c++/102228 - make lookup_anon_field O(1)

c++/102228 - make lookup_anon_field O(1)

For the testcase in PR101555 lookup_anon_field takes the majority
of parsing time followed by get_class_binding_direct/fields_linear_search
which is PR83309.  The situation with anon aggregates is particularly
dire when we need to build accesses to their members and the anon
aggregates are nested.  There for each such access we recursively
build sub-accesses to the anon aggregate FIELD_DECLs bottom-up,
DFS searching for them.  That's inefficient since as I believe
there's a 1:1 relationship between anon aggregate types and the
FIELD_DECL used to place them.

The patch below does away with the search in lookup_anon_field and
instead records the single FIELD_DECL in the anon aggregate types
lang-specific data, re-using the RTTI typeinfo_var field.  That
speeds up the compile of the testcase with -fsyntax-only from
about 4.5s to slightly less than 1s.

I tried to poke holes into the 1:1 relationship idea with my C++
knowledge but failed (which might not say much).  It also leaves
a hole for the case when the C++ FE itself duplicates such type
and places it at a semantically different position.  I've tried
to poke holes into it with the duplication mechanism I understand
(templates) but failed.

2021-09-08  Richard Biener  <rguenther@suse.de>

	PR c++/102228
gcc/cp/
	* cp-tree.h (ANON_AGGR_TYPE_FIELD): New define.
	* decl.c (fixup_anonymous_aggr): Wipe RTTI info put in
	place on invalid code.
	* decl2.c (reset_type_linkage): Guard CLASSTYPE_TYPEINFO_VAR
	access.
	* module.cc (trees_in::read_class_def): Likewise.  Reconstruct
	ANON_AGGR_TYPE_FIELD.
	* semantics.c (finish_member_declaration): Populate
	ANON_AGGR_TYPE_FIELD for anon aggregate typed members.
	* typeck.c (lookup_anon_field): Remove DFS search and return
	ANON_AGGR_TYPE_FIELD directly.