[Bug symtab/30826] New: [gdb/symtab] Review PU presence in objfile->compunits ()

["vries at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=30826

            Bug ID: 30826
           Summary: [gdb/symtab] Review PU presence in objfile->compunits
                    ()
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: symtab
          Assignee: unassigned at sourceware dot org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

[ First discussed here (
https://sourceware.org/pipermail/gdb-patches/2023-September/202161.html ) ]

The PUs are part of the linked list objfile->compunits (), and I've recently
committed a few fixes that skip PUs when walking the linked list:
...
$ git log --pretty=commit HEAD^^..HEAD
commit e061219f5d6 ("[gdb/symtab] Fix too many symbols in
gdbpy_lookup_static_symbols")
commit 7023b8d86c6 ("[gdb/symtab] Handle PU in iterate_over_some_symtabs")
...

The question is whether we can just drop the PUs from the linked list.

[ On a related note, I've been wondered about adding a sorted vector alongside
objfile->compunits (), which is sorted according to file offset, to address the
"insertion-order is search-order" problem.  If we would add such a vector, and
do all searches on it instead of the linked list, then rather than deleting PUs
from the linked list, we could not add them to the vector, which is slightly
easier. ]

Testing a WIP patch (see the gdb-patches thread mentioned above) revealed
regressions in gdb.cp/m-static.exp, where lookup_symbol_in_objfile_symtabs
walks the linked list but doesn't walk the corresponding includes for each CU.

However, it could be argued that the current way of visiting PUs is more
efficient.  It guarantees that PUs are visited only once, and if we visit all
includes for each CU, then a PU might be visited twice.

Note how the efficient approach differs depending on whether we lookup one
match or all matches:
- if we lookup one match, we want to not skip PUs, but skip includes, and
- if we lookup all matches, we want to skip PUs, but not skip includes.

In the former case, when finding the match in a PU, it's virtually a match in
the top-level canonical includer CU (the one you get when walking the user
chain to the top).

In the latter case, a PU can be visited more than once, which is potentially
inefficient.  This could be addressed with caching, perhaps this is already
done.

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