Fix TAB-completion + .gdb_index slowness (generalize filename_seen_cache)

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.

1 files changed, 63 insertions, 0 deletions

diff --git a/gdb/filename-seen-cache.h b/gdb/filename-seen-cache.h
new file mode 100644
index 00000000000..dd9cf7a8d79
--- /dev/null
+++ b/gdb/filename-seen-cache.h
@@ -0,0 +1,63 @@
+/* Filename-seen cache for the GNU debugger, GDB.
+
+   Copyright (C) 1986-2017 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include "defs.h"
+#include "common/function-view.h"
+
+/* Cache to watch for file names already seen.  */
+
+class filename_seen_cache
+{
+public:
+  filename_seen_cache ();
+  ~filename_seen_cache ();
+
+  /* Disable copy.  */
+  filename_seen_cache (const filename_seen_cache &) = delete;
+  void operator= (const filename_seen_cache &) = delete;
+
+  /* Empty the cache, but do not delete it.  */
+  void clear ();
+
+  /* If FILE is not already in the table of files in CACHE, add it and
+     return false; otherwise return true.
+
+     NOTE: We don't manage space for FILE, we assume FILE lives as
+     long as the caller needs.  */
+  bool seen (const char *file);
+
+  /* Traverse all cache entries, calling CALLBACK on each.  The
+     filename is passed as argument to CALLBACK.  */
+  void traverse (gdb::function_view<void (const char *filename)> callback)
+  {
+    auto erased_cb = [] (void **slot, void *info) -> int
+      {
+	auto filename = (const char *) *slot;
+	auto restored_cb = (decltype (callback) *) info;
+	(*restored_cb) (filename);
+	return 1;
+      };
+
+    htab_traverse_noresize (m_tab, erased_cb, &callback);
+  }
+
+private:
+  /* Table of files seen so far.  */
+  htab_t m_tab;
+};