[PATCH v2 00/28] Search symbols via quick API

Tom Tromey
2025-04-02T23:51:34Z

This series changes how symbols are looked up in gdb.

Currently, symbol lookup is done in two phases. In one phase, gdb
searches all existing symtabs for a symbol. In another phase, gdb
will call expand_symtabs_matching to expand some symtabs.

Different spots in gdb may order these phases differently -- so some
places will expand symtabs and then search the compunits, but other
places will first search expanded symtabs and then use the
expand_symtabs_matching callback to add further results.

This approach has a few drawbacks.

* The double search means that some discrepancies between the indexer
and the full reader go unnoticed. This may arguably be a strength
of the approach, though frequently a carefully crafted test case can
show this as a bug. Essentially, though, some searches only work by
accident.

* Searching all expanded symtabs means that, as a debug session drags
on, searches will examine many irrelevant symtabs.

* In some places, the two phases use different code to perform the
actual search, meaning that the results can depend on previous CU
expansion decisions.

* Similarly, if just a single symbol is needed, then both approaches
are bad: expand-then-search will be unnecessarily inefficient, while
search-then-expand approach means that the result depends on which
CUs happen to have already been expanded.

This series changes all of this. Now, all symbol lookups are done via
the "quick" API, with the idea being that the final search of the
expanded symtab is done via the expand_symtabs_matching callback.

This fixes all the issues pointed out above:

* Only relevant symtabs are searched, because the index only considers
matching symbols.

* Some discrepancies between the indexer and the full reader show up
as symbol lookup failures. Others (if the indexer thinks a symbol
exists but the full reader does not) will just be inefficient -- but
we could add a verifier for this.

* Lookups are no longer dependent on symtab expansion state, because
again the indexer is pre-filtering the matches.

Re-reading the series -- it's been in development for quite a while
and I've already landed ~20 preliminary patches -- shows that there
are still a few cleanups in here that could perhaps have gone in
separately.

Anyway, the essential change is to make the implementations of
expand_symtabs_matching also call the callback when a symtab has
already been expanded. After this, most of the work is cleaning up
individual callers.

This change lead to some surprising places. For example, I had to
rewrite the .gdb_index reader, because the work done for
"templates.exp" simply never worked in this mode -- and the test
obscured the problems, a classic case of lookups working by accident.

I regression tested each patch in this series. Furthermore I
regression tested the series as a whole using the cc-with-debug-names
and cc-with-gdb-index target boards. All of this was done on x86-64
Fedora 40.

I think maybe I've separately submitted "Ada import functions not in
index" patch. Anyway it's part of this series now.

Signed-off-by: Tom Tromey <tom@tromey.com>
---
Changes in v2:
- Rebased for all the other recent DWARF changes
- Link to v1: https://inbox.sourceware.org/gdb-patches/20250311-search-in-psyms-v1-0-d73d9be20983@tromey.com

---
Tom Tromey (28):
Add another minor hack to cooked_index_entry::full_name
Change ada_decode to preserve upper-case in some situations
Emit some type declarations in .gdb_index
Ada import functions not in index
Fix index's handling of DW_TAG_imported_declaration
Put all CTF symbols in global scope
Restore "ingestion" of .debug_str when writing .debug_names
Entries from anon-struct.exp not in cooked index
Remove dwarf2_per_cu_data::mark
Have expand_symtabs_matching work for already-expanded CUs
Rewrite the .gdb_index reader
Convert default_collect_symbol_completion_matches_break_on
Convert gdbpy_lookup_static_symbols
Convert ada_add_global_exceptions
Convert ada_language_defn::collect_symbol_completion_matches
Convert ada-lang.c:map_matching_symbols
Remove expand_symtabs_matching
Simplify basic_lookup_transparent_type
Remove objfile::expand_symtabs_for_function
Convert linespec.c:iterate_over_all_matching_symtabs
Simplify block_lookup_symbol_primary
Pass lookup_name_info to block_lookup_symbol_primary
Simplify block_lookup_symbol
Add best_symbol_tracker
Convert lookup_symbol_via_quick_fns
Convert lookup_symbol_in_objfile
Make dw_expand_symtabs_matching_file_matcher static
Remove enter_symbol_lookup

gdb/ada-lang.c | 221 ++---
gdb/ada-lang.h | 15 +-
gdb/block.c | 57 +-
gdb/block.h | 27 +-
gdb/cp-support.c | 28 +-
gdb/ctfread.c | 6 +-
gdb/dwarf2/abbrev.c | 7 +-
gdb/dwarf2/abbrev.h | 8 +
gdb/dwarf2/cooked-index-entry.c | 7 +
gdb/dwarf2/cooked-index-shard.c | 13 +-
gdb/dwarf2/cooked-index-shard.h | 7 +
gdb/dwarf2/cooked-index-worker.h | 17 +-
gdb/dwarf2/cooked-indexer.c | 37 +-
gdb/dwarf2/index-write.c | 103 ++-
gdb/dwarf2/read-gdb-index.c | 1268 ++++++-----------------------
gdb/dwarf2/read-gdb-index.h | 14 +
gdb/dwarf2/read.c | 176 ++--
gdb/dwarf2/read.h | 48 +-
gdb/dwarf2/tag.h | 12 +
gdb/linespec.c | 17 +-
gdb/objfiles.h | 4 -
gdb/psymtab.c | 3 -
gdb/python/py-symbol.c | 29 +-
gdb/symfile-debug.c | 22 -
gdb/symfile.c | 25 -
gdb/symfile.h | 9 -
gdb/symtab.c | 223 ++---
gdb/symtab.h | 2 +-
gdb/testsuite/gdb.ada/import.exp | 28 +-
gdb/testsuite/gdb.ctf/cross-tu-cyclic.exp | 4 +-
30 files changed, 861 insertions(+), 1576 deletions(-)
---
base-commit: 60ac9c60fe5b6dc5a59a30a971c3fad020fecf45
change-id: 20250311-search-in-psyms-3eb1049177e0

Best regards,
--
Tom Tromey <tom@tromey.com>