Implement pahole-like 'ptype /o' option

This commit implements the pahole-like '/o' option for 'ptype', which
prints the offsets and sizes of struct fields, reporting whenever
there is a hole found.

The output is heavily based on pahole(1), with a few modifications
here and there to adjust it to our reality.  Here's an example:

  /* offset    |  size */  type = struct wer : public tuv {
			   public:
  /*   32      |    24 */    struct tyu {
  /*   32:31   |     4 */        int a1 : 1;
  /*   32:28   |     4 */        int a2 : 3;
  /*   32: 5   |     4 */        int a3 : 23;
  /*   35: 3   |     1 */        char a4 : 2;
  /* XXX  3-bit hole   */
  /* XXX  4-byte hole  */
  /*   40      |     8 */        int64_t a5;
  /*   48:27   |     4 */        int a6 : 5;
  /*   48:56   |     8 */        int64_t a7 : 3;

				 /* total size (bytes):   24 */
			     } a1;

			     /* total size (bytes):   56 */
			   }

A big part of this patch handles the formatting logic of 'ptype',
which is a bit messy.  The code to handle bitfield offsets, however,
took some time to craft.  My thanks to Pedro Alves for figuring things
out and pointing me to the right direction, as well as coming up with
a way to inspect the layout of structs with bitfields (see testcase
for comments).

After many discussions both on IRC and at the mailing list, I tried to
implement printing vtables and inherited classes.  Unfortunately the
code grew too complex and there were still a few corner cases failing
so I had to drop the attempt.  This should be implemented in a future
patch.

This patch is the start of a long-term work I'll do to flush the local
patches we carry for Fedora GDB.  In this specific case, I'm aiming at
upstreaming the feature implemented by the 'pahole.py' script that is
shipped with Fedora GDB:

  <https://src.fedoraproject.org/rpms/gdb/blob/master/f/gdb-archer.patch#_311>

This has been regression-tested on the BuildBot.  There's a new
testcase for it, along with an update to the documentation.  I also
thought it was worth mentioning this feature in the NEWS file.

gdb/ChangeLog:
2017-12-15  Sergio Durigan Junior  <sergiodj@redhat.com>
	    Pedro Alves  <palves@redhat.com>

	PR cli/16224
	* NEWS (Changes since GDB 8.0): Mention new '/o' flag.
	* c-typeprint.c (OFFSET_SPC_LEN): New define.
	(c_type_print_varspec_prefix): New argument 'struct
	print_offset_data *'.
	(c_type_print_base_1): New function and prototype.
	(c_print_type_1): New function, with code from 'c_print_type'.
	(c_print_type): Use 'c_print_type_1'.
	(c_type_print_varspec_prefix): New argument 'struct
	print_offset_data *'.  Use it.  Call 'c_type_print_base_1'
	instead of 'c_print_type_base'.
	(print_spaces_filtered_with_print_options): New function.
	(output_access_specifier): Take new argument FLAGS.  Modify
	function to call 'print_spaces_filtered_with_print_options'.
	(c_print_type_vtable_offset_marker): New function.
	(c_print_type_union_field_offset): New function.
	(c_print_type_struct_field_offset): New function.
	(c_print_type_no_offsets): New function.
	(c_type_print_base_struct_union): New argument 'struct
	print_offset_data *'.  Print offsets and sizes for
	struct/union/class fields.
	* typeprint.c (const struct type_print_options
	type_print_raw_options): Initialize 'print_offsets'.
	(static struct type_print_options default_ptype_flags):
	Likewise.
	(struct print_offset_data print_offset_default_data): New
	variable.
	(whatis_exp): Handle '/o' option.
	(_initialize_typeprint): Add '/o' flag to ptype's help.
	* typeprint.h (struct print_offset_data): New struct.
	(struct type_print_options) <print_offsets>: New field.

gdb/testsuite/ChangeLog:
2017-12-15  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR cli/16224
	* gdb.base/ptype-offsets.cc: New file.
	* gdb.base/ptype-offsets.exp: New file.

gdb/doc/ChangeLog:
2017-12-15  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR cli/16224
	* gdb.texinfo (ptype): Add documentation for new flag '/o'.

1 files changed, 17 insertions, 0 deletions

diff --git a/gdb/typeprint.h b/gdb/typeprint.h
index a2058b0..2e19006 100644
--- a/gdb/typeprint.h
+++ b/gdb/typeprint.h
@@ -24,6 +24,20 @@ struct ui_file;
 struct typedef_hash_table;
 struct ext_lang_type_printers;
 
+struct print_offset_data
+{
+  /* The offset to be applied to bitpos when PRINT_OFFSETS is true.
+     This is needed for when we are printing nested structs and want
+     to make sure that the printed offset for each field carries over
+     the offset of the outter struct.  */
+  unsigned int offset_bitpos = 0;
+
+  /* END_BITPOS is the one-past-the-end bit position of the previous
+     field (where we expect the current field to be if there is no
+     hole).  */
+  unsigned int end_bitpos = 0;
+};
+
 struct type_print_options
 {
   /* True means that no special printing flags should apply.  */
@@ -35,6 +49,9 @@ struct type_print_options
   /* True means print typedefs in a class.  */
   unsigned int print_typedefs : 1;
 
+  /* True means to print offsets, a la 'pahole'.  */
+  unsigned int print_offsets : 1;
+
   /* The number of nested type definitions to print.  -1 == all.  */
   int print_nested_type_limit;