p (tem1, tem2))
 383             return false;
 384         }
 385
 386       /* Fallthru.  Compatible enough.  */
 387     }
 388
 389   /* ???  We might want to emit a warning here if type qualification
 390      differences were spotted.  Do not do this unconditionally though.  */
 391
 392   /* There is no point in comparing too many details of the decls here.
 393      The type compatibility checks or the completing of types has properly
 394      dealt with most issues.  */
 395
 396   /* The following should all not invoke fatal errors as in non-LTO
 397      mode the linker wouldn't complain either.  Just emit warnings.  */
 398
 399   /* Report a warning if user-specified alignments do not match.  */
 400   if ((DECL_USER_ALIGN (prevailing_decl) && DECL_USER_ALIGN (decl))
 401       && DECL_ALIGN (prevailing_decl) < DECL_ALIGN (decl))
 402     return false;
 403
 404   return true;
 405 }
 406
 407 /* Return true if the symtab entry E can be replaced by another symtab
 408    entry.  */
 409
 410 static bool
 411 lto_symtab_resolve_replaceable_p (lto_symtab_entry_t e)
 412 {
 413   if (DECL_EXTERNAL (e->decl)
 414       || DECL_COMDAT (e->decl)
 415       || DECL_ONE_ONLY (e->decl)
 416       || DECL_WEAK (e->decl))
 417     return true;
 418
 419   if (TREE_CODE (e->decl) == VAR_DECL)
 420     return (DECL_COMMON (e->decl)
 421             || (!flag_no_common && !DECL_INITIAL (e->decl)));
 422
 423   return false;
 424 }
 425
 426 /* Return true if the symtab entry E can be the prevailing one.  */
 427
 428 static bool
 429 lto_symtab_resolve_can_prevail_p (lto_symtab_entry_t e)
 430 {
 431   /* The C++ frontend ends up neither setting TREE_STATIC nor
 432      DECL_EXTERNAL on virtual methods but only TREE_PUBLIC.
 433      So do not reject !TREE_STATIC here but only DECL_EXTERNAL.  */
 434   if (DECL_EXTERNAL (e->decl))
 435     return false;
 436
 437   /* For functions we need a non-discarded body.  */
 438   if (TREE_CODE (e->decl) == FUNCTION_DECL)
 439     return (e->node && e->node->analyzed);
 440
 441   /* A variable should have a size.  */
 442   else if (TREE_CODE (e->decl) == VAR_DECL)
 443     {
 444       if (!e->vnode)
 445         return false;
 446       if (e->vnode->finalized)
 447         return true;
 448       return e->vnode->alias && e->vnode->extra_name->finalized;
 449     }
 450
 451   gcc_unreachable ();
 452 }
 453
 454 /* Resolve the symbol with the candidates in the chain *SLOT and store
 455    their resolutions.  */
 456
 457 static void
 458 lto_symtab_resolve_symbols (void **slot)
 459 {
 460   lto_symtab_entry_t e;
 461   lto_symtab_entry_t prevailing = NULL;
 462
 463   /* Always set e->node so that edges are updated to reflect decl merging. */
 464   for (e = (lto_symtab_entry_t) *slot; e; e = e->next)
 465     {
 466       if (TREE_CODE (e->decl) == FUNCTION_DECL)
 467         e->node = cgraph_get_node_or_alias (e->decl);
 468       else if (TREE_CODE (e->decl) == VAR_DECL)
 469         e->vnode = varpool_get_node (e->decl);
 470     }
 471
 472   e = (lto_symtab_entry_t) *slot;
 473
 474   /* If the chain is already resolved there is nothing else to do.  */
 475   if (e->resolution != LDPR_UNKNOWN)
 476     return;
 477
 478   /* Find the single non-replaceable prevailing symbol and
 479      diagnose ODR violations.  */
 480   for (e = (lto_symtab_entry_t) *slot; e; e = e->next)
 481     {
 482       if (!lto_symtab_resolve_can_prevail_p (e))
 483         {
 484           e->resolution = LDPR_RESOLVED_IR;
 485           e->guessed = true;
 486           continue;
 487         }
 488
 489       /* Set a default resolution - the final prevailing one will get
 490          adjusted later.  */
 491       e->resolution = LDPR_PREEMPTED_IR;
 492       e->guessed = true;
 493       if (!lto_symtab_resolve_replaceable_p (e))
 494         {
 495           if (prevailing)
 496             {
 497               error_at (DECL_SOURCE_LOCATION (e->decl),
 498                         "%qD has already been defined", e->decl);
 499               inform (DECL_SOURCE_LOCATION (prevailing->decl),
 500                       "previously defined here");
 501             }
 502           prevailing = e;
 503         }
 504     }
 505   if (prevailing)
 506     goto found;
 507
 508   /* Do a second round choosing one from the replaceable prevailing decls.  */
 509   for (e = (lto_symtab_entry_t) *slot; e; e = e->next)
 510     {
 511       if (e->resolution != LDPR_PREEMPTED_IR)
 512         continue;
 513
 514       /* Choose the first function that can prevail as prevailing.  */
 515       if (TREE_CODE (e->decl) == FUNCTION_DECL)
 516         {
 517           prevailing = e;
 518           break;
 519         }
 520
 521       /* From variables that can prevail choose the largest one.  */
 522       if (!prevailing
 523           || tree_int_cst_lt (DECL_SIZE (prevailing->decl),
 524                               DECL_SIZE (e->decl)))
 525         prevailing = e;
 526     }
 527
 528   if (!prevailing)
 529     return;
 530
 531 found:
 532   /* If current lto files represent the whole program,
 533     it is correct to use LDPR_PREVALING_DEF_IRONLY.
 534     If current lto files are part of whole program, internal
 535     resolver doesn't know if it is LDPR_PREVAILING_DEF
 536     or LDPR_PREVAILING_DEF_IRONLY.  Use IRONLY conforms to
 537     using -fwhole-program.  Otherwise, it doesn't
 538     matter using either LDPR_PREVAILING_DEF or
 539     LDPR_PREVAILING_DEF_IRONLY
 540
 541     FIXME: above workaround due to gold plugin makes some
 542     variables IRONLY, which are indeed PREVAILING_DEF in
 543     resolution file.  These variables still need manual
 544     externally_visible attribute.  */
 545     prevailing->resolution = LDPR_PREVAILING_DEF_IRONLY;
 546     prevailing->guessed = true;
 547 }
 548
 549 /* Merge all decls in the symbol table chain to the prevailing decl and
 550    issue diagnostics about type mismatches.  If DIAGNOSED_P is true
 551    do not issue further diagnostics.*/
 552
 553 static void
 554 lto_symtab_merge_decls_2 (void **slot, bool diagnosed_p)
 555 {
 556   lto_symtab_entry_t prevailing, e;
 557   VEC(tree, heap) *mismatches = NULL;
 558   unsigned i;
 559   tree decl;
 560
 561   /* Nothing to do for a single entry.  */
 562   prevailing = (lto_symtab_entry_t) *slot;
 563   if (!prevailing->next)
 564     return;
 565
 566   /* Try to merge each entry with the prevailing one.  */
 567   for (e = prevailing->next; e; e = e->next)
 568     {
 569       if (!lto_symtab_merge (prevailing, e)
 570           && !diagnosed_p)
 571         VEC_safe_push (tree, heap, mismatches, e->decl);
 572     }
 573   if (VEC_empty (tree, mismatches))
 574     return;
 575
 576   /* Diagnose all mismatched re-declarations.  */
 577   FOR_EACH_VEC_ELT (tree, mismatches, i, decl)
 578     {
 579       if (!types_compatible_p (TREE_TYPE (prevailing->decl), TREE_TYPE (decl)))
 580         diagnosed_p |= warning_at (DECL_SOURCE_LOCATION (decl), 0,
 581                                    "type of %qD does not match original "
 582                                    "declaration", decl);
 583
 584       else if ((DECL_USER_ALIGN (prevailing->decl) && DECL_USER_ALIGN (decl))
 585                && DECL_ALIGN (prevailing->decl) < DECL_ALIGN (decl))
 586         {
 587           diagnosed_p |= warning_at (DECL_SOURCE_LOCATION (decl), 0,
 588                                      "alignment of %qD is bigger than "
 589                                      "original declaration", decl);
 590         }
 591     }
 592   if (diagnosed_p)
 593     inform (DECL_SOURCE_LOCATION (prevailing->decl),
 594             "previously declared here");
 595
 596   VEC_free (tree, heap, mismatches);
 597 }
 598
 599 /* Helper to process the decl chain for the symbol table entry *SLOT.  */
 600
 601 static int
 602 lto_symtab_merge_decls_1 (void **slot, void *data ATTRIBUTE_UNUSED)
 603 {
 604   lto_symtab_entry_t e, prevailing;
 605   bool diagnosed_p = false;
 606
 607   /* Compute the symbol resolutions.  This is a no-op when using the
 608      linker plugin.  */
 609   lto_symtab_resolve_symbols (slot);
 610
 611   /* Find the prevailing decl.  */
 612   for (prevailing = (lto_symtab_entry_t) *slot;
 613        prevailing
 614        && prevailing->resolution != LDPR_PREVAILING_DEF_IRONLY
 615        && prevailing->resolution != LDPR_PREVAILING_DEF;
 616        prevailing = prevailing->next)
 617     ;
 618
 619   /* Assert it's the only one.  */
 620   if (prevailing)
 621     for (e = prevailing->next; e; e = e->next)
 622       {
 623         if (e->resolution == LDPR_PREVAILING_DEF_IRONLY
 624             || e->resolution == LDPR_PREVAILING_DEF)
 625           fatal_error ("multiple prevailing defs for %qE",
 626                        DECL_NAME (prevailing->decl));
 627       }
 628
 629   /* If there's not a prevailing symbol yet it's an external reference.
 630      Happens a lot during ltrans.  Choose the first symbol with a
 631      cgraph or a varpool node.  */
 632   if (!prevailing)
 633     {
 634       prevailing = (lto_symtab_entry_t) *slot;
 635       /* For functions choose one with a cgraph node.  */
 636       if (TREE_CODE (prevailing->decl) == FUNCTION_DECL)
 637         while (!prevailing->node
 638                && prevailing->next)
 639           prevailing = prevailing->next;
 640       /* For variables chose with a priority variant with vnode
 641          attached (i.e. from unit where external declaration of
 642          variable is actually used).
 643          When there are multiple variants, chose one with size.
 644          This is needed for C++ typeinfos, for example in
 645          lto/20081204-1 there are typeifos in both units, just
 646          one of them do have size.  */
 647       if (TREE_CODE (prevailing->decl) == VAR_DECL)
 648         {
 649           for (e = prevailing->next; e; e = e->next)
 650             if ((!prevailing->vnode && e->vnode)
 651                 || ((prevailing->vnode != NULL) == (e->vnode != NULL)
 652                     && !COMPLETE_TYPE_P (TREE_TYPE (prevailing->decl))
 653                     && COMPLETE_TYPE_P (TREE_TYPE (e->decl))))
 654               prevailing = e;
 655         }
 656     }
 657
 658   /* Move it first in the list.  */
 659   if ((lto_symtab_entry_t) *slot != prevailing)
 660     {
 661       for (e = (lto_symtab_entry_t) *slot; e->next != prevailing; e = e->next)
 662         ;
 663       e->next = prevailing->next;
 664       prevailing->next = (lto_symtab_entry_t) *slot;
 665       *slot = (void *) prevailing;
 666     }
 667
 668   /* Record the prevailing variable.  */
 669   if (TREE_CODE (prevailing->decl) == VAR_DECL)
 670     VEC_safe_push (tree, gc, lto_global_var_decls, prevailing->decl);
 671
 672   /* Diagnose mismatched objects.  */
 673   for (e = prevailing->next; e; e = e->next)
 674     {
 675       if (TREE_CODE (prevailing->decl) == TREE_CODE (e->decl))
 676         continue;
 677
 678       switch (TREE_CODE (prevailing->decl))
 679         {
 680         case VAR_DECL:
 681           gcc_assert (TREE_CODE (e->decl) == FUNCTION_DECL);
 682           error_at (DECL_SOURCE_LOCATION (e->decl),
 683                     "variable %qD redeclared as function", prevailing->decl);
 684           break;
 685
 686         case FUNCTION_DECL:
 687           gcc_assert (TREE_CODE (e->decl) == VAR_DECL);
 688           error_at (DECL_SOURCE_LOCATION (e->decl),
 689                     "function %qD redeclared as variable", prevailing->decl);
 690           break;
 691
 692         default:
 693           gcc_unreachable ();
 694         }
 695
 696       diagnosed_p = true;
 697     }
 698   if (diagnosed_p)
 699       inform (DECL_SOURCE_LOCATION (prevailing->decl),
 700               "previously declared here");
 701
 702   /* Merge the chain to the single prevailing decl and diagnose
 703      mismatches.  */
 704   lto_symtab_merge_decls_2 (slot, diagnosed_p);
 705
 706   /* Store resolution decision into the callgraph.
 707      In LTRANS don't overwrite information we stored into callgraph at
 708      WPA stage.
 709
 710      Do not bother to store guessed decisions.  Generic code knows how
 711      to handle UNKNOWN relocation well.
 712
 713      The problem with storing guessed decision is whether to use
 714      PREVAILING_DEF or PREVAILING_DEF_IRONLY.  First one would disable
 715      some whole program optimizations, while ther second would imply
 716      to many whole program assumptions.  */
 717   if (prevailing->node && !flag_ltrans && !prevailing->guessed)
 718     prevailing->node->resolution = prevailing->resolution;
 719   else if (prevailing->vnode && !flag_ltrans && !prevailing->guessed)
 720     prevailing->vnode->resolution = prevailing->resolution;
 721   return 1;
 722 }
 723
 724 /* Resolve and merge all symbol table chains to a prevailing decl.  */
 725
 726 void
 727 lto_symtab_merge_decls (void)
 728 {
 729   lto_symtab_maybe_init_hash_table ();
 730   htab_traverse (lto_symtab_identifiers, lto_symtab_merge_decls_1, NULL);
 731 }
 732
 733 /* Helper to process the decl chain for the symbol table entry *SLOT.  */
 734
 735 static int
 736 lto_symtab_merge_cgraph_nodes_1 (void **slot, void *data ATTRIBUTE_UNUSED)
 737 {
 738   lto_symtab_entry_t e, prevailing = (lto_symtab_entry_t) *slot;
 739
 740   if (!prevailing->next)
 741     return 1;
 742
 743   /* Replace the cgraph node of each entry with the prevailing one.  */
 744   for (e = prevailing->next; e; e = e->next)
 745     {
 746       if (e->node != NULL)
 747         {
 748           /* In case we prevail funcion by an alias, we can run into case
 749              that the alias has no cgraph node attached, since it was
 750              previously unused.  Create the node.  */
 751           if (!prevailing->node)
 752             {
 753               prevailing->node = cgraph_create_node (prevailing->decl);
 754               prevailing->node->alias = true;
 755             }
 756           lto_cgraph_replace_node (e->node, prevailing->node);
 757         }
 758       if (e->vnode != NULL)
 759         {
 760           if (!prevailing->vnode)
 761             {
 762               prevailing->vnode = varpool_node (prevailing->decl);
 763               prevailing->vnode->alias = true;
 764             }
 765           lto_varpool_replace_node (e->vnode, prevailing->vnode);
 766         }
 767     }
 768
 769   /* Drop all but the prevailing decl from the symtab.  */
 770   prevailing->next = NULL;
 771
 772   return 1;
 773 }
 774
 775 /* Merge cgraph nodes according to the symbol merging done by
 776    lto_symtab_merge_decls.  */
 777
 778 void
 779 lto_symtab_merge_cgraph_nodes (void)
 780 {
 781   struct cgraph_node *node;
 782   lto_symtab_maybe_init_hash_table ();
 783   htab_traverse (lto_symtab_identifiers, lto_symtab_merge_cgraph_nodes_1, NULL);
 784
 785   for (node = cgraph_nodes; node; node = node->next)
 786     if ((node->thunk.thunk_p || node->alias)
 787         && node->thunk.alias)
 788       node->thunk.alias = lto_symtab_prevailing_decl (node->thunk.alias);
 789 }
 790
 791 /* Given the decl DECL, return the prevailing decl with the same name. */
 792
 793 tree
 794 lto_symtab_prevailing_decl (tree decl)
 795 {
 796   lto_symtab_entry_t ret;
 797
 798   /* Builtins and local symbols are their own prevailing decl.  */
 799   if (!TREE_PUBLIC (decl) || is_builtin_fn (decl))
 800     return decl;
 801
 802   /* DECL_ABSTRACTs are their own prevailng decl.  */
 803   if (TREE_CODE (decl) == FUNCTION_DECL && DECL_ABSTRACT (decl))
 804     return decl;
 805
 806   /* Ensure DECL_ASSEMBLER_NAME will not set assembler name.  */
 807   gcc_assert (DECL_ASSEMBLER_NAME_SET_P (decl));
 808
 809   /* Walk through the list of candidates and return the one we merged to.  */
 810   ret = lto_symtab_get ((*targetm.asm_out.mangle_assembler_name)
 811                         (IDENTIFIER_POINTER (DECL_ASSEMBLER_NAME (decl))));
 812   if (!ret)
 813     return NULL_TREE;
 814
 815   return ret->decl;
 816 }
 817
 818 #include "gt-lto-symtab.h"