GCC Middle and Back End API Reference
ipa-devirt.c File Reference

Data Structures

struct  odr_type_d
struct  odr_hasher
struct  polymorphic_call_target_d
struct  polymorphic_call_target_hasher


typedef hash_table< odr_hasherodr_hash_type
typedef hash_table
< polymorphic_call_target_hasher


static bool polymorphic_type_binfo_p ()
hashval_t hash_type_name ()
static void add_type_duplicate ()
odr_type get_odr_type ()
static void dump_odr_type ()
static void dump_type_inheritance_graph ()
tree method_class_type ()
void build_type_inheritance_graph ()
static void maybe_record_node (vec< cgraph_node * > &nodes, tree target, pointer_set_t *inserted)
static void record_binfo (vec< cgraph_node * > &nodes, tree binfo, tree otr_type, tree type_binfo, HOST_WIDE_INT otr_token, pointer_set_t *inserted, pointer_set_t *matched_vtables, bool anonymous)
static void possible_polymorphic_call_targets_1 (vec< cgraph_node * > &nodes, pointer_set_t *inserted, pointer_set_t *matched_vtables, tree otr_type, odr_type type, HOST_WIDE_INT otr_token)
static void free_polymorphic_call_targets_hash ()
static void devirt_node_removal_hook ()
static void devirt_variable_node_removal_hook (struct varpool_node *n, void *d)
vec< cgraph_node * > possible_polymorphic_call_targets (tree otr_type, HOST_WIDE_INT otr_token, bool *finalp, void **cache_token)
void dump_possible_polymorphic_call_targets (FILE *f, tree otr_type, HOST_WIDE_INT otr_token)
bool possible_polymorphic_call_target_p (tree otr_type, HOST_WIDE_INT otr_token, struct cgraph_node *n)
void update_type_inheritance_graph ()
bool likely_target_p ()
static unsigned int ipa_devirt ()
static bool gate_ipa_devirt ()
ipa_opt_pass_dmake_pass_ipa_devirt ()


static pointer_set_tcached_polymorphic_call_targets
static odr_hash_type odr_hash
static vec< odr_type, va_gc > * odr_types_ptr

Typedef Documentation

   ODR type hash used to lookup ODR type based on tree type node.  
   Polymorphic call target query cache.  

Function Documentation

static void add_type_duplicate ( )
   TYPE is equivalent to VAL by ODR, but its tree representation differs
   from VAL->type.  This may happen in LTO where tree merging did not merge
   all variants of the same type.  It may or may not mean the ODR violation.
   Add it to the list of duplicates and warn on some violations.  
     See if this duplicate is new.  
         First we compare memory layout.  
         Next sanity check that bases are the same.  If not, we will end
         up producing wrong answers.  
         Regularize things a little.  During LTO same types may come with
         different BINFOs.  Either because their virtual table was
         not merged by tree merging and only later at decl merging or
         because one type comes with external vtable, while other
         with internal.  We want to merge equivalent binfos to conserve
         memory and streaming overhead.

         The external vtables are more harmful: they contain references
         to external declarations of methods that may be defined in the
         merged LTO unit.  For this reason we absolutely need to remove
         them and replace by internal variants. Not doing so will lead
         to incomplete answers from possible_polymorphic_call_targets.  
void build_type_inheritance_graph ( void  )
   Initialize IPA devirt and build inheritance tree graph.  
     We reconstruct the graph starting of types of all methods seen in the
     the unit.  

References pointer_set_insert().

static void devirt_node_removal_hook ( )
   When virtual function is removed, we may need to flush the cache.  

References hash_table< Descriptor, Allocator >::create(), pointer_set_create(), and polymorphic_call_target_hash.

Referenced by devirt_variable_node_removal_hook().

static void devirt_variable_node_removal_hook ( struct varpool_node n,
void *  d 
   When virtual table is removed, we may need to flush the cache.  

References cgraph_add_node_removal_hook(), devirt_node_removal_hook(), and varpool_add_node_removal_hook().

static void dump_odr_type ( )
   Dump ODR type T and all its derrived type.  INDENT specify indentation for
   recusive printing.  
void dump_possible_polymorphic_call_targets ( FILE *  f,
tree  otr_type,
HOST_WIDE_INT  otr_token 
   Dump all possible targets of a polymorphic call.  

References flags_from_decl_or_type(), lookup_attribute(), and NODE_FREQUENCY_NORMAL.

Referenced by varpool_finalize_decl().

static void dump_type_inheritance_graph ( )
   Dump the type inheritance graph.  
static void free_polymorphic_call_targets_hash ( )
   Destroy polymorphic call target query cache.  
static bool gate_ipa_devirt ( )
   Gate for IPCP optimization.  
odr_type get_odr_type ( )
   Get ODR type hash entry for TYPE.  If INSERT is true, create
   possibly new entry.  
     See if we already have entry for type.  
         With LTO we need to support multiple tree representation of
         the same ODR type.  
           For now record only polymorphic types. other are
           pointless for devirtualization and we can not precisely
           determine ODR equivalency of these during LTO.  
         First record bases, then add into array so ids are increasing.  
hashval_t hash_type_name ( )
   Produce hash based on type name.  
     If not in LTO, all main variants are unique, so we can do
     pointer hash.  
     Anonymous types are unique.  
     For polymorphic types, we can simply hash the virtual table.  
     Rest is not implemented yet.  

References iterative_hash_hashval_t().

static unsigned int ipa_devirt ( )
   The ipa-devirt pass.
   When polymorphic call has only one likely target in the unit,
   turn it into speculative call.  
                   When dumping see if we agree with speculation.  
               This is reached only when dumping; check if we agree or disagree
               with the speculation.  
               Do not introduce new references to external symbols.  While we
               can handle these just well, it is common for programs to
               incorrectly with headers defining methods they are linked
bool likely_target_p ( )
   Return true if N looks like likely target of a polymorphic call.
   Rule out cxa_pure_virtual, noreturns, function declared cold and
   other obvious cases.  
     cxa_pure_virtual and similar things are not likely.  
ipa_opt_pass_d* make_pass_ipa_devirt ( )
static void maybe_record_node ( vec< cgraph_node * > &  nodes,
tree  target,
pointer_set_t inserted 
   If TARGET has associated node, record it in the NODES array.  
         Those are used to mark impossible scenarios.  

References symtab_node_base::definition, gimple_get_virt_method_for_binfo(), pointer_set_insert(), polymorphic_type_binfo_p(), record_binfo(), types_same_for_odr(), and varpool_get_node().

tree method_class_type ( )
   Given method type T, return type of class it belongs to.
   Lookup this pointer and get its type.    

Referenced by varpool_finalize_decl(), and walk_polymorphic_call_targets().

static bool polymorphic_type_binfo_p ( )
   Return true if BINFO corresponds to a type with virtual methods. 

   Every type has several BINFOs.  One is the BINFO associated by the type
   while other represents bases of derived types.  The BINFOs representing
   bases do not have BINFO_VTABLE pointer set when this is the single
   inheritance (because vtables are shared).  Look up the BINFO of type
   and check presence of its vtable.  
     See if BINFO's type has an virtual table associtated with it.  

Referenced by maybe_record_node().

bool possible_polymorphic_call_target_p ( tree  otr_type,
HOST_WIDE_INT  otr_token,
struct cgraph_node n 
   Return true if N can be possibly target of a polymorphic call of
     At a moment we allow middle end to dig out new external declarations
     as a targets of polymorphic calls.  

References cgraph_node_name(), dump_file, cgraph_node::indirect_calls, cgraph_edge::indirect_info, cgraph_edge::next_callee, symtab_node_base::order, pointer_set_create(), and cgraph_indirect_call_info::polymorphic.

Referenced by dump_possible_polymorphic_call_targets(), and possible_polymorphic_call_target_p().

vec<cgraph_node *> possible_polymorphic_call_targets ( tree  otr_type,
HOST_WIDE_INT  otr_token,
bool *  finalp,
void **  cache_token 
   Return vector containing possible targets of polymorphic call of type
   OTR_TYPE caling method OTR_TOKEN with OFFSET.  If FINALp is non-NULL,
   store true if the list is complette. 
   CACHE_TOKEN (if non-NULL) will get stored to an unique ID of entry
   in the target cache.  If user needs to visit every target list
   just once, it can memoize them.

   Returned vector is placed into cache.  It is NOT caller's responsibility
   to free it.  The vector can be freed on cgraph_remove_node call if
   the particular node is a virtual function present in the cache.  
     If we do not have type in our hash it means we never seen any method
     in it.  
     For anonymous namespace types we can attempt to build full type.
     All derivations must be in this unit.  
     Initialize query cache.  
     Lookup cached answer.  
     Do actual search.  
     First see virtual method of type itself.  
     TODO: If method is final, we can stop here and signaize that
     list is final.  We need C++ FE to pass our info about final
     methods and classes.  
     Walk recursively all derived types.  Here we need to lookup proper basetype
     via their BINFO walk that is done by record_binfo  
static void possible_polymorphic_call_targets_1 ( vec< cgraph_node * > &  nodes,
pointer_set_t inserted,
pointer_set_t matched_vtables,
tree  otr_type,
odr_type  type,
HOST_WIDE_INT  otr_token 
   Lookup virtual methods matching OTR_TYPE (with OFFSET and OTR_TOKEN)
   of TYPE, insert them to NODES, recurse into derived nodes. 
   INSERTED is used to avoid duplicate insertions of methods into NODES.
   MATCHED_VTABLES are used to avoid duplicate walking vtables.  

References iterative_hash_hashval_t().

static void record_binfo ( vec< cgraph_node * > &  nodes,
tree  binfo,
tree  otr_type,
tree  type_binfo,
HOST_WIDE_INT  otr_token,
pointer_set_t inserted,
pointer_set_t matched_vtables,
bool  anonymous 
   See if BINFO's type match OTR_TYPE.  If so, lookup method
   in vtable of TYPE_BINFO and insert method to NODES array.
   Otherwise recurse to base BINFOs.
   This match what get_binfo_at_offset does, but with offset
   being unknown.

   TYPE_BINFO is binfo holding an virtual table matching
   BINFO's type.  In the case of single inheritance, this
   is binfo of BINFO's type ancestor (vtable is shared),
   otherwise it is binfo of BINFO's type.

   MATCHED_VTABLES tracks virtual tables we already did lookup
   for virtual function in.

   ANONYMOUS is true if BINFO is part of anonymous namespace.
         For types in anonymous namespace first check if the respective vtable
         is alive. If not, we know the type can't be called.  
     Walk bases.  
       Walking bases that have no virtual method is pointless excercise.  
                       In the case of single inheritance, the virtual table
                       is shared with the outer type.  

Referenced by maybe_record_node().

void update_type_inheritance_graph ( void  )
   After callgraph construction new external nodes may appear.
   Add them into the graph.  
     We reconstruct the graph starting of types of all methods seen in the
     the unit.  

Variable Documentation

pointer_set_t* cached_polymorphic_call_targets

Basic IPA utilities for type inheritance graph construction and devirtualization. Copyright (C) 2013 Free Software Foundation, Inc. Contributed by Jan Hubicka

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/.

   Brief vocalburary:
     ODR = One Definition Rule
        In short, the ODR states that:
        1 In any translation unit, a template, type, function, or object can
          have no more than one definition. Some of these can have any number
          of declarations. A definition provides an instance.
        2 In the entire program, an object or non-inline function cannot have
          more than one definition; if an object or function is used, it must
          have exactly one definition. You can declare an object or function
          that is never used, in which case you don't have to provide
          a definition. In no event can there be more than one definition.
        3 Some things, like types, templates, and extern inline functions, can
          be defined in more than one translation unit. For a given entity,
          each definition must be the same. Non-extern objects and functions
          in different translation units are different entities, even if their
          names and types are the same.

       This is the Gimple representation of type information of a polymorphic call.
       It contains two parameters:
         otr_type is a type of class whose method is called.
         otr_token is the index into virtual table where address is taken.

       This is the type inheritance information attached to each tree
       RECORD_TYPE by the C++ frotend.  It provides information about base
       types and virtual tables.

       BINFO is linked to the RECORD_TYPE by TYPE_BINFO.
       BINFO also links to its type by BINFO_TYPE and to the virtual table by

       Base types of a given type are enumerated by BINFO_BASE_BINFO
       vector.  Members of this vectors are not BINFOs associated
       with a base type.  Rather they are new copies of BINFOs
       (base BINFOs). Their virtual tables may differ from
       virtual table of the base type.  Also BINFO_OFFSET specifies
       offset of the base within the type.

       In the case of single inheritance, the virtual table is shared
       and BINFO_VTABLE of base BINFO is NULL.  In the case of multiple
       inheritance the individual virtual tables are pointer to by
       BINFO_VTABLE of base binfos (that differs of BINFO_VTABLE of 
       binfo associated to the base type).

       BINFO lookup for a given base type and offset can be done by
       get_binfo_at_offset.  It returns proper BINFO whose virtual table
       can be used for lookup of virtual methods associated with the
       base type.

       This is an index of virtual method in virtual table associated
       to the type defining it. Token can be looked up from OBJ_TYPE_REF
       or from DECL_VINDEX of a given virtual table.

     polymorphic (indirect) call
       This is callgraph represention of virtual method call.  Every
       polymorphic call contains otr_type and otr_token taken from
       original OBJ_TYPE_REF at callgraph construction time.

   What we do here:

   build_type_inheritance_graph triggers a construction of the type inheritance

     We reconstruct it based on types of methods we see in the unit.
     This means that the graph is not complete. Types with no methods are not
     inserted into the graph.  Also types without virtual methods are not
     represented at all, though it may be easy to add this.
     The inheritance graph is represented as follows:

       Vertices are structures odr_type.  Every odr_type may correspond
       to one or more tree type nodes that are equivalent by ODR rule.
       (the multiple type nodes appear only with linktime optimization)

       Edges are represented by odr_type->base and odr_type->derived_types.
       At the moment we do not track offsets of types for multiple inheritance.
       Adding this is easy.

  possible_polymorphic_call_targets returns, given an parameters found in
  indirect polymorphic edge all possible polymorphic call targets of the call.

  pass_ipa_devirt performs simple speculative devirtualization.
   Pointer set of all call targets appearing in the cache.  
odr_hash_type odr_hash
vec<odr_type, va_gc>* odr_types_ptr
   ODR types are also stored into ODR_TYPE vector to allow consistent
   walking.  Bases appear before derived types.  Vector is garbage collected
   so we won't end up visiting empty types.  
polymorphic_call_target_hash_type polymorphic_call_target_hash