path: root/gcc/lto-symtab.c

/* LTO symbol table.
   Copyright 2009 Free Software Foundation, Inc.
   Contributed by CodeSourcery, Inc.

This file is part of GCC.

GCC is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free
Software Foundation; either version 3, or (at your option) any later
version.

GCC is distributed in the hope that it will be useful, but WITHOUT ANY
WARRANTY; without even the implied warranty of MERCHANTABILITY or
FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
for more details.

You should have received a copy of the GNU General Public License
along with GCC; see the file COPYING3.  If not see
<http://www.gnu.org/licenses/>.  */

#include "config.h"
#include "system.h"
#include "coretypes.h"
#include "toplev.h"
#include "tree.h"
#include "gimple.h"
#include "ggc.h"	/* lambda.h needs this */
#include "lambda.h"	/* gcd */
#include "hashtab.h"
#include "plugin-api.h"
#include "lto-streamer.h"

/* Vector to keep track of external variables we've seen so far.  */
VEC(tree,gc) *lto_global_var_decls;

/* Symbol table entry.  */

struct GTY(()) lto_symtab_entry_def
{
  /* The symbol table entry key, an IDENTIFIER.  */
  tree id;
  /* The symbol table entry, a DECL.  */
  tree decl;
  /* The cgraph node if decl is a function decl.  Filled in during the
     merging process.  */
  struct cgraph_node *node;
  /* LTO file-data and symbol resolution for this decl.  */
  struct lto_file_decl_data * GTY((skip (""))) file_data;
  enum ld_plugin_symbol_resolution resolution;
  /* Pointer to the next entry with the same key.  Before decl merging
     this links all symbols from the different TUs.  After decl merging
     this links merged but incompatible decls, thus all prevailing ones
     remaining.  */
  struct lto_symtab_entry_def *next;
};
typedef struct lto_symtab_entry_def *lto_symtab_entry_t;

/* A poor man's symbol table. This hashes identifier to prevailing DECL
   if there is one. */

static GTY ((if_marked ("lto_symtab_entry_marked_p"),
	     param_is (struct lto_symtab_entry_def)))
  htab_t lto_symtab_identifiers;

/* Return the hash value of an lto_symtab_entry_t object pointed to by P.  */

static hashval_t
lto_symtab_entry_hash (const void *p)
{
  const struct lto_symtab_entry_def *base =
    (const struct lto_symtab_entry_def *) p;
  return htab_hash_string (IDENTIFIER_POINTER (base->id));
}

/* Return non-zero if P1 and P2 points to lto_symtab_entry_def structs
   corresponding to the same symbol.  */

static int
lto_symtab_entry_eq (const void *p1, const void *p2)
{
  const struct lto_symtab_entry_def *base1 =
     (const struct lto_symtab_entry_def *) p1;
  const struct lto_symtab_entry_def *base2 =
     (const struct lto_symtab_entry_def *) p2;
  return (base1->id == base2->id);
}

/* Returns non-zero if P points to an lto_symtab_entry_def struct that needs
   to be marked for GC.  */ 

static int
lto_symtab_entry_marked_p (const void *p)
{
  const struct lto_symtab_entry_def *base =
     (const struct lto_symtab_entry_def *) p;

  /* Keep this only if the decl or the chain is marked.  */
  return (ggc_marked_p (base->decl)
	  || (base->next && ggc_marked_p (base->next)));
}

/* Lazily initialize resolution hash tables.  */

static void
lto_symtab_maybe_init_hash_table (void)
{
  if (lto_symtab_identifiers)
    return;

  lto_symtab_identifiers =
    htab_create_ggc (1021, lto_symtab_entry_hash,
		     lto_symtab_entry_eq, NULL);
}

/* Registers DECL with the LTO symbol table as having resolution RESOLUTION
   and read from FILE_DATA. */

void
lto_symtab_register_decl (tree decl,
			  ld_plugin_symbol_resolution_t resolution,
			  struct lto_file_decl_data *file_data)
{
  lto_symtab_entry_t new_entry;
  void **slot;

  /* Check that declarations reaching this function do not have
     properties inconsistent with having external linkage.  If any of
     these asertions fail, then the object file reader has failed to
     detect these cases and issue appropriate error messages.  */
  gcc_assert (decl
	      && TREE_PUBLIC (decl)
	      && (TREE_CODE (decl) == VAR_DECL
		  || TREE_CODE (decl) == FUNCTION_DECL)
	      && DECL_ASSEMBLER_NAME_SET_P (decl));
  if (TREE_CODE (decl) == VAR_DECL
      && DECL_INITIAL (decl))
    gcc_assert (!DECL_EXTERNAL (decl)
		|| (TREE_STATIC (decl) && TREE_READONLY (decl)));
  if (TREE_CODE (decl) == FUNCTION_DECL)
    gcc_assert (!DECL_ABSTRACT (decl));

  new_entry = GGC_CNEW (struct lto_symtab_entry_def);
  new_entry->id = DECL_ASSEMBLER_NAME (decl);
  new_entry->decl = decl;
  new_entry->resolution = resolution;
  new_entry->file_data = file_data;

  lto_symtab_maybe_init_hash_table ();
  slot = htab_find_slot (lto_symtab_identifiers, new_entry, INSERT);
  new_entry->next = (lto_symtab_entry_t) *slot;
  *slot = new_entry;
}

/* Get the lto_symtab_entry_def struct associated with ID
   if there is one.  */

static lto_symtab_entry_t
lto_symtab_get (tree id)
{
  struct lto_symtab_entry_def temp;
  void **slot;

  lto_symtab_maybe_init_hash_table ();
  temp.id = id;
  slot = htab_find_slot (lto_symtab_identifiers, &temp, NO_INSERT);
  return slot ? (lto_symtab_entry_t) *slot : NULL;
}

/* Get the linker resolution for DECL.  */

enum ld_plugin_symbol_resolution
lto_symtab_get_resolution (tree decl)
{
  lto_symtab_entry_t e;

  gcc_assert (DECL_ASSEMBLER_NAME_SET_P (decl));

  e = lto_symtab_get (DECL_ASSEMBLER_NAME (decl));
  while (e && e->decl != decl)
    e = e->next;
  if (!e)
    return LDPR_UNKNOWN;

  return e->resolution;
}


/* Replace the cgraph node NODE with PREVAILING_NODE in the cgraph, merging
   all edges and removing the old node.  */

static void
lto_cgraph_replace_node (struct cgraph_node *node,
			 struct cgraph_node *prevailing_node)
{
  struct cgraph_edge *e, *next;

  /* Merge node flags.  */
  if (node->needed)
    cgraph_mark_needed_node (prevailing_node);
  if (node->reachable)
    cgraph_mark_reachable_node (prevailing_node);
  if (node->address_taken)
    {
      gcc_assert (!prevailing_node->global.inlined_to);
      cgraph_mark_address_taken_node (prevailing_node);
    }

  /* Redirect all incoming edges.  */
  for (e = node->callers; e; e = next)
    {
      next = e->next_caller;
      cgraph_redirect_edge_callee (e, prevailing_node);
    }

  /* There are not supposed to be any outgoing edges from a node we
     replace.  Still this can happen for multiple instances of weak
     functions.  */
  for (e = node->callees; e; e = next)
    {
      next = e->next_callee;
      cgraph_remove_edge (e);
    }

  /* Finally remove the replaced node.  */
  cgraph_remove_node (node);
}

/* Merge two variable or function symbol table entries PREVAILING and ENTRY.
   Return false if the symbols are not fully compatible and a diagnostic
   should be emitted.  */

static bool
lto_symtab_merge (lto_symtab_entry_t prevailing, lto_symtab_entry_t entry)
{
  tree prevailing_decl = prevailing->decl;
  tree decl = entry->decl;
  tree prevailing_type, type;

  /* Merge decl state in both directions, we may still end up using
     the new decl.  */
  TREE_ADDRESSABLE (prevailing_decl) |= TREE_ADDRESSABLE (decl);
  TREE_ADDRESSABLE (decl) |= TREE_ADDRESSABLE (prevailing_decl);

  /* The linker may ask us to combine two incompatible symbols.
     Detect this case and notify the caller of required diagnostics.  */

  if (TREE_CODE (decl) == FUNCTION_DECL)
    {
      if (TREE_TYPE (prevailing_decl) != TREE_TYPE (decl))
	/* If we don't have a merged type yet...sigh.  The linker
	   wouldn't complain if the types were mismatched, so we
	   probably shouldn't either.  Just use the type from
	   whichever decl appears to be associated with the
	   definition.  If for some odd reason neither decl is, the
	   older one wins.  */
	(void) 0;

      return true;
    }

  /* Now we exclusively deal with VAR_DECLs.  */

  /* Sharing a global symbol is a strong hint that two types are
     compatible.  We could use this information to complete
     incomplete pointed-to types more aggressively here, ignoring
     mismatches in both field and tag names.  It's difficult though
     to guarantee that this does not have side-effects on merging
     more compatible types from other translation units though.  */

  /* We can tolerate differences in type qualification, the
     qualification of the prevailing definition will prevail.
     ???  In principle we might want to only warn for structurally
     incompatible types here, but unless we have protective measures
     for TBAA in place that would hide useful information.  */
  prevailing_type = TYPE_MAIN_VARIANT (TREE_TYPE (prevailing_decl));
  type = TYPE_MAIN_VARIANT (TREE_TYPE (decl));

  /* We have to register and fetch canonical types here as the global
     fixup process didn't yet run.  */
  prevailing_type = gimple_register_type (prevailing_type);
  type = gimple_register_type (type);
  if (prevailing_type != type)
    {
      if (COMPLETE_TYPE_P (type))
	return false;

      /* If type is incomplete then avoid warnings in the cases
	 that TBAA handles just fine.  */

      if (TREE_CODE (prevailing_type) != TREE_CODE (type))
	return false;

      if (TREE_CODE (prevailing_type) == ARRAY_TYPE)
	{
	  tree tem1 = TREE_TYPE (prevailing_type);
	  tree tem2 = TREE_TYPE (type);
	  while (TREE_CODE (tem1) == ARRAY_TYPE
		 && TREE_CODE (tem2) == ARRAY_TYPE)
	    {
	      tem1 = TREE_TYPE (tem1);
	      tem2 = TREE_TYPE (tem2);
	    }

	  if (TREE_CODE (tem1) != TREE_CODE (tem2))
	    return false;

	  if (gimple_register_type (tem1) != gimple_register_type (tem2))
	    return false;
	}

      /* Fallthru.  Compatible enough.  */
    }

  /* ???  We might want to emit a warning here if type qualification
     differences were spotted.  Do not do this unconditionally though.  */

  /* There is no point in comparing too many details of the decls here.
     The type compatibility checks or the completing of types has properly
     dealt with most issues.  */

  /* The following should all not invoke fatal errors as in non-LTO
     mode the linker wouldn't complain either.  Just emit warnings.  */

  /* Report a warning if user-specified alignments do not match.  */
  if ((DECL_USER_ALIGN (prevailing_decl) && DECL_USER_ALIGN (decl))
      && DECL_ALIGN (prevailing_decl) < DECL_ALIGN (decl))
    return false;

  return true;
}

/* Return true if the symtab entry E can be replaced by another symtab
   entry.  */

static bool
lto_symtab_resolve_replaceable_p (lto_symtab_entry_t e)
{
  if (DECL_EXTERNAL (e->decl)
      || DECL_COMDAT (e->decl)
      || DECL_WEAK (e->decl))
    return true;

  if (TREE_CODE (e->decl) == VAR_DECL)
    return (DECL_COMMON (e->decl)
	    || (!flag_no_common && !DECL_INITIAL (e->decl)));

  return false;
}

/* Return true if the symtab entry E can be the prevailing one.  */

static bool
lto_symtab_resolve_can_prevail_p (lto_symtab_entry_t e)
{
  if (!TREE_STATIC (e->decl))
    return false;

  /* For functions we need a non-discarded body.  */
  if (TREE_CODE (e->decl) == FUNCTION_DECL)
    return (e->node && e->node->analyzed);

  /* A variable should have a size.  */
  else if (TREE_CODE (e->decl) == VAR_DECL)
    return (DECL_SIZE (e->decl) != NULL_TREE
	    /* The C++ frontend retains TREE_STATIC on the declaration
	       of foo_ in struct Foo { static Foo *foo_; }; but it is
	       not a definition.  g++.dg/lto/20090315_0.C.  */
	    && !DECL_EXTERNAL (e->decl));

  gcc_unreachable ();
}

/* Resolve the symbol with the candidates in the chain *SLOT and store
   their resolutions.  */

static void
lto_symtab_resolve_symbols (void **slot)
{
  lto_symtab_entry_t e = (lto_symtab_entry_t) *slot;
  lto_symtab_entry_t prevailing = NULL;

  /* If the chain is already resolved there is nothing to do.  */
  if (e->resolution != LDPR_UNKNOWN)
    return;

  /* Find the single non-replaceable prevailing symbol and
     diagnose ODR violations.  */
  for (; e; e = e->next)
    {
      if (TREE_CODE (e->decl) == FUNCTION_DECL)
	e->node = cgraph_get_node (e->decl);

      if (!lto_symtab_resolve_can_prevail_p (e))
	{
	  e->resolution = LDPR_RESOLVED_IR;
	  continue;
	}

      /* Set a default resolution - the final prevailing one will get
         adjusted later.  */
      e->resolution = LDPR_PREEMPTED_IR;
      if (!lto_symtab_resolve_replaceable_p (e))
	{
	  if (prevailing)
	    {
	      error_at (DECL_SOURCE_LOCATION (e->decl),
			"%qD has already been defined", e->decl);
	      inform (DECL_SOURCE_LOCATION (prevailing->decl),
		      "previously defined here");
	    }
	  prevailing = e;
	}
    }
  if (prevailing)
    goto found;

  /* Do a second round choosing one from the replaceable prevailing decls.  */
  for (e = (lto_symtab_entry_t) *slot; e; e = e->next)
    {
      if (e->resolution != LDPR_PREEMPTED_IR)
	continue;

      /* Choose the first function that can prevail as prevailing.  */
      if (TREE_CODE (e->decl) == FUNCTION_DECL)
	{
	  prevailing = e;
	  break;
	}

      /* From variables that can prevail choose the largest one.  */
      if (!prevailing
	  || tree_int_cst_lt (DECL_SIZE (prevailing->decl),
			      DECL_SIZE (e->decl)))
	prevailing = e;
    }

  if (!prevailing)
    return;

found:
  if (TREE_CODE (prevailing->decl) == VAR_DECL
      && TREE_READONLY (prevailing->decl))
    prevailing->resolution = LDPR_PREVAILING_DEF_IRONLY;
  else
    prevailing->resolution = LDPR_PREVAILING_DEF;
}

/* Merge all decls in the symbol table chain to the prevailing decl and
   issue diagnostics about type mismatches.  */

static void
lto_symtab_merge_decls_2 (void **slot)
{
  lto_symtab_entry_t prevailing, e;
  VEC(tree, heap) *mismatches = NULL;
  unsigned i;
  tree decl;
  bool diagnosed_p = false;

  /* Nothing to do for a single entry.  */
  prevailing = (lto_symtab_entry_t) *slot;
  if (!prevailing->next)
    return;

  /* Try to merge each entry with the prevailing one.  */
  for (e = prevailing->next; e; e = e->next)
    {
      if (!lto_symtab_merge (prevailing, e))
	VEC_safe_push (tree, heap, mismatches, e->decl);
    }
  if (VEC_empty (tree, mismatches))
    return;

  /* Diagnose all mismatched re-declarations.  */
  for (i = 0; VEC_iterate (tree, mismatches, i, decl); ++i)
    {
      if (TREE_TYPE (prevailing->decl) != TREE_TYPE (decl))
	diagnosed_p |= warning_at (DECL_SOURCE_LOCATION (decl), 0,
				   "type of %qD does not match original "
				   "declaration", decl);

      else if ((DECL_USER_ALIGN (prevailing->decl) && DECL_USER_ALIGN (decl))
	       && DECL_ALIGN (prevailing->decl) < DECL_ALIGN (decl))
	{
	  diagnosed_p |= warning_at (DECL_SOURCE_LOCATION (decl), 0,
				     "alignment of %qD is bigger than "
				     "original declaration", decl);
	}
    }
  if (diagnosed_p)
    inform (DECL_SOURCE_LOCATION (prevailing->decl),
	    "previously declared here");

  VEC_free (tree, heap, mismatches);
}

/* Helper to process the decl chain for the symbol table entry *SLOT.  */

static int
lto_symtab_merge_decls_1 (void **slot, void *data ATTRIBUTE_UNUSED)
{
  lto_symtab_entry_t e, prevailing;
  bool diagnosed_p = false;

  /* Compute the symbol resolutions.  This is a no-op when using the
     linker plugin.  */
  lto_symtab_resolve_symbols (slot);

  /* Find the prevailing decl.  */
  for (prevailing = (lto_symtab_entry_t) *slot;
       prevailing
       && prevailing->resolution != LDPR_PREVAILING_DEF_IRONLY
       && prevailing->resolution != LDPR_PREVAILING_DEF;
       prevailing = prevailing->next)
    ;

  /* Assert it's the only one.  */
  if (prevailing)
    for (e = prevailing->next; e; e = e->next)
      gcc_assert (e->resolution != LDPR_PREVAILING_DEF_IRONLY
		  && e->resolution != LDPR_PREVAILING_DEF);

  /* If there's not a prevailing symbol yet it's an external reference.
     Happens a lot during ltrans.  Choose the first symbol with a
     cgraph or a varpool node.  */
  if (!prevailing)
    {
      prevailing = (lto_symtab_entry_t) *slot;
      /* For functions choose one with a cgraph node.  */
      if (TREE_CODE (prevailing->decl) == FUNCTION_DECL)
	while (!prevailing->node
	       && prevailing->next)
	  prevailing = prevailing->next;
      /* We do not stream varpool nodes, so the first decl has to
	 be good enough for now.
	 ???  For QOI choose a variable with readonly initializer
	 if there is one.  This matches C++
	 struct Foo { static const int i = 1; }; without a real
	 definition.  */
      if (TREE_CODE (prevailing->decl) == VAR_DECL)
	while (!(TREE_READONLY (prevailing->decl)
		 && DECL_INITIAL (prevailing->decl))
	       && prevailing->next)
	  prevailing = prevailing->next;
    }

  /* Move it first in the list.  */
  if ((lto_symtab_entry_t) *slot != prevailing)
    {
      for (e = (lto_symtab_entry_t) *slot; e->next != prevailing; e = e->next)
	;
      e->next = prevailing->next;
      prevailing->next = (lto_symtab_entry_t) *slot;
      *slot = (void *) prevailing;
    }

  /* Record the prevailing variable.  */
  if (TREE_CODE (prevailing->decl) == VAR_DECL)
    VEC_safe_push (tree, gc, lto_global_var_decls, prevailing->decl);

  /* Diagnose mismatched objects.  */
  for (e = prevailing->next; e; e = e->next)
    {
      if (TREE_CODE (prevailing->decl) == TREE_CODE (e->decl))
	continue;

      switch (TREE_CODE (prevailing->decl))
	{
	case VAR_DECL:
	  gcc_assert (TREE_CODE (e->decl) == FUNCTION_DECL);
	  error_at (DECL_SOURCE_LOCATION (e->decl),
		    "variable %qD redeclared as function", prevailing->decl);
	  break;

	case FUNCTION_DECL:
	  gcc_assert (TREE_CODE (e->decl) == VAR_DECL);
	  error_at (DECL_SOURCE_LOCATION (e->decl),
		    "function %qD redeclared as variable", prevailing->decl);
	  break;

	default:
	  gcc_unreachable ();
	}

      diagnosed_p = true;
    }
  if (diagnosed_p)
      inform (DECL_SOURCE_LOCATION (prevailing->decl),
	      "previously declared here");

  /* Register and adjust types of the entries.  */
  for (e = (lto_symtab_entry_t) *slot; e; e = e->next)
    TREE_TYPE (e->decl) = gimple_register_type (TREE_TYPE (e->decl));

  /* Merge the chain to the single prevailing decl and diagnose
     mismatches.  */
  lto_symtab_merge_decls_2 (slot);

  /* Drop all but the prevailing decl from the symtab.  */
  if (TREE_CODE (prevailing->decl) != FUNCTION_DECL)
    prevailing->next = NULL;

  return 1;
}

/* Resolve and merge all symbol table chains to a prevailing decl.  */

void
lto_symtab_merge_decls (void)
{
  lto_symtab_maybe_init_hash_table ();
  htab_traverse (lto_symtab_identifiers, lto_symtab_merge_decls_1, NULL);
}

/* Helper to process the decl chain for the symbol table entry *SLOT.  */

static int
lto_symtab_merge_cgraph_nodes_1 (void **slot, void *data ATTRIBUTE_UNUSED)
{
  lto_symtab_entry_t e, prevailing = (lto_symtab_entry_t) *slot;

  if (!prevailing->next)
    return 1;

  gcc_assert (TREE_CODE (prevailing->decl) == FUNCTION_DECL);

  /* Replace the cgraph node of each entry with the prevailing one.  */
  for (e = prevailing->next; e; e = e->next)
    {
      if (e->node != NULL)
	lto_cgraph_replace_node (e->node, prevailing->node);
    }

  /* Drop all but the prevailing decl from the symtab.  */
  prevailing->next = NULL;

  return 1;
}

/* Merge cgraph nodes according to the symbol merging done by
   lto_symtab_merge_decls.  */

void
lto_symtab_merge_cgraph_nodes (void)
{
  lto_symtab_maybe_init_hash_table ();
  htab_traverse (lto_symtab_identifiers, lto_symtab_merge_cgraph_nodes_1, NULL);
}

/* Given the decl DECL, return the prevailing decl with the same name. */

tree
lto_symtab_prevailing_decl (tree decl)
{
  lto_symtab_entry_t ret;

  /* Builtins and local symbols are their own prevailing decl.  */
  if (!TREE_PUBLIC (decl) || is_builtin_fn (decl))
    return decl;

  /* DECL_ABSTRACTs are their own prevailng decl.  */
  if (TREE_CODE (decl) == FUNCTION_DECL && DECL_ABSTRACT (decl))
    return decl;

  /* Ensure DECL_ASSEMBLER_NAME will not set assembler name.  */
  gcc_assert (DECL_ASSEMBLER_NAME_SET_P (decl));

  /* Walk through the list of candidates and return the one we merged to.  */
  ret = lto_symtab_get (DECL_ASSEMBLER_NAME (decl));
  if (!ret)
    return NULL_TREE;

  return ret->decl;
}

#include "gt-lto-symtab.h"