gcc/lto-symtab.c

   1 /* LTO symbol table.
   2    Copyright 2009 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 "toplev.h"
  26 #include "tree.h"
  27 #include "gimple.h"
  28 #include "ggc.h"
  29 #include "lambda.h"     /* gcd */
  30 #include "hashtab.h"
  31 #include "plugin-api.h"
  32 #include "lto-streamer.h"
  33
  34 /* Vector to keep track of external variables we've seen so far.  */
  35 VEC(tree,gc) *lto_global_var_decls;
  36
  37 /* Symbol table entry.  */
  38
  39 struct GTY(()) lto_symtab_entry_def
  40 {
  41   /* The symbol table entry key, an IDENTIFIER.  */
  42   tree id;
  43   /* The symbol table entry, a DECL.  */
  44   tree decl;
  45   /* The cgraph node if decl is a function decl.  Filled in during the
  46      merging process.  */
  47   struct cgraph_node *node;
  48   /* The varpool node if decl is a variable decl.  Filled in during the
  49      merging process.  */
  50   struct varpool_node *vnode;
  51   /* LTO file-data and symbol resolution for this decl.  */
  52   struct lto_file_decl_data * GTY((skip (""))) file_data;
  53   enum ld_plugin_symbol_resolution resolution;
  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 = DECL_ASSEMBLER_NAME (decl);
 158   new_entry->decl = decl;
 159   new_entry->resolution = resolution;
 160   new_entry->file_data = file_data;
 161
 162   lto_symtab_maybe_init_hash_table ();
 163   slot = htab_find_slot (lto_symtab_identifiers, new_entry, INSERT);
 164   new_entry->next = (lto_symtab_entry_t) *slot;
 165   *slot = new_entry;
 166 }
 167
 168 /* Get the lto_symtab_entry_def struct associated with ID
 169    if there is one.  */
 170
 171 static lto_symtab_entry_t
 172 lto_symtab_get (tree id)
 173 {
 174   struct lto_symtab_entry_def temp;
 175   void **slot;
 176
 177   lto_symtab_maybe_init_hash_table ();
 178   temp.id = id;
 179   slot = htab_find_slot (lto_symtab_identifiers, &temp, NO_INSERT);
 180   return slot ? (lto_symtab_entry_t) *slot : NULL;
 181 }
 182
 183 /* Get the linker resolution for DECL.  */
 184
 185 enum ld_plugin_symbol_resolution
 186 lto_symtab_get_resolution (tree decl)
 187 {
 188   lto_symtab_entry_t e;
 189
 190   gcc_assert (DECL_ASSEMBLER_NAME_SET_P (decl));
 191
 192   e = lto_symtab_get (DECL_ASSEMBLER_NAME (decl));
 193   while (e && e->decl != decl)
 194     e = e->next;
 195   if (!e)
 196     return LDPR_UNKNOWN;
 197
 198   return e->resolution;
 199 }
 200
 201
 202 /* Replace the cgraph node NODE with PREVAILING_NODE in the cgraph, merging
 203    all edges and removing the old node.  */
 204
 205 static void
 206 lto_cgraph_replace_node (struct cgraph_node *node,
 207                          struct cgraph_node *prevailing_node)
 208 {
 209   struct cgraph_edge *e, *next;
 210   bool no_aliases_please = false;
 211
 212   if (cgraph_dump_file)
 213     {
 214       fprintf (cgraph_dump_file, "Replacing cgraph node %s/%i by %s/%i"
 215                " for symbol %s\n",
 216                cgraph_node_name (node), node->uid,
 217                cgraph_node_name (prevailing_node),
 218                prevailing_node->uid,
 219                IDENTIFIER_POINTER (DECL_ASSEMBLER_NAME (node->decl)));
 220     }
 221
 222   if (prevailing_node->same_body_alias)
 223     {
 224       if (prevailing_node->thunk.thunk_p)
 225         no_aliases_please = true;
 226       prevailing_node = prevailing_node->same_body;
 227     }
 228
 229   /* Merge node flags.  */
 230   if (node->needed)
 231     cgraph_mark_needed_node (prevailing_node);
 232   if (node->reachable)
 233     cgraph_mark_reachable_node (prevailing_node);
 234   if (node->address_taken)
 235     {
 236       gcc_assert (!prevailing_node->global.inlined_to);
 237       cgraph_mark_address_taken_node (prevailing_node);
 238     }
 239
 240   /* Redirect all incoming edges.  */
 241   for (e = node->callers; e; e = next)
 242     {
 243       next = e->next_caller;
 244       cgraph_redirect_edge_callee (e, prevailing_node);
 245     }
 246   /* Redirect incomming references.  */
 247   ipa_clone_refering (prevailing_node, NULL, &node->ref_list);
 248
 249   /* If we have aliases, redirect them to the prevailing node.  */
 250   if (!node->same_body_alias && node->same_body)
 251     {
 252       struct cgraph_node *alias, *last;
 253       /* We prevail aliases/tunks by a thunk.  This is doable but
 254          would need thunk combination.  Hopefully no ABI changes will
 255          every be crazy enough.  */
 256       gcc_assert (!no_aliases_please);
 257
 258       for (alias = node->same_body; alias; alias = alias->next)
 259         {
 260           last = alias;
 261           gcc_assert (alias->same_body_alias);
 262           alias->same_body = prevailing_node;
 263           alias->thunk.alias = prevailing_node->decl;
 264         }
 265       last->next = prevailing_node->same_body;
 266       /* Node with aliases is prevailed by alias.
 267          We could handle this, but combining thunks together will be tricky.
 268          Hopefully this does not happen.  */
 269       if (prevailing_node->same_body)
 270         prevailing_node->same_body->previous = last;
 271       prevailing_node->same_body = node->same_body;
 272       node->same_body = NULL;
 273     }
 274
 275   /* Finally remove the replaced node.  */
 276   if (node->same_body_alias)
 277     cgraph_remove_same_body_alias (node);
 278   else
 279     cgraph_remove_node (node);
 280 }
 281
 282 /* Replace the cgraph node NODE with PREVAILING_NODE in the cgraph, merging
 283    all edges and removing the old node.  */
 284
 285 static void
 286 lto_varpool_replace_node (struct varpool_node *vnode,
 287                           struct varpool_node *prevailing_node)
 288 {
 289   /* Merge node flags.  */
 290   if (vnode->needed)
 291     {
 292       gcc_assert (!vnode->analyzed || prevailing_node->analyzed);
 293       varpool_mark_needed_node (prevailing_node);
 294     }
 295   /* Relink aliases.  */
 296   if (vnode->extra_name && !vnode->alias)
 297     {
 298       struct varpool_node *alias, *last;
 299       for (alias = vnode->extra_name;
 300            alias; alias = alias->next)
 301         {
 302           last = alias;
 303           alias->extra_name = prevailing_node;
 304         }
 305
 306       if (prevailing_node->extra_name)
 307         {
 308           last->next = prevailing_node->extra_name;
 309           prevailing_node->extra_name->prev = last;
 310         }
 311       prevailing_node->extra_name = vnode->extra_name;
 312       vnode->extra_name = NULL;
 313     }
 314   gcc_assert (!vnode->finalized || prevailing_node->finalized);
 315   gcc_assert (!vnode->analyzed || prevailing_node->analyzed);
 316
 317   /* When replacing by an alias, the references goes to the original
 318      variable.  */
 319   if (prevailing_node->alias && prevailing_node->extra_name)
 320     prevailing_node = prevailing_node->extra_name;
 321   ipa_clone_refering (NULL, prevailing_node, &vnode->ref_list);
 322
 323   /* Be sure we can garbage collect the initializer.  */
 324   if (DECL_INITIAL (vnode->decl))
 325     DECL_INITIAL (vnode->decl) = error_mark_node;
 326   /* Finally remove the replaced node.  */
 327   varpool_remove_node (vnode);
 328 }
 329
 330 /* Merge two variable or function symbol table entries PREVAILING and ENTRY.
 331    Return false if the symbols are not fully compatible and a diagnostic
 332    should be emitted.  */
 333
 334 static bool
 335 lto_symtab_merge (lto_symtab_entry_t prevailing, lto_symtab_entry_t entry)
 336 {
 337   tree prevailing_decl = prevailing->decl;
 338   tree decl = entry->decl;
 339   tree prevailing_type, type;
 340
 341   /* Merge decl state in both directions, we may still end up using
 342      the new decl.  */
 343   TREE_ADDRESSABLE (prevailing_decl) |= TREE_ADDRESSABLE (decl);
 344   TREE_ADDRESSABLE (decl) |= TREE_ADDRESSABLE (prevailing_decl);
 345
 346   /* The linker may ask us to combine two incompatible symbols.
 347      Detect this case and notify the caller of required diagnostics.  */
 348
 349   if (TREE_CODE (decl) == FUNCTION_DECL)
 350     {
 351       if (TREE_TYPE (prevailing_decl) != TREE_TYPE (decl))
 352         /* If we don't have a merged type yet...sigh.  The linker
 353            wouldn't complain if the types were mismatched, so we
 354            probably shouldn't either.  Just use the type from
 355            whichever decl appears to be associated with the
 356            definition.  If for some odd reason neither decl is, the
 357            older one wins.  */
 358         (void) 0;
 359
 360       return true;
 361     }
 362
 363   /* Now we exclusively deal with VAR_DECLs.  */
 364
 365   /* Sharing a global symbol is a strong hint that two types are
 366      compatible.  We could use this information to complete
 367      incomplete pointed-to types more aggressively here, ignoring
 368      mismatches in both field and tag names.  It's difficult though
 369      to guarantee that this does not have side-effects on merging
 370      more compatible types from other translation units though.  */
 371
 372   /* We can tolerate differences in type qualification, the
 373      qualification of the prevailing definition will prevail.
 374      ???  In principle we might want to only warn for structurally
 375      incompatible types here, but unless we have protective measures
 376      for TBAA in place that would hide useful information.  */
 377   prevailing_type = TYPE_MAIN_VARIANT (TREE_TYPE (prevailing_decl));
 378   type = TYPE_MAIN_VARIANT (TREE_TYPE (decl));
 379
 380   /* We have to register and fetch canonical types here as the global
 381      fixup process didn't yet run.  */
 382   prevailing_type = gimple_register_type (prevailing_type);
 383   type = gimple_register_type (type);
 384   if (prevailing_type != type)
 385     {
 386       if (COMPLETE_TYPE_P (type))
 387         return false;
 388
 389       /* If type is incomplete then avoid warnings in the cases
 390          that TBAA handles just fine.  */
 391
 392       if (TREE_CODE (prevailing_type) != TREE_CODE (type))
 393         return false;
 394
 395       if (TREE_CODE (prevailing_type) == ARRAY_TYPE)
 396         {
 397           tree tem1 = TREE_TYPE (prevailing_type);
 398           tree tem2 = TREE_TYPE (type);
 399           while (TREE_CODE (tem1) == ARRAY_TYPE
 400                  && TREE_CODE (tem2) == ARRAY_TYPE)
 401             {
 402               tem1 = TREE_TYPE (tem1);
 403               tem2 = TREE_TYPE (tem2);
 404             }
 405
 406           if (TREE_CODE (tem1) != TREE_CODE (tem2))
 407             return false;
 408
 409           if (gimple_register_type (tem1) != gimple_register_type (tem2))
 410             return false;
 411         }
 412
 413       /* Fallthru.  Compatible enough.  */
 414     }
 415
 416   /* ???  We might want to emit a warning here if type qualification
 417      differences were spotted.  Do not do this unconditionally though.  */
 418
 419   /* There is no point in comparing too many details of the decls here.
 420      The type compatibility checks or the completing of types has properly
 421      dealt with most issues.  */
 422
 423   /* The following should all not invoke fatal errors as in non-LTO
 424      mode the linker wouldn't complain either.  Just emit warnings.  */
 425
 426   /* Report a warning if user-specified alignments do not match.  */
 427   if ((DECL_USER_ALIGN (prevailing_decl) && DECL_USER_ALIGN (decl))
 428       && DECL_ALIGN (prevailing_decl) < DECL_ALIGN (decl))
 429     return false;
 430
 431   return true;
 432 }
 433
 434 /* Return true if the symtab entry E can be replaced by another symtab
 435    entry.  */
 436
 437 static bool
 438 lto_symtab_resolve_replaceable_p (lto_symtab_entry_t e)
 439 {
 440   if (DECL_EXTERNAL (e->decl)
 441       || DECL_COMDAT (e->decl)
 442       || DECL_WEAK (e->decl))
 443     return true;
 444
 445   if (TREE_CODE (e->decl) == VAR_DECL)
 446     return (DECL_COMMON (e->decl)
 447             || (!flag_no_common && !DECL_INITIAL (e->decl)));
 448
 449   return false;
 450 }
 451
 452 /* Return true if the symtab entry E can be the prevailing one.  */
 453
 454 static bool
 455 lto_symtab_resolve_can_prevail_p (lto_symtab_entry_t e)
 456 {
 457   /* The C++ frontend ends up neither setting TREE_STATIC nor
 458      DECL_EXTERNAL on virtual methods but only TREE_PUBLIC.
 459      So do not reject !TREE_STATIC here but only DECL_EXTERNAL.  */
 460   if (DECL_EXTERNAL (e->decl))
 461     return false;
 462
 463   /* For functions we need a non-discarded body.  */
 464   if (TREE_CODE (e->decl) == FUNCTION_DECL)
 465     return (e->node
 466             && (e->node->analyzed
 467                 || (e->node->same_body_alias && e->node->same_body->analyzed)));
 468
 469   /* A variable should have a size.  */
 470   else if (TREE_CODE (e->decl) == VAR_DECL)
 471     {
 472       if (!e->vnode)
 473         return false;
 474       if (e->vnode->finalized)
 475         return true;
 476       return e->vnode->alias && e->vnode->extra_name->finalized;
 477     }
 478
 479   gcc_unreachable ();
 480 }
 481
 482 /* Resolve the symbol with the candidates in the chain *SLOT and store
 483    their resolutions.  */
 484
 485 static void
 486 lto_symtab_resolve_symbols (void **slot)
 487 {
 488   lto_symtab_entry_t e;
 489   lto_symtab_entry_t prevailing = NULL;
 490
 491   /* Always set e->node so that edges are updated to reflect decl merging. */
 492   for (e = (lto_symtab_entry_t) *slot; e; e = e->next)
 493     {
 494       if (TREE_CODE (e->decl) == FUNCTION_DECL)
 495         e->node = cgraph_get_node_or_alias (e->decl);
 496       else if (TREE_CODE (e->decl) == VAR_DECL)
 497         e->vnode = varpool_get_node (e->decl);
 498     }
 499
 500   e = (lto_symtab_entry_t) *slot;
 501
 502   /* If the chain is already resolved there is nothing else to do.  */
 503   if (e->resolution != LDPR_UNKNOWN)
 504     return;
 505
 506   /* Find the single non-replaceable prevailing symbol and
 507      diagnose ODR violations.  */
 508   for (e = (lto_symtab_entry_t) *slot; e; e = e->next)
 509     {
 510       if (!lto_symtab_resolve_can_prevail_p (e))
 511         {
 512           e->resolution = LDPR_RESOLVED_IR;
 513           continue;
 514         }
 515
 516       /* Set a default resolution - the final prevailing one will get
 517          adjusted later.  */
 518       e->resolution = LDPR_PREEMPTED_IR;
 519       if (!lto_symtab_resolve_replaceable_p (e))
 520         {
 521           if (prevailing)
 522             {
 523               error_at (DECL_SOURCE_LOCATION (e->decl),
 524                         "%qD has already been defined", e->decl);
 525               inform (DECL_SOURCE_LOCATION (prevailing->decl),
 526                       "previously defined here");
 527             }
 528           prevailing = e;
 529         }
 530     }
 531   if (prevailing)
 532     goto found;
 533
 534   /* Do a second round choosing one from the replaceable prevailing decls.  */
 535   for (e = (lto_symtab_entry_t) *slot; e; e = e->next)
 536     {
 537       if (e->resolution != LDPR_PREEMPTED_IR)
 538         continue;
 539
 540       /* Choose the first function that can prevail as prevailing.  */
 541       if (TREE_CODE (e->decl) == FUNCTION_DECL)
 542         {
 543           prevailing = e;
 544           break;
 545         }
 546
 547       /* From variables that can prevail choose the largest one.  */
 548       if (!prevailing
 549           || tree_int_cst_lt (DECL_SIZE (prevailing->decl),
 550                               DECL_SIZE (e->decl)))
 551         prevailing = e;
 552     }
 553
 554   if (!prevailing)
 555     return;
 556
 557 found:
 558   /* If current lto files represent the whole program,
 559     it is correct to use LDPR_PREVALING_DEF_IRONLY.
 560     If current lto files are part of whole program, internal
 561     resolver doesn't know if it is LDPR_PREVAILING_DEF
 562     or LDPR_PREVAILING_DEF_IRONLY.  Use IRONLY conforms to
 563     using -fwhole-program.  Otherwise, it doesn't
 564     matter using either LDPR_PREVAILING_DEF or
 565     LDPR_PREVAILING_DEF_IRONLY
 566
 567     FIXME: above workaround due to gold plugin makes some
 568     variables IRONLY, which are indeed PREVAILING_DEF in
 569     resolution file.  These variables still need manual
 570     externally_visible attribute.  */
 571     prevailing->resolution = LDPR_PREVAILING_DEF_IRONLY;
 572 }
 573
 574 /* Merge all decls in the symbol table chain to the prevailing decl and
 575    issue diagnostics about type mismatches.  */
 576
 577 static void
 578 lto_symtab_merge_decls_2 (void **slot)
 579 {
 580   lto_symtab_entry_t prevailing, e;
 581   VEC(tree, heap) *mismatches = NULL;
 582   unsigned i;
 583   tree decl;
 584   bool diagnosed_p = false;
 585
 586   /* Nothing to do for a single entry.  */
 587   prevailing = (lto_symtab_entry_t) *slot;
 588   if (!prevailing->next)
 589     return;
 590
 591   /* Try to merge each entry with the prevailing one.  */
 592   for (e = prevailing->next; e; e = e->next)
 593     {
 594       if (!lto_symtab_merge (prevailing, e))
 595         VEC_safe_push (tree, heap, mismatches, e->decl);
 596     }
 597   if (VEC_empty (tree, mismatches))
 598     return;
 599
 600   /* Diagnose all mismatched re-declarations.  */
 601   for (i = 0; VEC_iterate (tree, mismatches, i, decl); ++i)
 602     {
 603       if (TREE_TYPE (prevailing->decl) != TREE_TYPE (decl))
 604         diagnosed_p |= warning_at (DECL_SOURCE_LOCATION (decl), 0,
 605                                    "type of %qD does not match original "
 606                                    "declaration", decl);
 607
 608       else if ((DECL_USER_ALIGN (prevailing->decl) && DECL_USER_ALIGN (decl))
 609                && DECL_ALIGN (prevailing->decl) < DECL_ALIGN (decl))
 610         {
 611           diagnosed_p |= warning_at (DECL_SOURCE_LOCATION (decl), 0,
 612                                      "alignment of %qD is bigger than "
 613                                      "original declaration", decl);
 614         }
 615     }
 616   if (diagnosed_p)
 617     inform (DECL_SOURCE_LOCATION (prevailing->decl),
 618             "previously declared here");
 619
 620   VEC_free (tree, heap, mismatches);
 621 }
 622
 623 /* Helper to process the decl chain for the symbol table entry *SLOT.  */
 624
 625 static int
 626 lto_symtab_merge_decls_1 (void **slot, void *data ATTRIBUTE_UNUSED)
 627 {
 628   lto_symtab_entry_t e, prevailing;
 629   bool diagnosed_p = false;
 630
 631   /* Compute the symbol resolutions.  This is a no-op when using the
 632      linker plugin.  */
 633   lto_symtab_resolve_symbols (slot);
 634
 635   /* Find the prevailing decl.  */
 636   for (prevailing = (lto_symtab_entry_t) *slot;
 637        prevailing
 638        && prevailing->resolution != LDPR_PREVAILING_DEF_IRONLY
 639        && prevailing->resolution != LDPR_PREVAILING_DEF;
 640        prevailing = prevailing->next)
 641     ;
 642
 643   /* Assert it's the only one.  */
 644   if (prevailing)
 645     for (e = prevailing->next; e; e = e->next)
 646       {
 647         if (e->resolution == LDPR_PREVAILING_DEF_IRONLY
 648             || e->resolution == LDPR_PREVAILING_DEF)
 649           fatal_error ("multiple prevailing defs for %qE",
 650                        DECL_NAME (prevailing->decl));
 651       }
 652
 653   /* If there's not a prevailing symbol yet it's an external reference.
 654      Happens a lot during ltrans.  Choose the first symbol with a
 655      cgraph or a varpool node.  */
 656   if (!prevailing)
 657     {
 658       prevailing = (lto_symtab_entry_t) *slot;
 659       /* For functions choose one with a cgraph node.  */
 660       if (TREE_CODE (prevailing->decl) == FUNCTION_DECL)
 661         while (!prevailing->node
 662                && prevailing->next)
 663           prevailing = prevailing->next;
 664       /* For variables chose with a priority variant with vnode
 665          attached (i.e. from unit where external declaration of
 666          variable is actually used).
 667          When there are multiple variants, chose one with size.
 668          This is needed for C++ typeinfos, for example in
 669          lto/20081204-1 there are typeifos in both units, just
 670          one of them do have size.  */
 671       if (TREE_CODE (prevailing->decl) == VAR_DECL)
 672         {
 673           for (e = prevailing->next; e; e = e->next)
 674             if ((!prevailing->vnode && e->vnode)
 675                 || ((prevailing->vnode != NULL) == (e->vnode != NULL)
 676                     && !COMPLETE_TYPE_P (TREE_TYPE (prevailing->decl))
 677                     && COMPLETE_TYPE_P (TREE_TYPE (e->decl))))
 678               prevailing = e;
 679         }
 680     }
 681
 682   /* Move it first in the list.  */
 683   if ((lto_symtab_entry_t) *slot != prevailing)
 684     {
 685       for (e = (lto_symtab_entry_t) *slot; e->next != prevailing; e = e->next)
 686         ;
 687       e->next = prevailing->next;
 688       prevailing->next = (lto_symtab_entry_t) *slot;
 689       *slot = (void *) prevailing;
 690     }
 691
 692   /* Record the prevailing variable.  */
 693   if (TREE_CODE (prevailing->decl) == VAR_DECL)
 694     VEC_safe_push (tree, gc, lto_global_var_decls, prevailing->decl);
 695
 696   /* Diagnose mismatched objects.  */
 697   for (e = prevailing->next; e; e = e->next)
 698     {
 699       if (TREE_CODE (prevailing->decl) == TREE_CODE (e->decl))
 700         continue;
 701
 702       switch (TREE_CODE (prevailing->decl))
 703         {
 704         case VAR_DECL:
 705           gcc_assert (TREE_CODE (e->decl) == FUNCTION_DECL);
 706           error_at (DECL_SOURCE_LOCATION (e->decl),
 707                     "variable %qD redeclared as function", prevailing->decl);
 708           break;
 709
 710         case FUNCTION_DECL:
 711           gcc_assert (TREE_CODE (e->decl) == VAR_DECL);
 712           error_at (DECL_SOURCE_LOCATION (e->decl),
 713                     "function %qD redeclared as variable", prevailing->decl);
 714           break;
 715
 716         default:
 717           gcc_unreachable ();
 718         }
 719
 720       diagnosed_p = true;
 721     }
 722   if (diagnosed_p)
 723       inform (DECL_SOURCE_LOCATION (prevailing->decl),
 724               "previously declared here");
 725
 726   /* Register and adjust types of the entries.  */
 727   for (e = (lto_symtab_entry_t) *slot; e; e = e->next)
 728     TREE_TYPE (e->decl) = gimple_register_type (TREE_TYPE (e->decl));
 729
 730   /* Merge the chain to the single prevailing decl and diagnose
 731      mismatches.  */
 732   lto_symtab_merge_decls_2 (slot);
 733
 734   /* Drop all but the prevailing decl from the symtab.  */
 735   if (TREE_CODE (prevailing->decl) != FUNCTION_DECL
 736       && TREE_CODE (prevailing->decl) != VAR_DECL)
 737     prevailing->next = NULL;
 738
 739   /* Set used_from_object_file flags.  */
 740   if (prevailing->resolution == LDPR_PREVAILING_DEF
 741       || prevailing->resolution == LDPR_PREEMPTED_REG
 742       || prevailing->resolution == LDPR_RESOLVED_EXEC
 743       || prevailing->resolution == LDPR_RESOLVED_DYN)
 744     {
 745       if (TREE_CODE (prevailing->decl) == FUNCTION_DECL)
 746         {
 747           if (prevailing->node->same_body_alias)
 748             prevailing->node->same_body->local.used_from_object_file = true;
 749           else
 750             prevailing->node->local.used_from_object_file = true;
 751         }
 752       else
 753         {
 754           if (prevailing->vnode->alias)
 755             prevailing->vnode->extra_name->used_from_object_file = true;
 756           else
 757             prevailing->vnode->used_from_object_file = true;
 758         }
 759     }
 760   return 1;
 761 }
 762
 763 /* Resolve and merge all symbol table chains to a prevailing decl.  */
 764
 765 void
 766 lto_symtab_merge_decls (void)
 767 {
 768   lto_symtab_maybe_init_hash_table ();
 769   htab_traverse (lto_symtab_identifiers, lto_symtab_merge_decls_1, NULL);
 770 }
 771
 772 /* Helper to process the decl chain for the symbol table entry *SLOT.  */
 773
 774 static int
 775 lto_symtab_merge_cgraph_nodes_1 (void **slot, void *data ATTRIBUTE_UNUSED)
 776 {
 777   lto_symtab_entry_t e, prevailing = (lto_symtab_entry_t) *slot;
 778
 779   if (!prevailing->next)
 780     return 1;
 781
 782   /* Replace the cgraph node of each entry with the prevailing one.  */
 783   for (e = prevailing->next; e; e = e->next)
 784     {
 785       if (e->node != NULL)
 786         lto_cgraph_replace_node (e->node, prevailing->node);
 787       if (e->vnode != NULL)
 788         lto_varpool_replace_node (e->vnode, prevailing->vnode);
 789     }
 790
 791   /* Drop all but the prevailing decl from the symtab.  */
 792   prevailing->next = NULL;
 793
 794   return 1;
 795 }
 796
 797 /* Merge cgraph nodes according to the symbol merging done by
 798    lto_symtab_merge_decls.  */
 799
 800 void
 801 lto_symtab_merge_cgraph_nodes (void)
 802 {
 803   lto_symtab_maybe_init_hash_table ();
 804   htab_traverse (lto_symtab_identifiers, lto_symtab_merge_cgraph_nodes_1, NULL);
 805 }
 806
 807 /* Given the decl DECL, return the prevailing decl with the same name. */
 808
 809 tree
 810 lto_symtab_prevailing_decl (tree decl)
 811 {
 812   lto_symtab_entry_t ret;
 813
 814   /* Builtins and local symbols are their own prevailing decl.  */
 815   if (!TREE_PUBLIC (decl) || is_builtin_fn (decl))
 816     return decl;
 817
 818   /* DECL_ABSTRACTs are their own prevailng decl.  */
 819   if (TREE_CODE (decl) == FUNCTION_DECL && DECL_ABSTRACT (decl))
 820     return decl;
 821
 822   /* Ensure DECL_ASSEMBLER_NAME will not set assembler name.  */
 823   gcc_assert (DECL_ASSEMBLER_NAME_SET_P (decl));
 824
 825   /* Walk through the list of candidates and return the one we merged to.  */
 826   ret = lto_symtab_get (DECL_ASSEMBLER_NAME (decl));
 827   if (!ret)
 828     return NULL_TREE;
 829
 830   return ret->decl;
 831 }
 832
 833 #include "gt-lto-symtab.h"