[Bug c++/102228] New: lookup_anon_field is quadratic

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102228

            Bug ID: 102228
           Summary: lookup_anon_field is quadratic
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rguenth at gcc dot gnu.org
  Target Milestone: ---

When lookup_anon_field is called from build_class_member_access_expr with
the intent to build OBJECT.MEMBER in a way including the anon aggregates
COMPONENT_REFs then when there is a chain of anon aggregates to cover
the path is built bottom-up, repeating the expensive recursive walk of
lookup_anon_filed depth-of-anon-aggr times.

So instead of building OBJECT.<anon1>.<anon2>....<anonn>.MEMBER by
recursing to build_class_member_access_expr first with OBJECT
and <anonn> as MEMBER lookup_anon_field should return the full path
it found.

The PR101555 testcase with -fsyntax-only spends 66% in lookup_anon_field:

Samples: 8K of event 'cycles', Event count (approx.): 9385924074                
Overhead       Samples  Command  Shared Object     Symbol                       
  66.28%          5415  cc1plus  cc1plus           [.] lookup_anon_field
  10.04%           824  cc1plus  cc1plus           [.] get_class_binding_direct
   5.01%           411  cc1plus  cc1plus           [.] fields_linear_search

with -Os it's still the biggest hitter at nearly 15% compile time.