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bbf2f4dfae
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.
67 lines
1.6 KiB
C
67 lines
1.6 KiB
C
/* Filename-seen cache for the GNU debugger, GDB.
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Copyright (C) 1986-2017 Free Software Foundation, Inc.
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This file is part of GDB.
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>. */
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#include "defs.h"
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#include "filename-seen-cache.h"
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#include "filenames.h"
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/* Initial size of the table. It automagically grows from here. */
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#define INITIAL_FILENAME_SEEN_CACHE_SIZE 100
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/* filename_seen_cache constructor. */
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filename_seen_cache::filename_seen_cache ()
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{
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m_tab = htab_create_alloc (INITIAL_FILENAME_SEEN_CACHE_SIZE,
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filename_hash, filename_eq,
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NULL, xcalloc, xfree);
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}
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/* See filename-seen-cache.h. */
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void
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filename_seen_cache::clear ()
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{
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htab_empty (m_tab);
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}
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/* See filename-seen-cache.h. */
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filename_seen_cache::~filename_seen_cache ()
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{
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htab_delete (m_tab);
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}
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/* See filename-seen-cache.h. */
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bool
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filename_seen_cache::seen (const char *file)
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{
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void **slot;
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/* Is FILE in tab? */
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slot = htab_find_slot (m_tab, file, INSERT);
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if (*slot != NULL)
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return true;
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/* No; add it to tab. */
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*slot = (char *) file;
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return false;
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}
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