/* IPA visibility pass Copyright (C) 2003-2014 Free Software Foundation, 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 . */ /* This file implements two related passes: - pass_data_ipa_function_and_variable_visibility run just after symbol table, references and callgraph are built - pass_data_ipa_function_and_variable_visibility run as first proper IPA pass (that is after early optimization, or, (with LTO) as a first pass done at link-time. Purpose of both passes is to set correctly visibility properties of all symbols. This includes: - Symbol privatization: Some symbols that are declared public by frontend may be turned local (either by -fwhole-program flag, by linker plugin feedback or by other reasons) - Discovery of local functions: A local function is one whose calls can occur only in the current compilation unit and all its calls are explicit, so we can change its calling convention. We simply mark all static functions whose address is not taken as local. externally_visible flag is set for symbols that can not be privatized. For privatized symbols we clear TREE_PUBLIC flag and dismantle comdat group. - Dismantling of comdat groups: Comdat group represent a section that may be replaced by linker by a different copy of the same section from other unit. If we have resolution information (from linker plugin) and we know that a given comdat gorup is prevailing, we can dismantle it and turn symbols into normal symbols. If the resolution information says that the section was previaled by copy from non-LTO code, we can also dismantle it and turn all symbols into external. - Local aliases: Some symbols can be interposed by dynamic linker. Refering to these symbols is expensive, since it needs to be overwritable by the dynamic linker. In some cases we know that the interposition does not change semantic and we can always refer to a local copy (as in the case of inline function). In this case we produce a local alias and redirect calls to it. TODO: This should be done for references, too. - Removal of static ocnstructors and destructors that have no side effects. - Regularization of several oddities introduced by frontends that may be impractical later in the optimization queue. */ #include "config.h" #include "system.h" #include "coretypes.h" #include "tm.h" #include "tree.h" #include "cgraph.h" #include "tree-pass.h" #include "calls.h" #include "gimple-expr.h" #include "varasm.h" /* Return true when NODE can not be local. Worker for cgraph_local_node_p. */ bool cgraph_node::non_local_p (struct cgraph_node *node, void *data ATTRIBUTE_UNUSED) { /* FIXME: Aliases can be local, but i386 gets thunks wrong then. */ return !(node->only_called_directly_or_aliased_p () && !node->has_aliases_p () && node->definition && !DECL_EXTERNAL (node->decl) && !node->externally_visible && !node->used_from_other_partition && !node->in_other_partition); } /* Return true when function can be marked local. */ bool cgraph_node::local_p (void) { cgraph_node *n = ultimate_alias_target (); /* FIXME: thunks can be considered local, but we need prevent i386 from attempting to change calling convention of them. */ if (n->thunk.thunk_p) return false; return !n->call_for_symbol_thunks_and_aliases (cgraph_node::non_local_p, NULL, true); } /* Return true when there is a reference to node and it is not vtable. */ bool symtab_node::address_taken_from_non_vtable_p (void) { int i; struct ipa_ref *ref = NULL; for (i = 0; iterate_referring (i, ref); i++) if (ref->use == IPA_REF_ADDR) { varpool_node *node; if (is_a (ref->referring)) return true; node = dyn_cast (ref->referring); if (!DECL_VIRTUAL_P (node->decl)) return true; } return false; } /* A helper for comdat_can_be_unshared_p. */ static bool comdat_can_be_unshared_p_1 (symtab_node *node) { if (!node->externally_visible) return true; /* When address is taken, we don't know if equality comparison won't break eventually. Exception are virutal functions, C++ constructors/destructors and vtables, where this is not possible by language standard. */ if (!DECL_VIRTUAL_P (node->decl) && (TREE_CODE (node->decl) != FUNCTION_DECL || (!DECL_CXX_CONSTRUCTOR_P (node->decl) && !DECL_CXX_DESTRUCTOR_P (node->decl))) && node->address_taken_from_non_vtable_p ()) return false; /* If the symbol is used in some weird way, better to not touch it. */ if (node->force_output) return false; /* Explicit instantiations needs to be output when possibly used externally. */ if (node->forced_by_abi && TREE_PUBLIC (node->decl) && (node->resolution != LDPR_PREVAILING_DEF_IRONLY && !flag_whole_program)) return false; /* Non-readonly and volatile variables can not be duplicated. */ if (is_a (node) && (!TREE_READONLY (node->decl) || TREE_THIS_VOLATILE (node->decl))) return false; return true; } /* COMDAT functions must be shared only if they have address taken, otherwise we can produce our own private implementation with -fwhole-program. Return true when turning COMDAT functoin static can not lead to wrong code when the resulting object links with a library defining same COMDAT. Virtual functions do have their addresses taken from the vtables, but in C++ there is no way to compare their addresses for equality. */ static bool comdat_can_be_unshared_p (symtab_node *node) { if (!comdat_can_be_unshared_p_1 (node)) return false; if (node->same_comdat_group) { symtab_node *next; /* If more than one function is in the same COMDAT group, it must be shared even if just one function in the comdat group has address taken. */ for (next = node->same_comdat_group; next != node; next = next->same_comdat_group) if (!comdat_can_be_unshared_p_1 (next)) return false; } return true; } /* Return true when function NODE should be considered externally visible. */ static bool cgraph_externally_visible_p (struct cgraph_node *node, bool whole_program) { if (!node->definition) return false; if (!TREE_PUBLIC (node->decl) || DECL_EXTERNAL (node->decl)) return false; /* Do not try to localize built-in functions yet. One of problems is that we end up mangling their asm for WHOPR that makes it impossible to call them using the implicit built-in declarations anymore. Similarly this enables us to remove them as unreachable before actual calls may appear during expansion or folding. */ if (DECL_BUILT_IN (node->decl)) return true; /* If linker counts on us, we must preserve the function. */ if (node->used_from_object_file_p ()) return true; if (DECL_PRESERVE_P (node->decl)) return true; if (lookup_attribute ("externally_visible", DECL_ATTRIBUTES (node->decl))) return true; if (TARGET_DLLIMPORT_DECL_ATTRIBUTES && lookup_attribute ("dllexport", DECL_ATTRIBUTES (node->decl))) return true; if (node->resolution == LDPR_PREVAILING_DEF_IRONLY) return false; /* When doing LTO or whole program, we can bring COMDAT functoins static. This improves code quality and we know we will duplicate them at most twice (in the case that we are not using plugin and link with object file implementing same COMDAT) */ if ((in_lto_p || whole_program) && DECL_COMDAT (node->decl) && comdat_can_be_unshared_p (node)) return false; /* When doing link time optimizations, hidden symbols become local. */ if (in_lto_p && (DECL_VISIBILITY (node->decl) == VISIBILITY_HIDDEN || DECL_VISIBILITY (node->decl) == VISIBILITY_INTERNAL) /* Be sure that node is defined in IR file, not in other object file. In that case we don't set used_from_other_object_file. */ && node->definition) ; else if (!whole_program) return true; if (MAIN_NAME_P (DECL_NAME (node->decl))) return true; return false; } /* Return true when variable should be considered externally visible. */ bool varpool_node::externally_visible_p (void) { if (DECL_EXTERNAL (decl)) return true; if (!TREE_PUBLIC (decl)) return false; /* If linker counts on us, we must preserve the function. */ if (used_from_object_file_p ()) return true; if (DECL_HARD_REGISTER (decl)) return true; if (DECL_PRESERVE_P (decl)) return true; if (lookup_attribute ("externally_visible", DECL_ATTRIBUTES (decl))) return true; if (TARGET_DLLIMPORT_DECL_ATTRIBUTES && lookup_attribute ("dllexport", DECL_ATTRIBUTES (decl))) return true; /* See if we have linker information about symbol not being used or if we need to make guess based on the declaration. Even if the linker clams the symbol is unused, never bring internal symbols that are declared by user as used or externally visible. This is needed for i.e. references from asm statements. */ if (used_from_object_file_p ()) return true; if (resolution == LDPR_PREVAILING_DEF_IRONLY) return false; /* As a special case, the COMDAT virtual tables can be unshared. In LTO mode turn vtables into static variables. The variable is readonly, so this does not enable more optimization, but referring static var is faster for dynamic linking. Also this match logic hidding vtables from LTO symbol tables. */ if ((in_lto_p || flag_whole_program) && DECL_COMDAT (decl) && comdat_can_be_unshared_p (this)) return false; /* When doing link time optimizations, hidden symbols become local. */ if (in_lto_p && (DECL_VISIBILITY (decl) == VISIBILITY_HIDDEN || DECL_VISIBILITY (decl) == VISIBILITY_INTERNAL) /* Be sure that node is defined in IR file, not in other object file. In that case we don't set used_from_other_object_file. */ && definition) ; else if (!flag_whole_program) return true; /* Do not attempt to privatize COMDATS by default. This would break linking with C++ libraries sharing inline definitions. FIXME: We can do so for readonly vars with no address taken and possibly also for vtables since no direct pointer comparsion is done. It might be interesting to do so to reduce linking overhead. */ if (DECL_COMDAT (decl) || DECL_WEAK (decl)) return true; return false; } /* Return true if reference to NODE can be replaced by a local alias. Local aliases save dynamic linking overhead and enable more optimizations. */ bool can_replace_by_local_alias (symtab_node *node) { return (node->get_availability () > AVAIL_INTERPOSABLE && !decl_binds_to_current_def_p (node->decl) && !node->can_be_discarded_p ()); } /* Return true if we can replace refernece to NODE by local alias within a virtual table. Generally we can replace function pointers and virtual table pointers. */ bool can_replace_by_local_alias_in_vtable (symtab_node *node) { if (is_a (node) && !DECL_VIRTUAL_P (node->decl)) return false; return can_replace_by_local_alias (node); } /* walk_tree callback that rewrites initializer references. */ static tree update_vtable_references (tree *tp, int *walk_subtrees, void *data ATTRIBUTE_UNUSED) { if (TREE_CODE (*tp) == VAR_DECL || TREE_CODE (*tp) == FUNCTION_DECL) { if (can_replace_by_local_alias_in_vtable (symtab_node::get (*tp))) *tp = symtab_node::get (*tp)->noninterposable_alias ()->decl; *walk_subtrees = 0; } else if (IS_TYPE_OR_DECL_P (*tp)) *walk_subtrees = 0; return NULL; } /* In LTO we can remove COMDAT groups and weak symbols. Either turn them into normal symbols or external symbol depending on resolution info. */ static void update_visibility_by_resolution_info (symtab_node * node) { bool define; if (!node->externally_visible || (!DECL_WEAK (node->decl) && !DECL_ONE_ONLY (node->decl)) || node->resolution == LDPR_UNKNOWN) return; define = (node->resolution == LDPR_PREVAILING_DEF_IRONLY || node->resolution == LDPR_PREVAILING_DEF || node->resolution == LDPR_UNDEF || node->resolution == LDPR_PREVAILING_DEF_IRONLY_EXP); /* The linker decisions ought to agree in the whole group. */ if (node->same_comdat_group) for (symtab_node *next = node->same_comdat_group; next != node; next = next->same_comdat_group) gcc_assert (!next->externally_visible || define == (next->resolution == LDPR_PREVAILING_DEF_IRONLY || next->resolution == LDPR_PREVAILING_DEF || next->resolution == LDPR_UNDEF || next->resolution == LDPR_PREVAILING_DEF_IRONLY_EXP)); if (node->same_comdat_group) for (symtab_node *next = node->same_comdat_group; next != node; next = next->same_comdat_group) { next->set_comdat_group (NULL); DECL_WEAK (next->decl) = false; if (next->externally_visible && !define) DECL_EXTERNAL (next->decl) = true; } node->set_comdat_group (NULL); DECL_WEAK (node->decl) = false; if (!define) DECL_EXTERNAL (node->decl) = true; node->dissolve_same_comdat_group_list (); } /* Decide on visibility of all symbols. */ static unsigned int function_and_variable_visibility (bool whole_program) { struct cgraph_node *node; varpool_node *vnode; /* All aliases should be procssed at this point. */ gcc_checking_assert (!alias_pairs || !alias_pairs->length ()); FOR_EACH_FUNCTION (node) { int flags = flags_from_decl_or_type (node->decl); /* Optimize away PURE and CONST constructors and destructors. */ if (optimize && (flags & (ECF_CONST | ECF_PURE)) && !(flags & ECF_LOOPING_CONST_OR_PURE)) { DECL_STATIC_CONSTRUCTOR (node->decl) = 0; DECL_STATIC_DESTRUCTOR (node->decl) = 0; } /* Frontends and alias code marks nodes as needed before parsing is finished. We may end up marking as node external nodes where this flag is meaningless strip it. */ if (DECL_EXTERNAL (node->decl) || !node->definition) { node->force_output = 0; node->forced_by_abi = 0; } /* C++ FE on lack of COMDAT support create local COMDAT functions (that ought to be shared but can not due to object format limitations). It is necessary to keep the flag to make rest of C++ FE happy. Clear the flag here to avoid confusion in middle-end. */ if (DECL_COMDAT (node->decl) && !TREE_PUBLIC (node->decl)) DECL_COMDAT (node->decl) = 0; /* For external decls stop tracking same_comdat_group. It doesn't matter what comdat group they are in when they won't be emitted in this TU. */ if (node->same_comdat_group && DECL_EXTERNAL (node->decl)) { #ifdef ENABLE_CHECKING symtab_node *n; for (n = node->same_comdat_group; n != node; n = n->same_comdat_group) /* If at least one of same comdat group functions is external, all of them have to be, otherwise it is a front-end bug. */ gcc_assert (DECL_EXTERNAL (n->decl)); #endif node->dissolve_same_comdat_group_list (); } gcc_assert ((!DECL_WEAK (node->decl) && !DECL_COMDAT (node->decl)) || TREE_PUBLIC (node->decl) || node->weakref || DECL_EXTERNAL (node->decl)); if (cgraph_externally_visible_p (node, whole_program)) { gcc_assert (!node->global.inlined_to); node->externally_visible = true; } else { node->externally_visible = false; node->forced_by_abi = false; } if (!node->externally_visible && node->definition && !node->weakref && !DECL_EXTERNAL (node->decl)) { gcc_assert (whole_program || in_lto_p || !TREE_PUBLIC (node->decl)); node->unique_name = ((node->resolution == LDPR_PREVAILING_DEF_IRONLY || node->unique_name || node->resolution == LDPR_PREVAILING_DEF_IRONLY_EXP) && TREE_PUBLIC (node->decl)); node->resolution = LDPR_PREVAILING_DEF_IRONLY; if (node->same_comdat_group && TREE_PUBLIC (node->decl)) { symtab_node *next = node; /* Set all members of comdat group local. */ if (node->same_comdat_group) for (next = node->same_comdat_group; next != node; next = next->same_comdat_group) { next->set_comdat_group (NULL); if (!next->alias) next->set_section (NULL); next->make_decl_local (); next->unique_name = ((next->resolution == LDPR_PREVAILING_DEF_IRONLY || next->unique_name || next->resolution == LDPR_PREVAILING_DEF_IRONLY_EXP) && TREE_PUBLIC (next->decl)); } /* cgraph_externally_visible_p has already checked all other nodes in the group and they will all be made local. We need to dissolve the group at once so that the predicate does not segfault though. */ node->dissolve_same_comdat_group_list (); } if (TREE_PUBLIC (node->decl)) node->set_comdat_group (NULL); if (DECL_COMDAT (node->decl) && !node->alias) node->set_section (NULL); node->make_decl_local (); } if (node->thunk.thunk_p && TREE_PUBLIC (node->decl)) { struct cgraph_node *decl_node = node; decl_node = decl_node->callees->callee->function_symbol (); /* Thunks have the same visibility as function they are attached to. Make sure the C++ front end set this up properly. */ if (DECL_ONE_ONLY (decl_node->decl)) { gcc_checking_assert (DECL_COMDAT (node->decl) == DECL_COMDAT (decl_node->decl)); gcc_checking_assert (node->in_same_comdat_group_p (decl_node)); gcc_checking_assert (node->same_comdat_group); } node->forced_by_abi = decl_node->forced_by_abi; if (DECL_EXTERNAL (decl_node->decl)) DECL_EXTERNAL (node->decl) = 1; } update_visibility_by_resolution_info (node); } FOR_EACH_DEFINED_FUNCTION (node) { node->local.local |= node->local_p (); /* If we know that function can not be overwritten by a different semantics and moreover its section can not be discarded, replace all direct calls by calls to an noninterposable alias. This make dynamic linking cheaper and enable more optimization. TODO: We can also update virtual tables. */ if (node->callers && can_replace_by_local_alias (node)) { cgraph_node *alias = dyn_cast (node->noninterposable_alias ()); if (alias && alias != node) { while (node->callers) { struct cgraph_edge *e = node->callers; e->redirect_callee (alias); if (gimple_has_body_p (e->caller->decl)) { push_cfun (DECL_STRUCT_FUNCTION (e->caller->decl)); e->redirect_call_stmt_to_callee (); pop_cfun (); } } } } } FOR_EACH_VARIABLE (vnode) { /* weak flag makes no sense on local variables. */ gcc_assert (!DECL_WEAK (vnode->decl) || vnode->weakref || TREE_PUBLIC (vnode->decl) || DECL_EXTERNAL (vnode->decl)); /* In several cases declarations can not be common: - when declaration has initializer - when it is in weak - when it has specific section - when it resides in non-generic address space. - if declaration is local, it will get into .local common section so common flag is not needed. Frontends still produce these in certain cases, such as for: static int a __attribute__ ((common)) Canonicalize things here and clear the redundant flag. */ if (DECL_COMMON (vnode->decl) && (!(TREE_PUBLIC (vnode->decl) || DECL_EXTERNAL (vnode->decl)) || (DECL_INITIAL (vnode->decl) && DECL_INITIAL (vnode->decl) != error_mark_node) || DECL_WEAK (vnode->decl) || DECL_SECTION_NAME (vnode->decl) != NULL || ! (ADDR_SPACE_GENERIC_P (TYPE_ADDR_SPACE (TREE_TYPE (vnode->decl)))))) DECL_COMMON (vnode->decl) = 0; } FOR_EACH_DEFINED_VARIABLE (vnode) { if (!vnode->definition) continue; if (vnode->externally_visible_p ()) vnode->externally_visible = true; else { vnode->externally_visible = false; vnode->forced_by_abi = false; } if (!vnode->externally_visible && !vnode->weakref) { gcc_assert (in_lto_p || whole_program || !TREE_PUBLIC (vnode->decl)); vnode->unique_name = ((vnode->resolution == LDPR_PREVAILING_DEF_IRONLY || vnode->resolution == LDPR_PREVAILING_DEF_IRONLY_EXP) && TREE_PUBLIC (vnode->decl)); if (vnode->same_comdat_group && TREE_PUBLIC (vnode->decl)) { symtab_node *next = vnode; /* Set all members of comdat group local. */ if (vnode->same_comdat_group) for (next = vnode->same_comdat_group; next != vnode; next = next->same_comdat_group) { next->set_comdat_group (NULL); if (!next->alias) next->set_section (NULL); next->make_decl_local (); next->unique_name = ((next->resolution == LDPR_PREVAILING_DEF_IRONLY || next->unique_name || next->resolution == LDPR_PREVAILING_DEF_IRONLY_EXP) && TREE_PUBLIC (next->decl)); } vnode->dissolve_same_comdat_group_list (); } if (TREE_PUBLIC (vnode->decl)) vnode->set_comdat_group (NULL); if (DECL_COMDAT (vnode->decl) && !vnode->alias) vnode->set_section (NULL); vnode->make_decl_local (); vnode->resolution = LDPR_PREVAILING_DEF_IRONLY; } update_visibility_by_resolution_info (vnode); /* Update virtual tables to point to local aliases where possible. */ if (DECL_VIRTUAL_P (vnode->decl) && !DECL_EXTERNAL (vnode->decl)) { int i; struct ipa_ref *ref; bool found = false; /* See if there is something to update. */ for (i = 0; vnode->iterate_referring (i, ref); i++) if (ref->use == IPA_REF_ADDR && can_replace_by_local_alias_in_vtable (ref->referred)) { found = true; break; } if (found) { hash_set visited_nodes; vnode->get_constructor (); walk_tree (&DECL_INITIAL (vnode->decl), update_vtable_references, NULL, &visited_nodes); vnode->remove_all_references (); record_references_in_initializer (vnode->decl, false); } } } if (dump_file) { fprintf (dump_file, "\nMarking local functions:"); FOR_EACH_DEFINED_FUNCTION (node) if (node->local.local) fprintf (dump_file, " %s", node->name ()); fprintf (dump_file, "\n\n"); fprintf (dump_file, "\nMarking externally visible functions:"); FOR_EACH_DEFINED_FUNCTION (node) if (node->externally_visible) fprintf (dump_file, " %s", node->name ()); fprintf (dump_file, "\n\n"); fprintf (dump_file, "\nMarking externally visible variables:"); FOR_EACH_DEFINED_VARIABLE (vnode) if (vnode->externally_visible) fprintf (dump_file, " %s", vnode->name ()); fprintf (dump_file, "\n\n"); } symtab->function_flags_ready = true; return 0; } /* Local function pass handling visibilities. This happens before LTO streaming so in particular -fwhole-program should be ignored at this level. */ namespace { const pass_data pass_data_ipa_function_and_variable_visibility = { SIMPLE_IPA_PASS, /* type */ "visibility", /* name */ OPTGROUP_NONE, /* optinfo_flags */ TV_CGRAPHOPT, /* tv_id */ 0, /* properties_required */ 0, /* properties_provided */ 0, /* properties_destroyed */ 0, /* todo_flags_start */ ( TODO_remove_functions | TODO_dump_symtab ), /* todo_flags_finish */ }; /* Bring functions local at LTO time with -fwhole-program. */ static unsigned int whole_program_function_and_variable_visibility (void) { function_and_variable_visibility (flag_whole_program); if (optimize) ipa_discover_readonly_nonaddressable_vars (); return 0; } } // anon namespace namespace { const pass_data pass_data_ipa_whole_program_visibility = { IPA_PASS, /* type */ "whole-program", /* name */ OPTGROUP_NONE, /* optinfo_flags */ TV_CGRAPHOPT, /* tv_id */ 0, /* properties_required */ 0, /* properties_provided */ 0, /* properties_destroyed */ 0, /* todo_flags_start */ ( TODO_remove_functions | TODO_dump_symtab ), /* todo_flags_finish */ }; class pass_ipa_whole_program_visibility : public ipa_opt_pass_d { public: pass_ipa_whole_program_visibility (gcc::context *ctxt) : ipa_opt_pass_d (pass_data_ipa_whole_program_visibility, ctxt, NULL, /* generate_summary */ NULL, /* write_summary */ NULL, /* read_summary */ NULL, /* write_optimization_summary */ NULL, /* read_optimization_summary */ NULL, /* stmt_fixup */ 0, /* function_transform_todo_flags_start */ NULL, /* function_transform */ NULL) /* variable_transform */ {} /* opt_pass methods: */ virtual bool gate (function *) { /* Do not re-run on ltrans stage. */ return !flag_ltrans; } virtual unsigned int execute (function *) { return whole_program_function_and_variable_visibility (); } }; // class pass_ipa_whole_program_visibility } // anon namespace ipa_opt_pass_d * make_pass_ipa_whole_program_visibility (gcc::context *ctxt) { return new pass_ipa_whole_program_visibility (ctxt); } class pass_ipa_function_and_variable_visibility : public simple_ipa_opt_pass { public: pass_ipa_function_and_variable_visibility (gcc::context *ctxt) : simple_ipa_opt_pass (pass_data_ipa_function_and_variable_visibility, ctxt) {} /* opt_pass methods: */ virtual unsigned int execute (function *) { return function_and_variable_visibility (flag_whole_program && !flag_lto); } }; // class pass_ipa_function_and_variable_visibility simple_ipa_opt_pass * make_pass_ipa_function_and_variable_visibility (gcc::context *ctxt) { return new pass_ipa_function_and_variable_visibility (ctxt); }