[PATCH 26/40] Optimize .gdb_index symbol name searching

Pedro Alves
2017-06-02T12:39:58Z

As mentioned in the previous patch, .gdb_index name lookup got
significantly slower with the previous patch.

This patch addresses that, and in the process makes .gdb_index name
searching faster than what we had before the previous patch, even.
Using the same test:

$ cat script.cmd
set pagination off
set $count = 0
while $count < 400
complete b string_prin
printf "count = %d\n", $count
set $count = $count + 1
end

$ time gdb --batch -q ./gdb-with-index -ex "source script.cmd"

I got, before the previous patch (-O2, x86-64):

real 0m1.773s
user 0m1.737s
sys 0m0.040s

and after this patch:

real 0m1.361s
user 0m1.315s
sys 0m0.040s

The basic idea here is simple: instead of always iterating over all
the symbol names in the index, we build an accelerator/sorted name
table and binary search names in it.

Later in the series, we'll want to support wild matching for C++ too,
so this mechanism already considers that. For example, say that
you're looking up functions/methods named "func", no matter the
containing namespace/class. If we sorted the table by qualified name,
then we obviously wouldn't be able to find those symbols with a binary
search:

func
ns1::a::b::func
ns1::b::func
ns2::func

(function symbol names in .gdb_index have no parameter info, like psymbols)

To address that out, we put an entry for each name component in the
sorted table. something like this:

Table Entry Actual symbol
---------------------------------
func func

func ns1::a::b::func
b::func ns1::a::b::func
a::b::func ns1::a::b::func
ns1::a::b::func ns1::a::b::func

func ns1::b::func
b::func ns1::b::func
ns1::b::func ns1::b::func

func ns2::func
ns2::func ns2::func

Which sorted results in this:

Table Entry Actual symbol
---------------------------------
a::b::func ns1::a::b::func
b::func ns1::a::b::func
b::func ns1::b::func
func func
func ns1::a::b::func
func ns1::b::func
func ns2::func
ns1::a::b::func ns1::a::b::func
ns1::b::func ns1::b::func
ns2::func ns2::func

And we can binary search this.

Note that a binary search approach works for both completion and
regular lookup, while a name hashing approach only works for normal
symbol looking, since obviously "fun" and "func" have different
hashes.

At first I was a bit wary of these tables potentially growing GDB's
memory significantly. But I did an experiment that convinced it's not
a worry at all. I hacked gdb to count the total number of entries in
all the tables, attached that gdb to my system/Fedora's Firefox
(Fedora's debug packages uses .gdb_index), did "set max-completions
unlimited", and then hit "b [TAB]" to cause everything to expand.

That resulted in 1351355 name_components. Each entry takes 8 bytes,
so that's 10810840 bytes (ignoring std::vector overhead), or ~10.3 MB.
That's IMO too small to worry about, given GDB was using over 7400MB
total at that point. I.e., we're talking about 0.1% increase.

dw2_expand_symtabs_matching unit tests covering this will be added in
a follow up patch.

If the size of this table turns out to be a concern, I have an idea to
reduce the size of the table further at the expense of a bit more code
-- the vast majority of the name offsets are either 0 or fit in
8-bits:

total name_component = 1351355, of which,
name_component::name_offset instances need 0 bits = 679531
name_component::name_offset instances need 8 bits = 669526
name_component::name_offset instances need 16 bits = 2298
name_component::name_offset instances need 32 bits = 0
name_component::idx instances need 0 bits = 51
name_component::idx instances need 8 bits = 8361
name_component::idx instances need 16 bits = 280329
name_component::idx instances need 32 bits = 1062614

so we could have separate tables for 0 name_offset, 8-bit name_offset
and 32-bit name_offset. That'd give us roughly:

679531 * 0 + 669526 * 1 + 2298 * 4 + 1062614 * 4 = 4929174, or ~4.7MB

with only 8-bit and 32-bit tables, that'd be:

1349057 * 1 + 2298 * 4 + 4 * 1351355 = 6763669 bytes, or ~6.5MB.

I don't think we need to bother though.

I also timed:

$ time gdb --batch -q -p `pidof firefox`
$ time gdb --batch -q -p `pidof firefox` -ex "b main"
$ time gdb --batch -q -p `pidof firefox` -ex "set max-completion unlimited" -ex "complete b "

and compared before previous patch vs this patch, and I didn't see a
significant difference, seemingly because time to read debug info
dominates. The "complete b " variant of the test takes ~2min
currently... (I have a follow up series that speeds that up
somewhat.)

gdb/ChangeLog:
yyyy-mm-dd Pedro Alves <palves@redhat.com>

* dwarf2read.c (byte_swap, MAYBE_SWAP): Move higher up in file.
(struct name_component): New.
(mapped_index::name_components): New field.
(mapped_index::symbol_name_at): New method.
(create_addrmap_from_index): Call mapped_index ctor.
(dw2_map_matching_symbols): Add comment about name_components
table.
(dw2_expand_symtabs_matching): Factor part to...
(dw2_expand_symtabs_matching_symbol): ... this new function.
Build name components table, and lookup symbols in it before
calling the name matcher.
(dw2_expand_marked_cus): New, factored out from
dw2_expand_symtabs_matching.
(dwarf2_per_objfile_free): Call the mapped_index's dtor.
---
gdb/dwarf2read.c | 323 +++++++++++++++++++++++++++++++++++++++++++++++--------
1 file changed, 281 insertions(+), 42 deletions(-)

diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
index f523326..e955131 100644
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -178,6 +178,51 @@ DEF_VEC_I (offset_type);
GDB_INDEX_CU_SET_VALUE((cu_index), (value)); \
} while (0)

+#if WORDS_BIGENDIAN
+
+/* Convert VALUE between big- and little-endian. */
+static offset_type
+byte_swap (offset_type value)
+{
+ offset_type result;
+
+ result = (value & 0xff) << 24;
+ result |= (value & 0xff00) << 8;
+ result |= (value & 0xff0000) >> 8;
+ result |= (value & 0xff000000) >> 24;
+ return result;
+}
+
+#define MAYBE_SWAP(V) byte_swap (V)
+
+#else
+#define MAYBE_SWAP(V) (V)
+#endif /* WORDS_BIGENDIAN */
+
+/* An index into a (C++) symbol name component in a symbol name as
+ recorded in the mapped_index's symbol table. For each C++ symbol
+ in the symbol table, we record one entry for the start of each
+ component in the symbol in a table of name components, and then
+ sort the table, in order to be able to binary search symbol names,
+ ignoring leading namespaces, both completion and regular look up.
+ For example, for symbol "A::B::C", we'll have an entry that points
+ to "A::B::C", another that points to "B::C", and another for "C".
+ Note that function symbols in GDB index have no parameter
+ information, just the function/method names. You can convert a
+ name_component to a "const char *" using the
+ 'mapped_index::symbol_name_at(offset_type)' method. */
+struct name_component
+{
+ /* Offset in the symbol name where the component starts. Stored as
+ a (32-bit) offset instead of a pointer to save memory and improve
+ locality on 64-bit architectures. */
+ offset_type name_offset;
+
+ /* The symbol's index in the symbol and constant pool tables of a
+ mapped_index. */
+ offset_type idx;
+};
+
/* A description of the mapped index. The file format is described in
a comment by the code that writes the index. */
struct mapped_index
@@ -202,6 +247,15 @@ struct mapped_index

/* A pointer to the constant pool. */
const char *constant_pool;
+
+ /* The name_component table (a sorted vector). See name_component's
+ description above. */
+ std::vector<name_component> name_components;
+
+ /* Convenience method to get at the name of the symbol at IDX in the
+ symbol table. */
+ const char *symbol_name_at (offset_type idx) const
+ { return this->constant_pool + MAYBE_SWAP (this->symbol_table[idx]); }
};

typedef struct dwarf2_per_cu_data *dwarf2_per_cu_ptr;
@@ -2131,26 +2185,6 @@ line_header_eq_voidp (const void *item_lhs, const void *item_rhs)
}

\f
-#if WORDS_BIGENDIAN
-
-/* Convert VALUE between big- and little-endian. */
-static offset_type
-byte_swap (offset_type value)
-{
- offset_type result;
-
- result = (value & 0xff) << 24;
- result |= (value & 0xff00) << 8;
- result |= (value & 0xff0000) >> 8;
- result |= (value & 0xff000000) >> 24;
- return result;
-}
-
-#define MAYBE_SWAP(V) byte_swap (V)
-
-#else
-#define MAYBE_SWAP(V) (V)
-#endif /* WORDS_BIGENDIAN */

/* Read the given attribute value as an address, taking the attribute's
form into account. */
@@ -3390,6 +3424,7 @@ dwarf2_read_index (struct objfile *objfile)
create_addrmap_from_index (objfile, &local_map);

map = XOBNEW (&objfile->objfile_obstack, struct mapped_index);
+ map = new (map) mapped_index ();
*map = local_map;

dwarf2_per_objfile->index_table = map;
@@ -4016,7 +4051,11 @@ dw2_map_matching_symbols (struct objfile *objfile,

Since each language has its own symbol name matching algorithm,
and we don't know which language is the right one, we must match
- each symbol against all languages.
+ each symbol against all languages. This would be a potential
+ performance problem if it were not mitigated by the
+ mapped_index::name_components lookup table, which significantly
+ reduces the number of times we need to call into this matcher,
+ making it a non-issue.

- Symbol names in the index have no overload (parameter)
information. I.e., in C++, "foo(int)" and "foo(long)" both appear
@@ -4095,6 +4134,22 @@ gdb_index_symbol_name_matcher::matches (const char *symbol_name)
}

static void
+dw2_expand_marked_cus
+ (mapped_index &index, offset_type idx,
+ struct objfile *objfile,
+ gdb::function_view<expand_symtabs_file_matcher_ftype> file_matcher,
+ gdb::function_view<expand_symtabs_exp_notify_ftype> expansion_notify,
+ search_domain kind);
+
+static void
+dw2_expand_symtabs_matching_symbol
+ (mapped_index &index,
+ const lookup_name_info &lookup_name_in,
+ gdb::function_view<expand_symtabs_symbol_matcher_ftype> symbol_matcher,
+ enum search_domain kind,
+ gdb::function_view<void (offset_type)> on_match);
+
+static void
dw2_expand_symtabs_matching
(struct objfile *objfile,
gdb::function_view<expand_symtabs_file_matcher_ftype> file_matcher,
@@ -4105,14 +4160,12 @@ dw2_expand_symtabs_matching
{
int i;
offset_type iter;
- struct mapped_index *index;

dw2_setup (objfile);

/* index_table is NULL if OBJF_READNOW. */
if (!dwarf2_per_objfile->index_table)
return;
- index = dwarf2_per_objfile->index_table;

if (file_matcher != NULL)
{
@@ -4186,30 +4239,214 @@ dw2_expand_symtabs_matching
}
}

- gdb_index_symbol_name_matcher lookup_name_matcher (lookup_name);
+ mapped_index &index = *dwarf2_per_objfile->index_table;

- for (iter = 0; iter < index->symbol_table_slots; ++iter)
+ dw2_expand_symtabs_matching_symbol (index, lookup_name,
+ symbol_matcher,
+ kind, [&] (offset_type idx)
{
- offset_type idx = 2 * iter;
- const char *name;
- offset_type *vec, vec_len, vec_idx;
- int global_seen = 0;
+ dw2_expand_marked_cus (index, idx, objfile, file_matcher,
+ expansion_notify, kind);
+ });
+}

- QUIT;
+/* Helper for dw2_expand_symtabs_matching that works with a
+ mapped_index instead of the containing objfile. This is split to a
+ separate function in order to be able to unit test the
+ name_components matching using a mock mapped_index. For each
+ symbol name that matches, calls MATCH_CALLBACK, passing it the
+ symbol's index in the mapped_index symbol table. */

- if (index->symbol_table[idx] == 0 && index->symbol_table[idx + 1] == 0)
- continue;
+static void
+dw2_expand_symtabs_matching_symbol
+ (mapped_index &index,
+ const lookup_name_info &lookup_name,
+ gdb::function_view<expand_symtabs_symbol_matcher_ftype> symbol_matcher,
+ enum search_domain kind,
+ gdb::function_view<void (offset_type)> match_callback)
+{
+ gdb_index_symbol_name_matcher lookup_name_matcher
+ (lookup_name);
+
+ auto *name_cmp = case_sensitivity == case_sensitive_on ? strcmp : strcasecmp;
+
+ /* Build the symbol name component sorted vector, if we haven't yet.
+ The code below only knows how to break apart components of C++
+ symbol names (and other languages that use '::' as
+ namespace/module separator). If we add support for wild matching
+ to some language that uses some other operator (E.g., Ada, Go and
+ D use '.'), then we'll need to try splitting the symbol name
+ according to that language too. Note that Ada does support wild
+ matching, but doesn't currently support .gdb_index. */
+ if (index.name_components.empty ())
+ {
+ for (size_t iter = 0; iter < index.symbol_table_slots; ++iter)
+ {
+ offset_type idx = 2 * iter;
+
+ if (index.symbol_table[idx] == 0
+ && index.symbol_table[idx + 1] == 0)
+ continue;
+
+ const char *name = index.symbol_name_at (idx);
+
+ /* Add each name component to the name component table. */
+ unsigned int previous_len = 0;
+ for (unsigned int current_len = cp_find_first_component (name);
+ name[current_len] != '\0';
+ current_len += cp_find_first_component (name + current_len))
+ {
+ gdb_assert (name[current_len] == ':');
+ index.name_components.push_back ({previous_len, idx});
+ /* Skip the '::'. */
+ current_len += 2;
+ previous_len = current_len;
+ }
+ index.name_components.push_back ({previous_len, idx});
+ }
+
+ /* Sort name_comp elements by name. */
+ auto name_comp_compare = [&] (const name_component &left,
+ const name_component &right)
+ {
+ const char *left_qualified = index.symbol_name_at (left.idx);
+ const char *right_qualified = index.symbol_name_at (right.idx);
+
+ const char *left_name = left_qualified + left.name_offset;
+ const char *right_name = right_qualified + right.name_offset;
+
+ return name_cmp (left_name, right_name) < 0;
+ };
+
+ std::sort (index.name_components.begin (),
+ index.name_components.end (),
+ name_comp_compare);
+ }
+
+ const char *cplus
+ = lookup_name.cplus ().lookup_name ().c_str ();

- name = index->constant_pool + MAYBE_SWAP (index->symbol_table[idx]);
+ /* Comparison function object for lower_bound that matches against a
+ given symbol name. */
+ auto lookup_compare_lower = [&] (const name_component &elem,
+ const char *name)
+ {
+ const char *elem_qualified = index.symbol_name_at (elem.idx);
+ const char *elem_name = elem_qualified + elem.name_offset;
+ return name_cmp (elem_name, name) < 0;
+ };
+
+ /* Comparison function object for upper_bound that matches against a
+ given symbol name. */
+ auto lookup_compare_upper = [&] (const char *name,
+ const name_component &elem)
+ {
+ const char *elem_qualified = index.symbol_name_at (elem.idx);
+ const char *elem_name = elem_qualified + elem.name_offset;
+ return name_cmp (name, elem_name) < 0;
+ };
+
+ auto begin = index.name_components.begin ();
+ auto end = index.name_components.end ();
+
+ /* Find the lower bound. */
+ auto lower = [&] ()
+ {
+ if (lookup_name.completion_mode () && cplus[0] == '\0')
+ return begin;
+ else
+ return std::lower_bound (begin, end, cplus, lookup_compare_lower);
+ } ();

- if (!lookup_name_matcher.matches (name)
- || (symbol_matcher != NULL && !symbol_matcher (name)))
+ /* Find the upper bound. */
+ auto upper = [&] ()
+ {
+ if (lookup_name.completion_mode ())
+ {
+ /* The string frobbing below won't work if the string is
+ empty. We don't need it then, anyway -- if we're
+ completing an empty string, then we want to iterate over
+ the whole range. */
+ if (cplus[0] == '\0')
+ return end;
+
+ /* In completion mode, increment the last character because
+ we want UPPER to point past all symbols names that have
+ the same prefix. */
+ std::string after = cplus;
+
+ gdb_assert (after.back () != 0xff);
+ after.back ()++;
+
+ return std::upper_bound (lower, end, after.c_str (),
+ lookup_compare_upper);
+ }
+ else
+ return std::upper_bound (lower, end, cplus, lookup_compare_upper);
+ } ();
+
+ /* Now for each symbol name in range, check to see if we have a name
+ match, and if so, call the MATCH_CALLBACK callback. */
+
+ /* The same symbol may appear more than once in the range though.
+ E.g., if we're looking for symbols that complete "w", and we have
+ a symbol named "w1::w2", we'll find the two name components for
+ that same symbol in the range. To be sure we only call the
+ callback once per symbol, we first collect the symbol name
+ indexes that matched in a temporary vector and ignore
+ duplicates. */
+ std::vector<offset_type> matches;
+ matches.reserve (std::distance (lower, upper));
+
+ for (;lower != upper; ++lower)
+ {
+ const char *qualified = index.symbol_name_at (lower->idx);
+
+ if (!lookup_name_matcher.matches (qualified)
+ || (symbol_matcher != NULL && !symbol_matcher (qualified)))
continue;

- /* The name was matched, now expand corresponding CUs that were
- marked. */
- vec = (offset_type *) (index->constant_pool
- + MAYBE_SWAP (index->symbol_table[idx + 1]));
+ matches.push_back (lower->idx);
+ }
+
+ std::sort (matches.begin (), matches.end ());
+
+ /* Finally call the callback, once per match. */
+ ULONGEST prev = -1;
+ for (offset_type idx : matches)
+ {
+ if (prev != idx)
+ {
+ match_callback (idx);
+ prev = idx;
+ }
+ }
+
+ /* Above we use a type wider than idx's for 'prev', since 0 and
+ (offset_type)-1 are both possible values. */
+ static_assert (sizeof (prev) > sizeof (offset_type), "");
+}
+
+/* Helper for dw2_expand_matching symtabs. Called on each symbol
+ matched, to expand corresponding CUs that were marked. IDX is the
+ index of the symbol name that matched. */
+
+static void
+dw2_expand_marked_cus
+ (mapped_index &index, offset_type idx,
+ struct objfile *objfile,
+ gdb::function_view<expand_symtabs_file_matcher_ftype> file_matcher,
+ gdb::function_view<expand_symtabs_exp_notify_ftype> expansion_notify,
+ search_domain kind)
+{
+ const char *name;
+ offset_type *vec, vec_len, vec_idx;
+ bool global_seen = false;
+
+ /* XXX reindent code below before pushing. */
+
+ vec = (offset_type *) (index.constant_pool
+ + MAYBE_SWAP (index.symbol_table[idx + 1]));
vec_len = MAYBE_SWAP (vec[0]);
for (vec_idx = 0; vec_idx < vec_len; ++vec_idx)
{
@@ -4225,7 +4462,7 @@ dw2_expand_symtabs_matching
and indices >= 7 may elide them for certain symbols
(gold does this). */
int attrs_valid =
- (index->version >= 7
+ (index.version >= 7
&& symbol_kind != GDB_INDEX_SYMBOL_KIND_NONE);

/* Work around gold/15646. */
@@ -4234,7 +4471,7 @@ dw2_expand_symtabs_matching
if (!is_static && global_seen)
continue;
if (!is_static)
- global_seen = 1;
+ global_seen = true;
}

/* Only check the symbol's kind if it has one. */
@@ -4285,7 +4522,6 @@ dw2_expand_symtabs_matching
}
}
}
- }
}

/* A helper for dw2_find_pc_sect_compunit_symtab which finds the most specific
@@ -23285,6 +23521,9 @@ dwarf2_per_objfile_free (struct objfile *objfile, void *d)

if (data->dwz_file && data->dwz_file->dwz_bfd)
gdb_bfd_unref (data->dwz_file->dwz_bfd);
+
+ if (data->index_table != NULL)
+ data->index_table->~mapped_index ();
}

\f
--
2.5.5