Re: [RFA 2/6] Allocate abbrev_table with new

Simon Marchi
2018-01-07T05:29:10Z

On 2018-01-05 07:26 PM, Tom Tromey wrote:
> This changes dwarf2read.c to allocate abbrev tables using "new", and
> then updates the users.
>
> This is somewhat complicated because ownership rules for abbrev tables
> are obscure and involve a more than usual amount of cleanup
> manipulation.

Hi Tom,

After staring at this code for longer than I had planned, I came to the
conclusion that it would be much simpler if dwarf2_cu did not contain a
pointer to the abbrev_table. In fact, I think that dwarf2_cu::abbrev_table
is actually a disguised function parameter that should be passed alongside
the cu, not in it.

This is made clear by the fact that we are setting cu->abbrev_table just
before calling the DIE-reading function, and reset it just after (with a
cleanup). The dwarf2_cu::abbrev_table is only useful for the duration of
a function call, and then reset.

The dwarf2_cu structure never actually owns the abbrev_table, it's always
some frame in the call stack that does. But they own it indirectly via
dwarf2_cu::abbrev_table and some cleanup that will free the table and
reset that field (dwarf2_free_abbrev_table, or auto_free_abbrev_table with
your patch).

The parameters for the DIE-reading operations are passed through the
die_reader_specs structure, so I think it would make sense to put the
reference to the abbrev_table there (this structure really exists to
avoid having super long parameter lists). This way, each frame that
reads in an abbrev_table owns it through a unique_ptr and passes it to
DIE-reading functions through parameter (through die_reader_specs). That
seems more straightforward.

Here's a patch that applies over this one, that illustrates the idea. Let
me know what you think.

Simon

From 60ec3e20ef44be977ea402d8dfb7050317857c39 Mon Sep 17 00:00:00 2001
From: Simon Marchi <simon.marchi@ericsson.com>
Date: Sat, 6 Jan 2018 22:44:29 -0500
Subject: [PATCH] Suggestion

---
gdb/dwarf2read.c | 188 +++++++++++++++++--------------------------------------
1 file changed, 59 insertions(+), 129 deletions(-)

diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
index 7fd68c5..fb19de8 100644
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -668,11 +668,6 @@ struct dwarf2_cu
distinguish these in buildsym.c. */
struct pending **list_in_scope;

- /* The abbrev table for this CU.
- Normally this points to the abbrev table in the objfile.
- But if DWO_UNIT is non-NULL this is the abbrev table in the DWO file. */
- struct abbrev_table *abbrev_table;
-
/* Hash table holding all the loaded partial DIEs
with partial_die->offset.SECT_OFF as hash. */
htab_t partial_dies;
@@ -774,28 +769,6 @@ struct dwarf2_cu
unsigned int processing_has_namespace_info : 1;
};

-static void dwarf2_free_abbrev_table (struct dwarf2_cu *);
-
-/* Free an abbrev table on destruction. */
-
-class auto_free_abbrev_table
-{
-public:
- auto_free_abbrev_table (struct dwarf2_cu *cu)
- : m_cu (cu)
- {
- }
-
- ~auto_free_abbrev_table ()
- {
- dwarf2_free_abbrev_table (m_cu);
- }
-
-private:
-
- struct dwarf2_cu *m_cu;
-};
-
/* Persistent data held for a compilation unit, even when not
processing it. We put a pointer to this structure in the
read_symtab_private field of the psymtab. */
@@ -1263,6 +1236,9 @@ struct die_reader_specs

/* The value of the DW_AT_comp_dir attribute. */
const char *comp_dir;
+
+ /* The abbreviation table to use when reading the DIEs. */
+ struct abbrev_table *abbrev_table;
};

/* Type of function passed to init_cutu_and_read_dies, et.al. */
@@ -1556,6 +1532,8 @@ struct abbrev_table
struct abbrev_info **abbrevs;
};

+typedef std::unique_ptr<struct abbrev_table> abbrev_table_up;
+
/* Attributes have a name and a value. */
struct attribute
{
@@ -1784,12 +1762,9 @@ static void dwarf2_read_symtab (struct partial_symtab *,

static void psymtab_to_symtab_1 (struct partial_symtab *);

-static std::unique_ptr<struct abbrev_table> abbrev_table_read_table
+static abbrev_table_up abbrev_table_read_table
(struct dwarf2_section_info *, sect_offset);

-static void dwarf2_read_abbrevs (struct dwarf2_cu *,
- struct dwarf2_section_info *);
-
static unsigned int peek_abbrev_code (bfd *, const gdb_byte *);

static struct partial_die_info *load_partial_dies
@@ -7377,7 +7352,8 @@ static void
init_cu_die_reader (struct die_reader_specs *reader,
struct dwarf2_cu *cu,
struct dwarf2_section_info *section,
- struct dwo_file *dwo_file)
+ struct dwo_file *dwo_file,
+ struct abbrev_table *abbrev_table)
{
gdb_assert (section->readin && section->buffer != NULL);
reader->abfd = get_section_bfd_owner (section);
@@ -7387,6 +7363,7 @@ init_cu_die_reader (struct die_reader_specs *reader,
reader->buffer = section->buffer;
reader->buffer_end = section->buffer + section->size;
reader->comp_dir = NULL;
+ reader->abbrev_table = abbrev_table;
}

/* Subroutine of init_cutu_and_read_dies to simplify it.
@@ -7402,26 +7379,25 @@ init_cu_die_reader (struct die_reader_specs *reader,
STUB_COMP_DIR may be non-NULL.
*RESULT_READER,*RESULT_INFO_PTR,*RESULT_COMP_UNIT_DIE,*RESULT_HAS_CHILDREN
are filled in with the info of the DIE from the DWO file.
- ABBREV_TABLE_PROVIDED is non-zero if the caller of init_cutu_and_read_dies
- provided an abbrev table to use.
- *ABBREV_TABLE may be filled in with a new abbrev table.
+ *RESULT_DWO_ABBREV_TABLE will be filled in with the abbrev table allocated
+ from the dwo. Since *RESULT_READER references this abbrev table, it must be
+ kept around for at least as long as *RESULT_READER.
+
The result is non-zero if a valid (non-dummy) DIE was found. */

static int
read_cutu_die_from_dwo (struct dwarf2_per_cu_data *this_cu,
struct dwo_unit *dwo_unit,
- int abbrev_table_provided,
struct die_info *stub_comp_unit_die,
const char *stub_comp_dir,
struct die_reader_specs *result_reader,
const gdb_byte **result_info_ptr,
struct die_info **result_comp_unit_die,
int *result_has_children,
- gdb::optional<auto_free_abbrev_table> *abbrev_table)
+ abbrev_table_up *result_dwo_abbrev_table)
{
struct objfile *objfile = dwarf2_per_objfile->objfile;
struct dwarf2_cu *cu = this_cu->cu;
- struct dwarf2_section_info *section;
bfd *abfd;
const gdb_byte *begin_info_ptr, *info_ptr;
struct attribute *comp_dir, *stmt_list, *low_pc, *high_pc, *ranges;
@@ -7484,13 +7460,12 @@ read_cutu_die_from_dwo (struct dwarf2_per_cu_data *this_cu,

/* Set up for reading the DWO CU/TU. */
cu->dwo_unit = dwo_unit;
- section = dwo_unit->section;
+ dwarf2_section_info *section = dwo_unit->section;
dwarf2_read_section (objfile, section);
abfd = get_section_bfd_owner (section);
begin_info_ptr = info_ptr = (section->buffer
+ to_underlying (dwo_unit->sect_off));
dwo_abbrev_section = &dwo_unit->dwo_file->sections.abbrev;
- init_cu_die_reader (result_reader, cu, section, dwo_unit->dwo_file);

if (this_cu->is_debug_types)
{
@@ -7531,22 +7506,10 @@ read_cutu_die_from_dwo (struct dwarf2_per_cu_data *this_cu,
dwo_unit->length = get_cu_length (&cu->header);
}

- /* Replace the CU's original abbrev table with the DWO's.
- Reminder: We can't read the abbrev table until we've read the header. */
- if (abbrev_table_provided)
- {
- /* Don't free the provided abbrev table, the caller of
- init_cutu_and_read_dies owns it. */
- dwarf2_read_abbrevs (cu, dwo_abbrev_section);
- /* Ensure the DWO abbrev table gets freed. */
- abbrev_table->emplace (cu);
- }
- else
- {
- dwarf2_free_abbrev_table (cu);
- dwarf2_read_abbrevs (cu, dwo_abbrev_section);
- /* Leave any existing abbrev table cleanup as is. */
- }
+ *result_dwo_abbrev_table
+ = abbrev_table_read_table (dwo_abbrev_section, cu->header.abbrev_sect_off);
+ init_cu_die_reader (result_reader, cu, section, dwo_unit->dwo_file,
+ result_dwo_abbrev_table->get ());

/* Read in the die, but leave space to copy over the attributes
from the stub. This has the benefit of simplifying the rest of
@@ -7699,14 +7662,16 @@ init_tu_and_read_dwo_dies (struct dwarf2_per_cu_data *this_cu,
abbrev table. When reading DWOs with skeletonless TUs, all the TUs
could share abbrev tables. */

- gdb::optional<auto_free_abbrev_table> abbrev_table;
+ /* The abbreviation table used by READER, this must live at least as long as
+ READER. */
+ abbrev_table_up dwo_abbrev_table;
+
if (read_cutu_die_from_dwo (this_cu, sig_type->dwo_unit,
- 0 /* abbrev_table_provided */,
NULL /* stub_comp_unit_die */,
sig_type->dwo_unit->dwo_file->comp_dir,
&reader, &info_ptr,
&comp_unit_die, &has_children,
- &abbrev_table) == 0)
+ &dwo_abbrev_table) == 0)
{
/* Dummy die. */
do_cleanups (cleanups);
@@ -7885,37 +7850,31 @@ init_cutu_and_read_dies (struct dwarf2_per_cu_data *this_cu,

/* If we don't have them yet, read the abbrevs for this compilation unit.
And if we need to read them now, make sure they're freed when we're
- done. Note that it's important that if the CU had an abbrev table
- on entry we don't free it when we're done: Somewhere up the call stack
- it may be in use. */
- gdb::optional<auto_free_abbrev_table> abbrev_table_freer;
+ done (own the table through ABBREV_TABLE_HOLDER). */
+ abbrev_table_up abbrev_table_holder;
if (abbrev_table != NULL)
+ gdb_assert (cu->header.abbrev_sect_off == abbrev_table->sect_off);
+ else
{
- gdb_assert (cu->abbrev_table == NULL);
- gdb_assert (cu->header.abbrev_sect_off == abbrev_table->sect_off);
- cu->abbrev_table = abbrev_table;
- }
- else if (cu->abbrev_table == NULL)
- {
- dwarf2_read_abbrevs (cu, abbrev_section);
- abbrev_table_freer.emplace (cu);
- }
- else if (rereading_dwo_cu)
- {
- dwarf2_free_abbrev_table (cu);
- dwarf2_read_abbrevs (cu, abbrev_section);
+ abbrev_table_holder
+ = abbrev_table_read_table (abbrev_section, cu->header.abbrev_sect_off);
+ abbrev_table = abbrev_table_holder.get ();
}

/* Read the top level CU/TU die. */
- init_cu_die_reader (&reader, cu, section, NULL);
+ init_cu_die_reader (&reader, cu, section, NULL, abbrev_table);
info_ptr = read_full_die (&reader, &comp_unit_die, info_ptr, &has_children);

/* If we are in a DWO stub, process it and then read in the "real" CU/TU
- from the DWO file.
+ from the DWO file. read_cutu_die_from_dwo will allocate the abbreviation
+ table from the DWO file and pass the ownership over to us. It will be
+ referenced from READER, so we must make sure to free it after we're done
+ with READER.
+
Note that if USE_EXISTING_OK != 0, and THIS_CU->cu already contains a
DWO CU, that this test will fail (the attribute will not be present). */
attr = dwarf2_attr (comp_unit_die, DW_AT_GNU_dwo_name, cu);
- gdb::optional<auto_free_abbrev_table> abbrev_table_freer_2;
+ abbrev_table_up dwo_abbrev_table;
if (attr)
{
struct dwo_unit *dwo_unit;
@@ -7932,11 +7891,10 @@ init_cutu_and_read_dies (struct dwarf2_per_cu_data *this_cu,
if (dwo_unit != NULL)
{
if (read_cutu_die_from_dwo (this_cu, dwo_unit,
- abbrev_table != NULL,
comp_unit_die, NULL,
&reader, &info_ptr,
&dwo_comp_unit_die, &has_children,
- &abbrev_table_freer_2) == 0)
+ &dwo_abbrev_table) == 0)
{
/* Dummy die. */
do_cleanups (cleanups);
@@ -8045,10 +8003,10 @@ init_cutu_and_read_dies_no_follow (struct dwarf2_per_cu_data *this_cu,
return;
}

- dwarf2_read_abbrevs (&cu, abbrev_section);
- auto_free_abbrev_table abbrev_table_freer (&cu);
+ abbrev_table_up abbrev_table
+ = abbrev_table_read_table (abbrev_section, cu.header.abbrev_sect_off);

- init_cu_die_reader (&reader, &cu, section, dwo_file);
+ init_cu_die_reader (&reader, &cu, section, dwo_file, abbrev_table.get ());
info_ptr = read_full_die (&reader, &comp_unit_die, info_ptr, &has_children);

die_reader_func (&reader, info_ptr, comp_unit_die, has_children, data);
@@ -8518,7 +8476,7 @@ build_type_psymtabs_1 (void)
{
struct tu_stats *tu_stats = &dwarf2_per_objfile->tu_stats;
struct cleanup *cleanups;
- std::unique_ptr<struct abbrev_table> abbrev_table;
+ abbrev_table_up abbrev_table;
sect_offset abbrev_offset;
struct tu_abbrev_offset *sorted_by_abbrev;
int i;
@@ -9522,25 +9480,26 @@ peek_abbrev_code (bfd *abfd, const gdb_byte *info_ptr)
return read_unsigned_leb128 (abfd, info_ptr, &bytes_read);
}

-/* Read the initial uleb128 in the die at INFO_PTR in compilation unit CU.
+/* Read the initial uleb128 in the die at INFO_PTR in compilation unit
+ READER::CU. Use READER::ABBREV_TABLE to lookup any abbreviation.
+
Return the corresponding abbrev, or NULL if the number is zero (indicating
an empty DIE). In either case *BYTES_READ will be set to the length of
the initial number. */

static struct abbrev_info *
-peek_die_abbrev (const gdb_byte *info_ptr, unsigned int *bytes_read,
- struct dwarf2_cu *cu)
+peek_die_abbrev (const die_reader_specs &reader,
+ const gdb_byte *info_ptr, unsigned int *bytes_read)
{
+ dwarf2_cu *cu = reader.cu;
bfd *abfd = cu->objfile->obfd;
- unsigned int abbrev_number;
- struct abbrev_info *abbrev;
-
- abbrev_number = read_unsigned_leb128 (abfd, info_ptr, bytes_read);
+ unsigned int abbrev_number
+ = read_unsigned_leb128 (abfd, info_ptr, bytes_read);

if (abbrev_number == 0)
return NULL;

- abbrev = cu->abbrev_table->lookup_abbrev (abbrev_number);
+ abbrev_info *abbrev = reader.abbrev_table->lookup_abbrev (abbrev_number);
if (!abbrev)
{
error (_("Dwarf Error: Could not find abbrev number %d in %s"
@@ -9559,13 +9518,11 @@ peek_die_abbrev (const gdb_byte *info_ptr, unsigned int *bytes_read,
static const gdb_byte *
skip_children (const struct die_reader_specs *reader, const gdb_byte *info_ptr)
{
- struct dwarf2_cu *cu = reader->cu;
- struct abbrev_info *abbrev;
- unsigned int bytes_read;
-
while (1)
{
- abbrev = peek_die_abbrev (info_ptr, &bytes_read, cu);
+ unsigned int bytes_read;
+ abbrev_info *abbrev = peek_die_abbrev (*reader, info_ptr, &bytes_read);
+
if (abbrev == NULL)
return info_ptr + bytes_read;
else
@@ -17877,7 +17834,7 @@ read_full_die_1 (const struct die_reader_specs *reader,
return info_ptr;
}

- abbrev = cu->abbrev_table->lookup_abbrev (abbrev_number);
+ abbrev = reader->abbrev_table->lookup_abbrev (abbrev_number);
if (!abbrev)
error (_("Dwarf Error: could not find abbrev number %d [in module %s]"),
abbrev_number,
@@ -17985,7 +17942,7 @@ abbrev_table::lookup_abbrev (unsigned int abbrev_number)

/* Read in an abbrev table. */

-static std::unique_ptr<struct abbrev_table>
+static abbrev_table_up
abbrev_table_read_table (struct dwarf2_section_info *section,
sect_offset sect_off)
{
@@ -17998,8 +17955,7 @@ abbrev_table_read_table (struct dwarf2_section_info *section,
struct attr_abbrev *cur_attrs;
unsigned int allocated_attrs;

- std::unique_ptr<struct abbrev_table> abbrev_table
- (new struct abbrev_table (sect_off));
+ abbrev_table_up abbrev_table (new struct abbrev_table (sect_off));

dwarf2_read_section (objfile, section);
abbrev_ptr = section->buffer + to_underlying (sect_off);
@@ -18088,31 +18044,6 @@ abbrev_table_read_table (struct dwarf2_section_info *section,
return abbrev_table;
}

-/* Read the abbrev table for CU from ABBREV_SECTION. */
-
-static void
-dwarf2_read_abbrevs (struct dwarf2_cu *cu,
- struct dwarf2_section_info *abbrev_section)
-{
- cu->abbrev_table
- = (abbrev_table_read_table (abbrev_section, cu->header.abbrev_sect_off)
- .release ());
-}
-
-/* Release the memory used by the abbrev table for a compilation unit. */
-
-static void
-dwarf2_free_abbrev_table (struct dwarf2_cu *cu)
-{
- if (cu->abbrev_table != NULL)
- {
- delete cu->abbrev_table;
- /* Set this to NULL so that we SEGV if we try to read it later,
- and also because free_comp_unit verifies this is NULL. */
- cu->abbrev_table = NULL;
- }
-}
-\f
/* Returns nonzero if TAG represents a type that we might generate a partial
symbol for. */

@@ -18155,7 +18086,6 @@ load_partial_dies (const struct die_reader_specs *reader,
struct objfile *objfile = cu->objfile;
struct partial_die_info *part_die;
struct partial_die_info *parent_die, *lasÃ©t_die, *first_die = NULL;
- struct abbrev_info *abbrev;
unsigned int bytes_read;
unsigned int load_all = 0;
int nesting_level = 1;
@@ -18180,7 +18110,7 @@ load_partial_dies (const struct die_reader_specs *reader,

while (1)
{
- abbrev = peek_die_abbrev (info_ptr, &bytes_read, cu);
+ abbrev_info *abbrev = peek_die_abbrev (*reader, info_ptr, &bytes_read);

/* A NULL abbrev means the end of a series of children. */
if (abbrev == NULL)
--
2.7.4