On 9/16/21 05:25, Feng Xue OS via Gcc-patches wrote:
This and following patches are composed to enable full devirtualization
under whole program assumption (so also called whole-program
devirtualization, WPD for short), which is an enhancement to current
speculative devirtualization. The base of the optimization is how to
identify class type that is local in terms of whole-program scope, at
least those class types in libstdc++ must be excluded in some way.
Our means is to use typeinfo symbol as identity marker of a class since
it is unique and always generated once the class or its derived type
is instantiated somewhere, and rely on symbol resolution by
lto-linker-plugin to detect whether a typeinfo is referenced by regular
object/library, which indirectly tells class types are escaped or not.
The RFC at https://gcc.gnu.org/pipermail/gcc/2021-August/237132.html
gives more details on that.
Bootstrapped/regtested on x86_64-linux and aarch64-linux.
Thanks,
Feng
----
2021-09-07 Feng Xue <f...@os.amperecomputing.com>
gcc/
* common.opt (-fdevirtualize-fully): New option.
* class.c (build_rtti_vtbl_entries): Force generation of typeinfo
even -fno-rtti is specificied under full devirtualization.
This makes -fno-rtti useless; rather than this, you should warn about
the combination of flags and force flag_rtti on. It also sounds like
you depend on the library not being built with -fno-rtti.
* cgraph.c (cgraph_update_edges_for_call_stmt): Add an assertion
to check node to be traversed.
* cgraphclones.c (cgraph_node::find_replacement): Record
former_clone_of on replacement node.
* cgraphunit.c (symtab_node::needed_p): Always output vtable for
full devirtualization.
(analyze_functions): Force generation of primary vtables for all
base classes.
* ipa-devirt.c (odr_type_d::whole_program_local): New field.
(odr_type_d::has_virtual_base): Likewise.
(odr_type_d::all_derivations_known): Removed.
(odr_type_d::whole_program_local_p): New member function.
(odr_type_d::all_derivations_known_p): Likewise.
(odr_type_d::possibly_instantiated_p): Likewise.
(odr_type_d::set_has_virtual_base): Likewise.
(get_odr_type): Set "whole_program_local" and "has_virtual_base"
when adding a type.
(type_all_derivations_known_p): Replace implementation by a call
to odr_type_d::all_derivations_known_p.
(type_possibly_instantiated_p): Replace implementation by a call
to odr_type_d::possibly_instantiated_p.
(type_known_to_have_no_derivations_p): Replace call to
type_possibly_instantiated_p with call to
odr_type_d::possibly_instantiated_p.
(type_all_ctors_visible_p): Removed.
(type_whole_program_local_p): New function.
(get_type_vtable): Likewise.
(extract_typeinfo_in_vtable): Likewise.
(identify_whole_program_local_types): Likewise.
(dump_odr_type): Dump has_virtual_base and whole_program_local_p()
of type.
(maybe_record_node): Resort to type_whole_program_local_p to
check whether a class has been optimized away.
(record_target_from_binfo): Remove parameter "anonymous", add
a new parameter "possibly_instantiated", and adjust code
accordingly.
(devirt_variable_node_removal_hook): Replace call to
"type_in_anonymous_namespace_p" with "type_whole_program_local_p".
(possible_polymorphic_call_targets): Replace call to
"type_possibly_instantiated_p" with "possibly_instantiated_p",
replace flag check on "all_derivations_known" with call to
"all_derivations_known_p".
* ipa-icf.c (filter_removed_items): Disable folding on vtable
under full devirtualization.
* ipa-polymorphic-call.c (restrict_to_inner_class): Move odr
type check to type_known_to_have_no_derivations_p.
* ipa-utils.h (identify_whole_program_local_types): New
declaration.
(type_all_derivations_known_p): Parameter type adjustment.
* ipa.c (walk_polymorphic_call_targets): Do not mark vcall
targets as reachable for full devirtualization.
(can_remove_vtable_if_no_refs_p): New function.
(symbol_table::remove_unreachable_nodes): Add defined vtables
to reachable list under full devirtualization.
* lto-symtab.c (lto_symtab_merge_symbols): Identify whole
program local types after symbol table merge.
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