In some cases we've been replacing heap-allocated gdb_byte buffers
managed with xmalloc/make_cleanup(xfree) with gdb::vector<gdb_byte>.
That usually pessimizes the code a little bit because std::vector
value-initializes elements (which for gdb_byte means
zero-initialization), while if you're creating a temporary buffer,
you're most certaintly going to fill it in with some data. An
alternative is to use
unique_ptr<gdb_byte[]> buf (new gdb_byte[size]);
but it looks like that's not very popular.
Recently, a use of obstacks in dwarf2read.c was replaced with
std::vector<gdb_byte> and that as well introduced a pessimization for
always memsetting the buffer when it's garanteed that the zeros will
be overwritten immediately. (see dwarf2read.c change in this patch to
find it.)
So here's a different take at addressing this issue "by design":
#1 - Introduce default_init_allocator<T>
I.e., a custom allocator that does default construction using default
initialization, meaning, no more zero initialization. That's the
default_init_allocation<T> class added in this patch.
See "Notes" at
<http://en.cppreference.com/w/cpp/container/vector/resize>.
#2 - Introduce def_vector<T>
I.e., a convenience typedef, because typing the allocator is annoying:
using def_vector<T> = std::vector<T, gdb::default_init_allocator<T>>;
#3 - Introduce byte_vector
Because gdb_byte vectors will be the common thing, add a convenience
"byte_vector" typedef:
using byte_vector = def_vector<gdb_byte>;
which is really the same as:
std::vector<gdb_byte, gdb::default_init_allocator<gdb_byte>>;
The intent then is to make "gdb::byte_vector" be the go-to for dynamic
byte buffers. So the less friction, the better.
#4 - Adjust current code to use it.
To set the example going forward. Replace std::vector uses and also
unique_ptr<byte[]> uses.
One nice thing is that with this allocator, for changes like these:
-std::unique_ptr<byte[]> buf (new gdb_byte[some_size]);
+gdb::byte_vector buf (some_size);
fill_with_data (buf.data (), buf.size ());
the generated code is the same as before. I.e., the compiler
de-structures the vector and gets rid of the unused "reserved vs size"
related fields.
The other nice thing is that it's easier to write
gdb::byte_vector buf (size);
than
std::unique_ptr<gdb_byte[]> buf (new gdb_byte[size]);
or even (C++14):
auto buf = std::make_unique<gdb_byte[]> (size); // zero-initializes...
#5 - Suggest s/std::vector<gdb_byte>/gdb::byte_vector/ going forward.
Note that this commit actually fixes a couple of bugs where the current
code is incorrectly using "std::vector::reserve(new_size)" and then
accessing the vector's internal buffer beyond the vector's size: see
dwarf2loc.c and charset.c. That's undefined behavior and may trigger
debug mode assertion failures. With default_init_allocator,
"resize()" behaves like "reserve()" performance wise, in that it
leaves new elements with unspecified values, but, it does that safely
without triggering undefined behavior when you access those values.
gdb/ChangeLog:
2017-06-14 Pedro Alves <palves@redhat.com>
* ada-lang.c: Include "common/byte-vector.h".
(ada_value_primitive_packed_val): Use gdb::byte_vector.
* charset.c (wchar_iterator::iterate): Resize the vector instead
of reserving it.
* common/byte-vector.h: Include "common/def-vector.h".
(wchar_iterator::m_out): Now a gdb::def_vector<gdb_wchar_t>.
* cli/cli-dump.c: Include "common/byte-vector.h".
(dump_memory_to_file, restore_binary_file): Use gdb::byte_vector.
* common/byte-vector.h: New file.
* common/def-vector.h: New file.
* common/default-init-alloc.h: New file.
* dwarf2loc.c: Include "common/byte-vector.h".
(rw_pieced_value): Use gdb::byte_vector, and resize the vector
instead of reserving it.
* dwarf2read.c: Include "common/byte-vector.h".
(data_buf::m_vec): Now a gdb::byte_vector.
* gdb_regex.c: Include "common/def-vector.h".
(compiled_regex::compiled_regex): Use gdb::def_vector<char>.
* mi/mi-main.c: Include "common/byte-vector.h".
(mi_cmd_data_read_memory): Use gdb::byte_vector.
* printcmd.c: Include "common/byte-vector.h".
(print_scalar_formatted): Use gdb::byte_vector.
* valprint.c: Include "common/byte-vector.h".
(maybe_negate_by_bytes, print_decimal_chars): Use
gdb::byte_vector.
This patch replaces compile_rx_or_error and make_regfree_cleanup with
a class that wraps a regex_t.
gdb/ChangeLog:
2017-06-07 Pedro Alves <palves@redhat.com>
* Makefile.in (SFILES): Add gdb_regex.c.
(COMMON_OBS): Add gdb_regex.o.
* ada-lang.c (ada_add_standard_exceptions)
(ada_add_exceptions_from_frame, name_matches_regex)
(ada_add_global_exceptions, ada_exceptions_list_1): Change regex
parameter type to compiled_regex. Adjust.
(ada_exceptions_list): Use compiled_regex.
* break-catch-throw.c (exception_catchpoint::pattern): Now a
std::unique_ptr<compiled_regex>.
(exception_catchpoint::~exception_catchpoint): Remove regfree
call.
(check_status_exception_catchpoint): Adjust to use compiled_regex.
(handle_gnu_v3_exceptions): Adjust to use compiled_regex.
* breakpoint.c (solib_catchpoint::compiled): Now a
std::unique_ptr<compiled_regex>.
(solib_catchpoint::~solib_catchpoint): Remove regfree call.
(check_status_catch_solib): Adjust to use compiled_regex.
(add_solib_catchpoint): Adjust to use compiled_regex.
* cli/cli-cmds.c (apropos_command): Use compiled_regex.
* cli/cli-decode.c (apropos_cmd): Change regex parameter to
compiled_regex reference. Adjust to use it.
* cli/cli-decode.h: Remove struct re_pattern_buffer forward
declaration. Include "gdb_regex.h".
(apropos_cmd): Change regex parameter to compiled_regex reference.
* gdb_regex.c: New file.
* gdb_regex.h (make_regfree_cleanup, get_regcomp_error): Delete
declarations.
(class compiled_regex): New.
* linux-tdep.c: Include "common/gdb_optional.h".
(struct mapping_regexes): New, factored out from
mapping_is_anonymous_p, and adjusted to use compiled_regex.
(mapping_is_anonymous_p): Use mapping_regexes wrapped in a
gdb::optional and remove cleanups. Adjust to compiled_regex.
* probe.c: Include "common/gdb_optional.h".
(collect_probes): Use compiled_regex and gdb::optional and remove
cleanups.
* skip.c: Include "common/gdb_optional.h".
(skiplist_entry::compiled_function_regexp): Now a
gdb::optional<compiled_regex>.
(skiplist_entry::compiled_function_regexp_is_valid): Delete field.
(free_skiplist_entry): Remove regfree call.
(compile_skip_regexp, skip_rfunction_p): Adjust to use
compiled_regex and gdb::optional.
* symtab.c: Include "common/gdb_optional.h".
(search_symbols): Use compiled_regex and gdb::optional.
* utils.c (do_regfree_cleanup, make_regfree_cleanup)
(get_regcomp_error, compile_rx_or_error): Delete. Some bits moved
to gdb_regex.c.
Pedro Alves