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                xstrdup (cgraph_node_name (node)), node->uid,
 219                xstrdup (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->symbol.decl)))));
 223     }
 224
 225   /* Merge node flags.  */
 226   if (node->symbol.force_output)
 227     cgraph_mark_force_output_node (prevailing_node);
 228   if (node->symbol.address_taken)
 229     {
 230       gcc_assert (!prevailing_node->global.inlined_to);
 231       cgraph_mark_address_taken_node (prevailing_node);
 232     }
 233
 234   /* Redirect all incoming edges.  */
 235   compatible_p
 236     = types_compatible_p (TREE_TYPE (TREE_TYPE (prevailing_node->symbol.decl)),
 237                           TREE_TYPE (TREE_TYPE (node->symbol.decl)));
 238   for (e = node->callers; e; e = next)
 239     {
 240       next = e->next_caller;
 241       cgraph_redirect_edge_callee (e, prevailing_node);
 242       /* If there is a mismatch between the supposed callee return type and
 243          the real one do not attempt to inline this function.
 244          ???  We really need a way to match function signatures for ABI
 245          compatibility and perform related promotions at inlining time.  */
 246       if (!compatible_p)
 247         e->call_stmt_cannot_inline_p = 1;
 248     }
 249   /* Redirect incomming references.  */
 250   ipa_clone_referring ((symtab_node)prevailing_node, &node->symbol.ref_list);
 251
 252   /* Finally remove the replaced node.  */
 253   cgraph_remove_node (node);
 254 }
 255
 256 /* Replace the cgraph node NODE with PREVAILING_NODE in the cgraph, merging
 257    all edges and removing the old node.  */
 258
 259 static void
 260 lto_varpool_replace_node (struct varpool_node *vnode,
 261                           struct varpool_node *prevailing_node)
 262 {
 263   gcc_assert (!vnode->finalized || prevailing_node->finalized);
 264   gcc_assert (!vnode->analyzed || prevailing_node->analyzed);
 265
 266   ipa_clone_referring ((symtab_node)prevailing_node, &vnode->symbol.ref_list);
 267
 268   /* Be sure we can garbage collect the initializer.  */
 269   if (DECL_INITIAL (vnode->symbol.decl))
 270     DECL_INITIAL (vnode->symbol.decl) = error_mark_node;
 271   /* Finally remove the replaced node.  */
 272   varpool_remove_node (vnode);
 273 }
 274
 275 /* Merge two variable or function symbol table entries PREVAILING and ENTRY.
 276    Return false if the symbols are not fully compatible and a diagnostic
 277    should be emitted.  */
 278
 279 static bool
 280 lto_symtab_merge (lto_symtab_entry_t prevailing, lto_symtab_entry_t entry)
 281 {
 282   tree prevailing_decl = prevailing->decl;
 283   tree decl = entry->decl;
 284   tree prevailing_type, type;
 285
 286   /* Merge decl state in both directions, we may still end up using
 287      the new decl.  */
 288   TREE_ADDRESSABLE (prevailing_decl) |= TREE_ADDRESSABLE (decl);
 289   TREE_ADDRESSABLE (decl) |= TREE_ADDRESSABLE (prevailing_decl);
 290
 291   /* The linker may ask us to combine two incompatible symbols.
 292      Detect this case and notify the caller of required diagnostics.  */
 293
 294   if (TREE_CODE (decl) == FUNCTION_DECL)
 295     {
 296       if (!types_compatible_p (TREE_TYPE (prevailing_decl),
 297                                TREE_TYPE (decl)))
 298         /* If we don't have a merged type yet...sigh.  The linker
 299            wouldn't complain if the types were mismatched, so we
 300            probably shouldn't either.  Just use the type from
 301            whichever decl appears to be associated with the
 302            definition.  If for some odd reason neither decl is, the
 303            older one wins.  */
 304         (void) 0;
 305
 306       return true;
 307     }
 308
 309   /* Now we exclusively deal with VAR_DECLs.  */
 310
 311   /* Sharing a global symbol is a strong hint that two types are
 312      compatible.  We could use this information to complete
 313      incomplete pointed-to types more aggressively here, ignoring
 314      mismatches in both field and tag names.  It's difficult though
 315      to guarantee that this does not have side-effects on merging
 316      more compatible types from other translation units though.  */
 317
 318   /* We can tolerate differences in type qualification, the
 319      qualification of the prevailing definition will prevail.
 320      ???  In principle we might want to only warn for structurally
 321      incompatible types here, but unless we have protective measures
 322      for TBAA in place that would hide useful information.  */
 323   prevailing_type = TYPE_MAIN_VARIANT (TREE_TYPE (prevailing_decl));
 324   type = TYPE_MAIN_VARIANT (TREE_TYPE (decl));
 325
 326   if (!types_compatible_p (prevailing_type, type))
 327     {
 328       if (COMPLETE_TYPE_P (type))
 329         return false;
 330
 331       /* If type is incomplete then avoid warnings in the cases
 332          that TBAA handles just fine.  */
 333
 334       if (TREE_CODE (prevailing_type) != TREE_CODE (type))
 335         return false;
 336
 337       if (TREE_CODE (prevailing_type) == ARRAY_TYPE)
 338         {
 339           tree tem1 = TREE_TYPE (prevailing_type);
 340           tree tem2 = TREE_TYPE (type);
 341           while (TREE_CODE (tem1) == ARRAY_TYPE
 342                  && TREE_CODE (tem2) == ARRAY_TYPE)
 343             {
 344               tem1 = TREE_TYPE (tem1);
 345               tem2 = TREE_TYPE (tem2);
 346             }
 347
 348           if (TREE_CODE (tem1) != TREE_CODE (tem2))
 349             return false;
 350
 351           if (!types_compatible_p (tem1, tem2))
 352             return false;
 353         }
 354
 355       /* Fallthru.  Compatible enough.  */
 356     }
 357
 358   /* ???  We might want to emit a warning here if type qualification
 359      differences were spotted.  Do not do this unconditionally though.  */
 360
 361   /* There is no point in comparing too many details of the decls here.
 362      The type compatibility checks or the completing of types has properly
 363      dealt with most issues.  */
 364
 365   /* The following should all not invoke fatal errors as in non-LTO
 366      mode the linker wouldn't complain either.  Just emit warnings.  */
 367
 368   /* Report a warning if user-specified alignments do not match.  */
 369   if ((DECL_USER_ALIGN (prevailing_decl) && DECL_USER_ALIGN (decl))
 370       && DECL_ALIGN (prevailing_decl) < DECL_ALIGN (decl))
 371     return false;
 372
 373   return true;
 374 }
 375
 376 /* Return true if the symtab entry E can be replaced by another symtab
 377    entry.  */
 378
 379 static bool
 380 lto_symtab_resolve_replaceable_p (lto_symtab_entry_t e)
 381 {
 382   if (DECL_EXTERNAL (e->decl)
 383       || DECL_COMDAT (e->decl)
 384       || DECL_ONE_ONLY (e->decl)
 385       || DECL_WEAK (e->decl))
 386     return true;
 387
 388   if (TREE_CODE (e->decl) == VAR_DECL)
 389     return (DECL_COMMON (e->decl)
 390             || (!flag_no_common && !DECL_INITIAL (e->decl)));
 391
 392   return false;
 393 }
 394
 395 /* Return true if the symtab entry E can be the prevailing one.  */
 396
 397 static bool
 398 lto_symtab_resolve_can_prevail_p (lto_symtab_entry_t e)
 399 {
 400   /* The C++ frontend ends up neither setting TREE_STATIC nor
 401      DECL_EXTERNAL on virtual methods but only TREE_PUBLIC.
 402      So do not reject !TREE_STATIC here but only DECL_EXTERNAL.  */
 403   if (DECL_EXTERNAL (e->decl))
 404     return false;
 405
 406   /* For functions we need a non-discarded body.  */
 407   if (TREE_CODE (e->decl) == FUNCTION_DECL)
 408     return (e->node && e->node->analyzed);
 409
 410   else if (TREE_CODE (e->decl) == VAR_DECL)
 411     {
 412       if (!e->vnode)
 413         return false;
 414       return e->vnode->finalized;
 415     }
 416
 417   gcc_unreachable ();
 418 }
 419
 420 /* Resolve the symbol with the candidates in the chain *SLOT and store
 421    their resolutions.  */
 422
 423 static void
 424 lto_symtab_resolve_symbols (void **slot)
 425 {
 426   lto_symtab_entry_t e;
 427   lto_symtab_entry_t prevailing = NULL;
 428
 429   /* Always set e->node so that edges are updated to reflect decl merging. */
 430   for (e = (lto_symtab_entry_t) *slot; e; e = e->next)
 431     {
 432       if (TREE_CODE (e->decl) == FUNCTION_DECL)
 433         e->node = cgraph_get_node (e->decl);
 434       else if (TREE_CODE (e->decl) == VAR_DECL)
 435         e->vnode = varpool_get_node (e->decl);
 436       if (e->resolution == LDPR_PREVAILING_DEF_IRONLY
 437           || e->resolution == LDPR_PREVAILING_DEF_IRONLY_EXP
 438           || e->resolution == LDPR_PREVAILING_DEF)
 439         prevailing = e;
 440     }
 441
 442   /* If the chain is already resolved there is nothing else to do.  */
 443   if (prevailing)
 444     return;
 445
 446   /* Find the single non-replaceable prevailing symbol and
 447      diagnose ODR violations.  */
 448   for (e = (lto_symtab_entry_t) *slot; e; e = e->next)
 449     {
 450       if (!lto_symtab_resolve_can_prevail_p (e))
 451         {
 452           e->resolution = LDPR_RESOLVED_IR;
 453           e->guessed = true;
 454           continue;
 455         }
 456
 457       /* Set a default resolution - the final prevailing one will get
 458          adjusted later.  */
 459       e->resolution = LDPR_PREEMPTED_IR;
 460       e->guessed = true;
 461       if (!lto_symtab_resolve_replaceable_p (e))
 462         {
 463           if (prevailing)
 464             {
 465               error_at (DECL_SOURCE_LOCATION (e->decl),
 466                         "%qD has already been defined", e->decl);
 467               inform (DECL_SOURCE_LOCATION (prevailing->decl),
 468                       "previously defined here");
 469             }
 470           prevailing = e;
 471         }
 472     }
 473   if (prevailing)
 474     goto found;
 475
 476   /* Do a second round choosing one from the replaceable prevailing decls.  */
 477   for (e = (lto_symtab_entry_t) *slot; e; e = e->next)
 478     {
 479       if (e->resolution != LDPR_PREEMPTED_IR)
 480         continue;
 481
 482       /* Choose the first function that can prevail as prevailing.  */
 483       if (TREE_CODE (e->decl) == FUNCTION_DECL)
 484         {
 485           prevailing = e;
 486           break;
 487         }
 488
 489       /* From variables that can prevail choose the largest one.  */
 490       if (!prevailing
 491           || tree_int_cst_lt (DECL_SIZE (prevailing->decl),
 492                               DECL_SIZE (e->decl))
 493           /* When variables are equivalent try to chose one that has useful
 494              DECL_INITIAL.  This makes sense for keyed vtables that are
 495              DECL_EXTERNAL but initialized.  In units that do not need them
 496              we replace the initializer by error_mark_node to conserve
 497              memory.
 498
 499              We know that the vtable is keyed outside the LTO unit - otherwise
 500              the keyed instance would prevail.  We still can preserve useful
 501              info in the initializer.  */
 502           || (DECL_SIZE (prevailing->decl) == DECL_SIZE (e->decl)
 503               && (DECL_INITIAL (e->decl)
 504                   && DECL_INITIAL (e->decl) != error_mark_node)
 505               && (!DECL_INITIAL (prevailing->decl)
 506                   || DECL_INITIAL (prevailing->decl) == error_mark_node)))
 507         prevailing = e;
 508     }
 509
 510   if (!prevailing)
 511     return;
 512
 513 found:
 514   /* If current lto files represent the whole program,
 515     it is correct to use LDPR_PREVALING_DEF_IRONLY.
 516     If current lto files are part of whole program, internal
 517     resolver doesn't know if it is LDPR_PREVAILING_DEF
 518     or LDPR_PREVAILING_DEF_IRONLY.  Use IRONLY conforms to
 519     using -fwhole-program.  Otherwise, it doesn't
 520     matter using either LDPR_PREVAILING_DEF or
 521     LDPR_PREVAILING_DEF_IRONLY
 522
 523     FIXME: above workaround due to gold plugin makes some
 524     variables IRONLY, which are indeed PREVAILING_DEF in
 525     resolution file.  These variables still need manual
 526     externally_visible attribute.  */
 527     prevailing->resolution = LDPR_PREVAILING_DEF_IRONLY;
 528     prevailing->guessed = true;
 529 }
 530
 531 /* Merge all decls in the symbol table chain to the prevailing decl and
 532    issue diagnostics about type mismatches.  If DIAGNOSED_P is true
 533    do not issue further diagnostics.*/
 534
 535 static void
 536 lto_symtab_merge_decls_2 (void **slot, bool diagnosed_p)
 537 {
 538   lto_symtab_entry_t prevailing, e;
 539   VEC(tree, heap) *mismatches = NULL;
 540   unsigned i;
 541   tree decl;
 542
 543   /* Nothing to do for a single entry.  */
 544   prevailing = (lto_symtab_entry_t) *slot;
 545   if (!prevailing->next)
 546     return;
 547
 548   /* Try to merge each entry with the prevailing one.  */
 549   for (e = prevailing->next; e; e = e->next)
 550     {
 551       if (!lto_symtab_merge (prevailing, e)
 552           && !diagnosed_p)
 553         VEC_safe_push (tree, heap, mismatches, e->decl);
 554     }
 555   if (VEC_empty (tree, mismatches))
 556     return;
 557
 558   /* Diagnose all mismatched re-declarations.  */
 559   FOR_EACH_VEC_ELT (tree, mismatches, i, decl)
 560     {
 561       if (!types_compatible_p (TREE_TYPE (prevailing->decl), TREE_TYPE (decl)))
 562         diagnosed_p |= warning_at (DECL_SOURCE_LOCATION (decl), 0,
 563                                    "type of %qD does not match original "
 564                                    "declaration", decl);
 565
 566       else if ((DECL_USER_ALIGN (prevailing->decl) && DECL_USER_ALIGN (decl))
 567                && DECL_ALIGN (prevailing->decl) < DECL_ALIGN (decl))
 568         {
 569           diagnosed_p |= warning_at (DECL_SOURCE_LOCATION (decl), 0,
 570                                      "alignment of %qD is bigger than "
 571                                      "original declaration", decl);
 572         }
 573     }
 574   if (diagnosed_p)
 575     inform (DECL_SOURCE_LOCATION (prevailing->decl),
 576             "previously declared here");
 577
 578   VEC_free (tree, heap, mismatches);
 579 }
 580
 581 /* Helper to process the decl chain for the symbol table entry *SLOT.  */
 582
 583 static int
 584 lto_symtab_merge_decls_1 (void **slot, void *data ATTRIBUTE_UNUSED)
 585 {
 586   lto_symtab_entry_t e, prevailing;
 587   bool diagnosed_p = false;
 588
 589   /* Compute the symbol resolutions.  This is a no-op when using the
 590      linker plugin.  */
 591   lto_symtab_resolve_symbols (slot);
 592
 593   /* Find the prevailing decl.  */
 594   for (prevailing = (lto_symtab_entry_t) *slot;
 595        prevailing
 596        && prevailing->resolution != LDPR_PREVAILING_DEF_IRONLY
 597        && prevailing->resolution != LDPR_PREVAILING_DEF_IRONLY_EXP
 598        && prevailing->resolution != LDPR_PREVAILING_DEF;
 599        prevailing = prevailing->next)
 600     ;
 601
 602   /* Assert it's the only one.  */
 603   if (prevailing)
 604     for (e = prevailing->next; e; e = e->next)
 605       {
 606         if (e->resolution == LDPR_PREVAILING_DEF_IRONLY
 607             || e->resolution == LDPR_PREVAILING_DEF_IRONLY_EXP
 608             || e->resolution == LDPR_PREVAILING_DEF)
 609           fatal_error ("multiple prevailing defs for %qE",
 610                        DECL_NAME (prevailing->decl));
 611       }
 612
 613   /* If there's not a prevailing symbol yet it's an external reference.
 614      Happens a lot during ltrans.  Choose the first symbol with a
 615      cgraph or a varpool node.  */
 616   if (!prevailing)
 617     {
 618       prevailing = (lto_symtab_entry_t) *slot;
 619       /* For functions choose one with a cgraph node.  */
 620       if (TREE_CODE (prevailing->decl) == FUNCTION_DECL)
 621         while (!prevailing->node
 622                && prevailing->next)
 623           prevailing = prevailing->next;
 624       /* For variables chose with a priority variant with vnode
 625          attached (i.e. from unit where external declaration of
 626          variable is actually used).
 627          When there are multiple variants, chose one with size.
 628          This is needed for C++ typeinfos, for example in
 629          lto/20081204-1 there are typeifos in both units, just
 630          one of them do have size.  */
 631       if (TREE_CODE (prevailing->decl) == VAR_DECL)
 632         {
 633           for (e = prevailing->next; e; e = e->next)
 634             if ((!prevailing->vnode && e->vnode)
 635                 || ((prevailing->vnode != NULL) == (e->vnode != NULL)
 636                     && !COMPLETE_TYPE_P (TREE_TYPE (prevailing->decl))
 637                     && COMPLETE_TYPE_P (TREE_TYPE (e->decl))))
 638               prevailing = e;
 639         }
 640     }
 641
 642   /* Move it first in the list.  */
 643   if ((lto_symtab_entry_t) *slot != prevailing)
 644     {
 645       for (e = (lto_symtab_entry_t) *slot; e->next != prevailing; e = e->next)
 646         ;
 647       e->next = prevailing->next;
 648       prevailing->next = (lto_symtab_entry_t) *slot;
 649       *slot = (void *) prevailing;
 650     }
 651
 652   /* Record the prevailing variable.  */
 653   if (TREE_CODE (prevailing->decl) == VAR_DECL)
 654     VEC_safe_push (tree, gc, lto_global_var_decls, prevailing->decl);
 655
 656   /* Diagnose mismatched objects.  */
 657   for (e = prevailing->next; e; e = e->next)
 658     {
 659       if (TREE_CODE (prevailing->decl) == TREE_CODE (e->decl))
 660         continue;
 661
 662       switch (TREE_CODE (prevailing->decl))
 663         {
 664         case VAR_DECL:
 665           gcc_assert (TREE_CODE (e->decl) == FUNCTION_DECL);
 666           error_at (DECL_SOURCE_LOCATION (e->decl),
 667                     "variable %qD redeclared as function", prevailing->decl);
 668           break;
 669
 670         case FUNCTION_DECL:
 671           gcc_assert (TREE_CODE (e->decl) == VAR_DECL);
 672           error_at (DECL_SOURCE_LOCATION (e->decl),
 673                     "function %qD redeclared as variable", prevailing->decl);
 674           break;
 675
 676         default:
 677           gcc_unreachable ();
 678         }
 679
 680       diagnosed_p = true;
 681     }
 682   if (diagnosed_p)
 683       inform (DECL_SOURCE_LOCATION (prevailing->decl),
 684               "previously declared here");
 685
 686   /* Merge the chain to the single prevailing decl and diagnose
 687      mismatches.  */
 688   lto_symtab_merge_decls_2 (slot, diagnosed_p);
 689
 690   /* Store resolution decision into the callgraph.
 691      In LTRANS don't overwrite information we stored into callgraph at
 692      WPA stage.
 693
 694      Do not bother to store guessed decisions.  Generic code knows how
 695      to handle UNKNOWN relocation well.
 696
 697      The problem with storing guessed decision is whether to use
 698      PREVAILING_DEF, PREVAILING_DEF_IRONLY, PREVAILING_DEF_IRONLY_EXP.
 699      First one would disable some whole program optimizations, while
 700      ther second would imply to many whole program assumptions.  */
 701   if (prevailing->node && !flag_ltrans && !prevailing->guessed)
 702     prevailing->node->symbol.resolution = prevailing->resolution;
 703   else if (prevailing->vnode && !flag_ltrans && !prevailing->guessed)
 704     prevailing->vnode->symbol.resolution = prevailing->resolution;
 705   return 1;
 706 }
 707
 708 /* Resolve and merge all symbol table chains to a prevailing decl.  */
 709
 710 void
 711 lto_symtab_merge_decls (void)
 712 {
 713   lto_symtab_maybe_init_hash_table ();
 714   htab_traverse (lto_symtab_identifiers, lto_symtab_merge_decls_1, NULL);
 715 }
 716
 717 /* Helper to process the decl chain for the symbol table entry *SLOT.  */
 718
 719 static int
 720 lto_symtab_merge_cgraph_nodes_1 (void **slot, void *data ATTRIBUTE_UNUSED)
 721 {
 722   lto_symtab_entry_t e, prevailing = (lto_symtab_entry_t) *slot;
 723
 724   if (!prevailing->next)
 725     return 1;
 726
 727   /* Replace the cgraph node of each entry with the prevailing one.  */
 728   for (e = prevailing->next; e; e = e->next)
 729     {
 730       if (e->node != NULL)
 731         {
 732           /* In case we prevail funcion by an alias, we can run into case
 733              that the alias has no cgraph node attached, since it was
 734              previously unused.  Create the node.  */
 735           if (!prevailing->node)
 736             {
 737               prevailing->node = cgraph_create_node (prevailing->decl);
 738               prevailing->node->alias = true;
 739             }
 740           lto_cgraph_replace_node (e->node, prevailing->node);
 741         }
 742       if (e->vnode != NULL)
 743         {
 744           if (!prevailing->vnode)
 745             {
 746               prevailing->vnode = varpool_node (prevailing->decl);
 747               prevailing->vnode->alias = true;
 748             }
 749           lto_varpool_replace_node (e->vnode, prevailing->vnode);
 750         }
 751     }
 752
 753   /* Drop all but the prevailing decl from the symtab.  */
 754   prevailing->next = NULL;
 755
 756   return 1;
 757 }
 758
 759 /* Merge cgraph nodes according to the symbol merging done by
 760    lto_symtab_merge_decls.  */
 761
 762 void
 763 lto_symtab_merge_cgraph_nodes (void)
 764 {
 765   struct cgraph_node *node;
 766   struct varpool_node *vnode;
 767   lto_symtab_maybe_init_hash_table ();
 768   htab_traverse (lto_symtab_identifiers, lto_symtab_merge_cgraph_nodes_1, NULL);
 769
 770   FOR_EACH_FUNCTION (node)
 771     if ((node->thunk.thunk_p || node->alias)
 772         && node->thunk.alias)
 773       node->thunk.alias = lto_symtab_prevailing_decl (node->thunk.alias);
 774   FOR_EACH_VARIABLE (vnode)
 775     if (vnode->alias_of)
 776       vnode->alias_of = lto_symtab_prevailing_decl (vnode->alias_of);
 777 }
 778
 779 /* Given the decl DECL, return the prevailing decl with the same name. */
 780
 781 tree
 782 lto_symtab_prevailing_decl (tree decl)
 783 {
 784   lto_symtab_entry_t ret;
 785
 786   /* Builtins and local symbols are their own prevailing decl.  */
 787   if (!TREE_PUBLIC (decl) || is_builtin_fn (decl))
 788     return decl;
 789
 790   /* DECL_ABSTRACTs are their own prevailng decl.  */
 791   if (TREE_CODE (decl) == FUNCTION_DECL && DECL_ABSTRACT (decl))
 792     return decl;
 793
 794   /* Ensure DECL_ASSEMBLER_NAME will not set assembler name.  */
 795   gcc_assert (DECL_ASSEMBLER_NAME_SET_P (decl));
 796
 797   /* Walk through the list of candidates and return the one we merged to.  */
 798   ret = lto_symtab_get ((*targetm.asm_out.mangle_assembler_name)
 799                         (IDENTIFIER_POINTER (DECL_ASSEMBLER_NAME (decl))));
 800   if (!ret)
 801     return NULL_TREE;
 802
 803   return ret->decl;
 804 }
 805
 806 #include "gt-lto-symtab.h"