Re: [patch] Improve symbol lookup performance noted in PR 15519

Doug Evans
2013-06-05T22:31:20Z

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On Mon, Jun 3, 2013 at 10:27 AM, Pedro Alves <palves@redhat.com> wrote:
> What I guess I'm still missing to understand it,
> is a short blurb describing what is being skipped and why is
> it safe to be skipped. :-)
>
> Just to make sure I understand the change -- I see
> cp_lookup_symbol_namespace does:
>
> ...
> /* Search for name in namespaces imported to this and parent
> blocks. */
> while (block != NULL)
> {
> sym = cp_lookup_symbol_imports (scope, name, block,
> domain, 0, 1);
>
> if (sym)
> return sym;
>
> block = BLOCK_SUPERBLOCK (block);
> }
>
> and a chunk of the speedup comes from skipping that, correct?
> That is, it is supposedly unnecessary to look symbol imports
> when looking up a symbol in a baseclass, right?

Right.
There are two kinds of "using"s: directives and declarations.
"using directives" cannot appear in class scope, and that is what
cp_lookup_symbol_namespace handles.
And, AFAICT, gdb doesn't support "using declarations" yet,
which does affect base class lookup here.

> The patch then also replaces a lookup_symbol_static with a specific
> block call followed by a fallback lookup_static_symbol_aux search over all
> objfiles, by always doing the lookup_static_symbol_aux search over
> all objfiles. It makes me wonder if it was there for a reason things were
> done that way before, like for something like the same named class/methods
> being implemented/hidden/private in different sos/dlls (therefore not
> really subject to ODR), therefore making it desirable to lookup in the
> same context first. I have no idea, probably not. :-) I guess I'm just
> after getting the analysis/conclusion that led to the change
> recorded for posterity. :-)

It's kind of a toss up.
Even if someone played games with visibility the previous lookup only
checks the current symtab; there's no guarantee the needed debug info
isn't in another symtab in that objfile (e.g., due to gcc's aggressive
debug info pruning).
Ultimately, I think what we want is to search the current objfile, and
then search the remaining objfiles, but I left that for another day.
Still, searching the current symtab isn't expensive enough that
re-searching it is a problem and it sometimes will win.
So I've modified the patch to keep that behaviour.
Still have to search the world if block != NULL though. Improving
that's also left for another day.

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2013-06-05 Doug Evans <dje@google.com>
Keith Seitz <keiths@redhat.com>

PR 15519
* cp-namespace.c (find_symbol_in_baseclass): Call
cp_lookup_symbol_in_namespace instead of cp_lookup_symbol_namespace.
Check result of call to lookup_symbol_static.
Call lookup_static_symbol_aux unconditionally.
Call check_typedef on base types before accessing them.
(cp_lookup_nested_symbol): Fix comment.

testsuite/
* gdb.cp/derivation2.cc: New file.
* gdb.cp/derivation.cc (main): Call foo2.
* gdb.cp/derivation.exp: Add tests for typedefs in another
file, and when there's an active block.

Index: cp-namespace.c
===================================================================
RCS file: /cvs/src/src/gdb/cp-namespace.c,v
retrieving revision 1.67
diff -u -p -r1.67 cp-namespace.c
--- cp-namespace.c 30 May 2013 17:29:06 -0000 1.67
+++ cp-namespace.c 5 Jun 2013 22:21:17 -0000
@@ -720,36 +720,40 @@ find_symbol_in_baseclass (struct type *p
for (i = 0; i < TYPE_N_BASECLASSES (parent_type); ++i)
{
size_t len;
+ struct type *base_type = TYPE_BASECLASS (parent_type, i);
const char *base_name = TYPE_BASECLASS_NAME (parent_type, i);

if (base_name == NULL)
continue;

/* Search this particular base class. */
- sym = cp_lookup_symbol_namespace (base_name, name, block, VAR_DOMAIN);
+ sym = cp_lookup_symbol_in_namespace (base_name, name, block,
+ VAR_DOMAIN, 0);
if (sym != NULL)
break;

+ /* Now search all static file-level symbols. We have to do this for
+ things like typedefs in the class. First search in this symtab,
+ what we want is possibly there. */
len = strlen (base_name) + 2 + strlen (name) + 1;
concatenated_name = xrealloc (concatenated_name, len);
xsnprintf (concatenated_name, len, "%s::%s", base_name, name);
sym = lookup_symbol_static (concatenated_name, block, VAR_DOMAIN);
+ if (sym != NULL)
+ break;

- /* If there is currently no BLOCK, e.g., the inferior hasn't yet
- been started, then try searching all STATIC_BLOCK symbols in
- all objfiles. */
- if (block == NULL)
- {
- sym = lookup_static_symbol_aux (concatenated_name, VAR_DOMAIN);
- if (sym != NULL)
- break;
- }
+ /* Nope. We now have to search all static blocks in all objfiles,
+ even if block != NULL, because there's no guarantees as to which
+ symtab the symbol we want is in. */
+ sym = lookup_static_symbol_aux (concatenated_name, VAR_DOMAIN);
+ if (sym != NULL)
+ break;

/* If this class has base classes, search them next. */
- if (TYPE_N_BASECLASSES (TYPE_BASECLASS (parent_type, i)) > 0)
+ CHECK_TYPEDEF (base_type);
+ if (TYPE_N_BASECLASSES (base_type) > 0)
{
- sym = find_symbol_in_baseclass (TYPE_BASECLASS (parent_type, i),
- name, block);
+ sym = find_symbol_in_baseclass (base_type, name, block);
if (sym != NULL)
break;
}
@@ -797,8 +801,8 @@ cp_lookup_nested_symbol (struct type *pa
if (sym != NULL)
return sym;

- /* Now search all static file-level symbols. Not strictly
- correct, but more useful than an error. We do not try to
+ /* Now search all static file-level symbols. We have to do this
+ for things like typedefs in the class. We do not try to
guess any imported namespace as even the fully specified
namespace search is already not C++ compliant and more
assumptions could make it too magic. */
Index: testsuite/gdb.cp/derivation.cc
===================================================================
RCS file: /cvs/src/src/gdb/testsuite/gdb.cp/derivation.cc,v
retrieving revision 1.5
diff -u -p -r1.5 derivation.cc
--- testsuite/gdb.cp/derivation.cc 1 Jan 2013 06:33:27 -0000 1.5
+++ testsuite/gdb.cp/derivation.cc 5 Jun 2013 22:21:18 -0000
@@ -16,6 +16,8 @@
along with this program. If not, see <http://www.gnu.org/licenses/>.
*/

+extern void foo2 (); /* from derivation2.cc */
+
namespace N {
typedef double value_type;
struct Base { typedef int value_type; };
@@ -306,9 +308,7 @@ int main(void)
N::Derived::value_type d = 1;
N::value_type n = 3.0;
dobj.doit ();
+ foo2 ();
return 0;

}
-
-
-
Index: testsuite/gdb.cp/derivation.exp
===================================================================
RCS file: /cvs/src/src/gdb/testsuite/gdb.cp/derivation.exp,v
retrieving revision 1.24
diff -u -p -r1.24 derivation.exp
--- testsuite/gdb.cp/derivation.exp 1 Jan 2013 06:33:27 -0000 1.24
+++ testsuite/gdb.cp/derivation.exp 5 Jun 2013 22:21:18 -0000
@@ -32,14 +32,15 @@ if { [skip_cplus_tests] } { continue }

load_lib "cp-support.exp"

-standard_testfile .cc
+standard_testfile derivation.cc derivation2.cc

-if {[prepare_for_testing $testfile.exp $testfile $srcfile {debug c++}]} {
+if {[prepare_for_testing $testfile.exp $testfile \
+ [list $srcfile $srcfile2] {debug c++}]} {
return -1
}

# Check inheritance of typedefs.
-foreach klass {"A" "D" "E" "F"} {
+foreach klass {"A" "D" "E" "F" "A2" "D2"} {
gdb_test "ptype ${klass}::value_type" "type = int"
gdb_test "whatis ${klass}::value_type" "type = int"
gdb_test "p (${klass}::value_type) 0" " = 0"
@@ -57,6 +58,13 @@ if ![runto 'marker1'] then {
continue
}

+# Check inheritance of typedefs again, but this time with an active block.
+foreach klass {"A" "D" "A2" "D2"} {
+ gdb_test "ptype ${klass}::value_type" "type = int"
+ gdb_test "whatis ${klass}::value_type" "type = int"
+ gdb_test "p (${klass}::value_type) 0" " = 0"
+}
+
gdb_test "up" ".*main.*" "up from marker1"

# Print class types and values.
Index: testsuite/gdb.cp/derivation2.cc
===================================================================
RCS file: testsuite/gdb.cp/derivation2.cc
diff -N testsuite/gdb.cp/derivation2.cc
--- /dev/null 1 Jan 1970 00:00:00 -0000
+++ testsuite/gdb.cp/derivation2.cc 5 Jun 2013 22:21:18 -0000
@@ -0,0 +1,49 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+ Copyright 2013 Free Software Foundation, Inc.
+
+ 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/>.
+ */
+
+/* A copy of some classes in derivation.cc so that we can test symbol lookup
+ in other CUs. */
+
+class A2 {
+public:
+ typedef int value_type;
+ value_type a;
+
+ A2()
+ {
+ a=1;
+ }
+};
+
+class D2 : public A2 {
+public:
+ value_type d;
+
+ D2()
+ {
+ d=7;
+ }
+};
+
+void
+foo2 ()
+{
+ D2 d2_instance;
+ d2_instance.a = 42;
+ d2_instance.d = 43;
+}