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Tab completion when debugging a program binary that uses GDB index is
surprisingly much slower than when GDB uses psymtabs instead. Around
1.5x/3x slower. That's surprising, because the whole point of GDB
index is to speed things up...
For example, with:
set pagination off
set $count = 0
while $count < 400
complete b string_prin # matches gdb's string_printf
printf "count = %d\n", $count
set $count = $count + 1
end
$ time ./gdb --batch -q ./gdb-with-index -ex "source script.cmd"
real 0m11.042s
user 0m10.920s
sys 0m0.042s
$ time ./gdb --batch -q ./gdb-without-index -ex "source script.cmd"
real 0m4.635s
user 0m4.590s
sys 0m0.037s
Same but with:
- complete b string_prin
+ complete b zzzzzz
to exercise the no-matches worst case, master currently gets you
something like:
with index without index
real 0m11.971s 0m8.413s
user 0m11.912s 0m8.355s
sys 0m0.035s 0m0.035s
Running gdb under perf shows 80% spent inside
maybe_add_partial_symtab_filename, and 20% spent in the lbasename
inside that.
The problem that tab completion walks over all compunit symtabs, and
for each, walks the contained file symtabs. And there a huge number
of file symtabs (each included system header, etc.) that appear in
each compunit symtab's file symtab list. As in, when debugging GDB, I
have 367381 symtabs iterated, when of those only 5371 filenames are
unique...
This was a regression from the earlier (nice) split of symtabs in
compunit symtabs + file symtabs.
The fix here is to add a cache of unique filenames per objfile so that
the walk / uniquing is only done once. There's already a abstraction
for this in symtab.c; this patch moves that code out to a separate
file and C++ifies it bit.
This makes the worst-case scenario above consistently drop to ~2.5s
(1.5s for the "string_prin" hit case), making it over 3.3x times
faster than psymtabs in this use case (7x in the "string_prin" hit
case).
gdb/ChangeLog:
2017-07-17 Pedro Alves <palves@redhat.com>
* Makefile.in (COMMON_OBS): Add filename-seen-cache.o.
* dwarf2read.c: Include "filename-seen-cache.h".
* dwarf2read.c (dwarf2_per_objfile) <filenames_cache>: New field.
(dw2_map_symbol_filenames): Build and use a filenames_seen_cache.
* filename-seen-cache.c: New file.
* filename-seen-cache.h: New file.
* symtab.c: Include "filename-seen-cache.h".
(struct filename_seen_cache, INITIAL_FILENAME_SEEN_CACHE_SIZE)
(create_filename_seen_cache, clear_filename_seen_cache)
(delete_filename_seen_cache, filename_seen): Delete, parts moved
to filename-seen-cache.h/filename-seen-cache.c.
(output_source_filename, sources_info)
(maybe_add_partial_symtab_filename)
(make_source_files_completion_list): Adjust to use
filename_seen_cache.
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