From d5722aa2fe9e1d76d98865a9ab77a7b9388743c9 Mon Sep 17 00:00:00 2001 From: Pedro Alves Date: Wed, 14 Jun 2017 11:08:52 +0100 Subject: Introduce gdb::byte_vector, add allocator that default-initializes In some cases we've been replacing heap-allocated gdb_byte buffers managed with xmalloc/make_cleanup(xfree) with gdb::vector. 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 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 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 I.e., a custom allocator that does default construction using default initialization, meaning, no more zero initialization. That's the default_init_allocation class added in this patch. See "Notes" at . #2 - Introduce def_vector I.e., a convenience typedef, because typing the allocator is annoying: using def_vector = std::vector>; #3 - Introduce byte_vector Because gdb_byte vectors will be the common thing, add a convenience "byte_vector" typedef: using byte_vector = def_vector; which is really the same as: std::vector>; 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 uses. One nice thing is that with this allocator, for changes like these: -std::unique_ptr 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 buf (new gdb_byte[size]); or even (C++14): auto buf = std::make_unique (size); // zero-initializes... #5 - Suggest s/std::vector/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 * 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. * 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. * 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. --- gdb/valprint.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) (limited to 'gdb/valprint.c') diff --git a/gdb/valprint.c b/gdb/valprint.c index e627e85..c10dade 100644 --- a/gdb/valprint.c +++ b/gdb/valprint.c @@ -37,6 +37,7 @@ #include "typeprint.h" #include #include +#include "common/byte-vector.h" /* Maximum number of wchars returned from wchar_iterate. */ #define MAX_WCHARS 4 @@ -1721,7 +1722,7 @@ print_octal_chars (struct ui_file *stream, const gdb_byte *valaddr, static bool maybe_negate_by_bytes (const gdb_byte *bytes, unsigned len, enum bfd_endian byte_order, - std::vector *out_vec) + gdb::byte_vector *out_vec) { gdb_byte sign_byte; if (byte_order == BFD_ENDIAN_BIG) @@ -1780,7 +1781,7 @@ print_decimal_chars (struct ui_file *stream, const gdb_byte *valaddr, int dummy; int flip; - std::vector negated_bytes; + gdb::byte_vector negated_bytes; if (is_signed && maybe_negate_by_bytes (valaddr, len, byte_order, &negated_bytes)) { -- cgit v1.1