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<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.

gdb/cli/cli-dump.c

@@ -31,7 +31,7 @@
 #include "cli/cli-utils.h"
 #include "gdb_bfd.h"
 #include "filestuff.h"
-
+#include "common/byte-vector.h"
 
 static const char *
 scan_expression_with_cleanup (const char **cmd, const char *def)
@@ -230,17 +230,17 @@ dump_memory_to_file (const char *cmd, const char *mode, const char *file_format)
 
   /* FIXME: Should use read_memory_partial() and a magic blocking
      value.  */
-  std::unique_ptr<gdb_byte[]> buf (new gdb_byte[count]);
-  read_memory (lo, buf.get (), count);
+  gdb::byte_vector buf (count);
+  read_memory (lo, buf.data (), count);
   
   /* Have everything.  Open/write the data.  */
   if (file_format == NULL || strcmp (file_format, "binary") == 0)
     {
-      dump_binary_file (filename, mode, buf.get (), count);
+      dump_binary_file (filename, mode, buf.data (), count);
     }
   else
     {
-      dump_bfd_file (filename, mode, file_format, lo, buf.get (), count);
+      dump_bfd_file (filename, mode, file_format, lo, buf.data (), count);
     }
 
   do_cleanups (old_cleanups);
@@ -545,13 +545,13 @@ restore_binary_file (const char *filename, struct callback_data *data)
     perror_with_name (filename);
 
   /* Now allocate a buffer and read the file contents.  */
-  std::unique_ptr<gdb_byte[]> buf (new gdb_byte[len]);
-  if (fread (buf.get (), 1, len, file) != len)
+  gdb::byte_vector buf (len);
+  if (fread (buf.data (), 1, len, file) != len)
     perror_with_name (filename);
 
   /* Now write the buffer into target memory.  */
   len = target_write_memory (data->load_start + data->load_offset,
-			     buf.get (), len);
+			     buf.data (), len);
   if (len != 0)
     warning (_("restore: memory write failed (%s)."), safe_strerror (len));
   do_cleanups (cleanup);