[PATCH v2 42/42] Share DWARF partial symtabs

Simon Marchi
2020-05-12T21:19:13Z

From: Tom Tromey <tom@tromey.com>

This changes the DWARF reader to share partial symtabs (or indices if
they are available) across objfiles. This has a few parts.

* If multiple objfiles backed by the same BFD can share partial symtabs
(see below), a single dwarf2_per_bfd is created. It is stored in the
per-bfd `dwarf2_per_bfd_bfd_data_key` registry. Multiple
dwarf2_per_objfile objects will point to the same instance. The
lifetime of these dwarf2_per_bfd objects is naturally handled. When
all the objfiles using the BFD are destroyed, the BFD's refount drops
to 0, which triggers the removal of the corresponding dwarf2_per_bfd
object from the registry and its destruction.

* If multiple objfiles backed by the same BFD can't share partial
symtabs (see below), one dwarf2_per_bfd object is created for each
objfile. Each dwarf2_per_objfile will point to their own instance of
dwarf2_per_bfd. These instances of dwarf2_per_bfd are kept in a
per-objfile registry, meaning that when the objfile gets destroyed,
the dwarf2_per_bfd instance gets destroyed as well.

* objfile::partial_symtabs is changed to be a shared_ptr again. This
lets us stash a second reference in dwarf2_per_bfd; if the DWARF
data is being shared, we can simply copy this value to the new
objfile.

* Two dwarf2_per_objfile objects backed by the same BFD may share a
dwarf2_per_bfd instance if:

* No other symbol reader has found symbols, and
* No BFD section rqeuires relocation

YYYY-MM-DD Tom Tromey <tom@tromey.com>
YYYY-MM-DD Simon Marchi <simon.marchi@efficios.com>

* objfiles.h (struct objfile) <partial_symtabs>: Now a
shared_ptr.
* dwarf2/read.h (struct dwarf2_per_objfile) <partial_symtabs>: New
member.
* dwarf2/read.c (dwarf2_per_bfd_bfd_data_key,
dwarf2_per_bfd_objfile_data_key>: New globals.
(dwarf2_has_info): Use shared dwarf2_per_bfd if possible.
(dwarf2_get_section_info): Use get_dwarf2_per_objfile.
(dwarf2_initialize_objfile): Consider cases where per_bfd can be
shared.
(dwarf2_build_psymtabs): Set objfile::partial_symtabs and
short-circuit when sharing.
(dwarf2_build_psymtabs): Set dwarf2_per_objfile::partial_symtabs.
(dwarf2_psymtab::expand_psymtab): Use free_cached_comp_units.
---
gdb/dwarf2/read.c | 105 +++++++++++++++++++++++++++++++++++++++-------
gdb/dwarf2/read.h | 5 +++
gdb/objfiles.h | 2 +-
3 files changed, 97 insertions(+), 15 deletions(-)

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index 3f93a51381f..2de4ead004f 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -105,7 +105,19 @@ static bool check_physname = false;
/* When true, do not reject deprecated .gdb_index sections. */
static bool use_deprecated_index_sections = false;

-static const struct objfile_key<dwarf2_per_objfile> dwarf2_objfile_data_key;
+/* This is used to store the data that is always per objfile. */
+static const objfile_key<dwarf2_per_objfile> dwarf2_objfile_data_key;
+
+/* These are used to store the dwarf2_per_bfd objects.
+
+ objfiles having the same BFD, which doesn't require relocations, are going to
+ share a dwarf2_per_bfd object, which is held in the _bfd_data_key version.
+
+ Other objfiles are not going to share a dwarf2_per_bfd with any other
+ objfiles, so they'll have their own version kept in the _objfile_data_key
+ version. */
+static const struct bfd_key<dwarf2_per_bfd> dwarf2_per_bfd_bfd_data_key;
+static const struct objfile_key<dwarf2_per_bfd> dwarf2_per_bfd_objfile_data_key;

/* The "aclass" indices for various kinds of computed DWARF symbols. */

@@ -1853,9 +1865,30 @@ dwarf2_has_info (struct objfile *objfile,

if (dwarf2_per_objfile == NULL)
{
- /* For now, each dwarf2_per_objfile owns its own dwarf2_per_bfd (no
- sharing yet). */
- dwarf2_per_bfd *per_bfd = new dwarf2_per_bfd (objfile->obfd, names, can_copy);
+ dwarf2_per_bfd *per_bfd;
+
+ /* We can share a "dwarf2_per_bfd" with other objfiles if the BFD
+ doesn't require relocations and if there aren't partial symbols
+ from some other reader. */
+ if (!objfile_has_partial_symbols (objfile)
+ && !gdb_bfd_requires_relocations (objfile->obfd))
+ {
+ /* See if one has been created for this BFD yet. */
+ per_bfd = dwarf2_per_bfd_bfd_data_key.get (objfile->obfd);
+
+ if (per_bfd == nullptr)
+ {
+ /* No, create it now. */
+ per_bfd = new dwarf2_per_bfd (objfile->obfd, names, can_copy);
+ dwarf2_per_bfd_bfd_data_key.set (objfile->obfd, per_bfd);
+ }
+ }
+ else
+ {
+ /* No sharing possible, create one specifically for this objfile. */
+ per_bfd = new dwarf2_per_bfd (objfile->obfd, names, can_copy);
+ dwarf2_per_bfd_objfile_data_key.set (objfile, per_bfd);
+ }

dwarf2_per_objfile = dwarf2_objfile_data_key.emplace (objfile, objfile, per_bfd);
}
@@ -2017,7 +2050,7 @@ dwarf2_get_section_info (struct objfile *objfile,
asection **sectp, const gdb_byte **bufp,
bfd_size_type *sizep)
{
- struct dwarf2_per_objfile *data = dwarf2_objfile_data_key.get (objfile);
+ struct dwarf2_per_objfile *data = get_dwarf2_per_objfile (objfile);
struct dwarf2_section_info *info;

/* We may see an objfile without any DWARF, in which case we just
@@ -5874,6 +5907,7 @@ dwarf2_initialize_objfile (struct objfile *objfile, dw_index_kind *index_kind)
{
struct dwarf2_per_objfile *dwarf2_per_objfile
= get_dwarf2_per_objfile (objfile);
+ dwarf2_per_bfd *per_bfd = dwarf2_per_objfile->per_bfd;

/* If we're about to read full symbols, don't bother with the
indices. In this case we also don't care if some other debug
@@ -5881,20 +5915,28 @@ dwarf2_initialize_objfile (struct objfile *objfile, dw_index_kind *index_kind)
expanded anyway. */
if ((objfile->flags & OBJF_READNOW))
{
- dwarf2_per_objfile->per_bfd->using_index = 1;
+ /* When using READNOW, the using_index flag (set below) indicates that
+ PER_BFD was already initialized, when we loaded some other objfile. */
+ if (per_bfd->using_index)
+ {
+ *index_kind = dw_index_kind::GDB_INDEX;
+ dwarf2_per_objfile->resize_symtabs ();
+ return true;
+ }
+
+ per_bfd->using_index = 1;
create_all_comp_units (dwarf2_per_objfile);
create_all_type_units (dwarf2_per_objfile);
- dwarf2_per_objfile->per_bfd->quick_file_names_table
- = create_quick_file_names_table
- (dwarf2_per_objfile->per_bfd->all_comp_units.size ());
+ per_bfd->quick_file_names_table
+ = create_quick_file_names_table (per_bfd->all_comp_units.size ());
dwarf2_per_objfile->resize_symtabs ();

- for (int i = 0; i < (dwarf2_per_objfile->per_bfd->all_comp_units.size ()
- + dwarf2_per_objfile->per_bfd->all_type_units.size ()); ++i)
+ for (int i = 0; i < (per_bfd->all_comp_units.size ()
+ + per_bfd->all_type_units.size ()); ++i)
{
- dwarf2_per_cu_data *per_cu = dwarf2_per_objfile->per_bfd->get_cutu (i);
+ dwarf2_per_cu_data *per_cu = per_bfd->get_cutu (i);

- per_cu->v.quick = OBSTACK_ZALLOC (&dwarf2_per_objfile->per_bfd->obstack,
+ per_cu->v.quick = OBSTACK_ZALLOC (&per_bfd->obstack,
struct dwarf2_per_cu_quick_data);
}

@@ -5905,6 +5947,24 @@ dwarf2_initialize_objfile (struct objfile *objfile, dw_index_kind *index_kind)
return true;
}

+ /* Was a debug names index already read when we processed an objfile sharing
+ PER_BFD? */
+ if (per_bfd->debug_names_table != nullptr)
+ {
+ *index_kind = dw_index_kind::DEBUG_NAMES;
+ dwarf2_per_objfile->resize_symtabs ();
+ return true;
+ }
+
+ /* Was a GDB index already read when we processed an objfile sharing
+ PER_BFD? */
+ if (per_bfd->index_table != nullptr)
+ {
+ *index_kind = dw_index_kind::GDB_INDEX;
+ dwarf2_per_objfile->resize_symtabs ();
+ return true;
+ }
+
if (dwarf2_read_debug_names (dwarf2_per_objfile))
{
*index_kind = dw_index_kind::DEBUG_NAMES;
@@ -5945,6 +6005,16 @@ dwarf2_build_psymtabs (struct objfile *objfile)
{
struct dwarf2_per_objfile *dwarf2_per_objfile
= get_dwarf2_per_objfile (objfile);
+ dwarf2_per_bfd *per_bfd = dwarf2_per_objfile->per_bfd;
+
+ if (per_bfd->partial_symtabs != nullptr)
+ {
+ /* Partial symbols were already read, so now we can simply
+ attach them. */
+ objfile->partial_symtabs = per_bfd->partial_symtabs;
+ dwarf2_per_objfile->resize_symtabs ();
+ return;
+ }

init_psymbol_list (objfile, 1024);

@@ -5966,6 +6036,12 @@ dwarf2_build_psymtabs (struct objfile *objfile)
{
exception_print (gdb_stderr, except);
}
+
+ /* Finish by setting the local reference to partial symtabs, so that
+ we don't try to read them again if reading another objfile with the same
+ BFD. If we can't in fact share, this won't make a difference anyway as
+ the dwarf2_per_bfd object won't be shared. */
+ per_bfd->partial_symtabs = objfile->partial_symtabs;
}

/* Find the base address of the compilation unit for range lists and
@@ -9014,9 +9090,10 @@ dwarf2_psymtab::expand_psymtab (struct objfile *objfile)
{
gdb_assert (!readin_p (objfile));

+ dwarf2_per_objfile *per_objfile = get_dwarf2_per_objfile (objfile);
+ free_cached_comp_units freer (per_objfile);
expand_dependencies (objfile);

- dwarf2_per_objfile *per_objfile = get_dwarf2_per_objfile (objfile);
dw2_do_instantiate_symtab (per_cu_data, per_objfile, false);
gdb_assert (get_compunit_symtab (objfile) != nullptr);
}
diff --git a/gdb/dwarf2/read.h b/gdb/dwarf2/read.h
index 996cf55af22..53f0c7361a3 100644
--- a/gdb/dwarf2/read.h
+++ b/gdb/dwarf2/read.h
@@ -254,6 +254,11 @@ struct dwarf2_per_bfd
/* CUs that are queued to be read. */
std::queue<dwarf2_queue_item> queue;

+ /* We keep a separate reference to the partial symtabs, in case we
+ are sharing them between objfiles. This is only set after
+ partial symbols have been read the first time. */
+ std::shared_ptr<psymtab_storage> partial_symtabs;
+
private:

/* The total number of per_cu and signatured_type objects that have
diff --git a/gdb/objfiles.h b/gdb/objfiles.h
index 0b47bd0c1e2..56ff52119dc 100644
--- a/gdb/objfiles.h
+++ b/gdb/objfiles.h
@@ -576,7 +576,7 @@ struct objfile

/* The partial symbol tables. */

- std::unique_ptr<psymtab_storage> partial_symtabs;
+ std::shared_ptr<psymtab_storage> partial_symtabs;

/* The object file's BFD. Can be null if the objfile contains only
minimal symbols, e.g. the run time common symbols for SunOS4. */
--
2.26.2