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, 193 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.base/ptype-offsets.cc b/gdb/testsuite/gdb.base/ptype-offsets.cc
new file mode 100644
index 0000000000..86735203ad
--- /dev/null
+++ b/gdb/testsuite/gdb.base/ptype-offsets.cc
@@ -0,0 +1,193 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2017 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include <stdint.h>
+
+/* A struct with many types of fields, in order to test 'ptype
+   /o'.  */
+
+struct abc
+{
+  /* Virtual destructor.  */
+  virtual ~abc ()
+  {}
+
+  /* 8-byte address.  Because of the virtual destructor above, this
+     field's offset will be 8.  */
+  void *field1;
+
+  /* No hole here.  */
+
+  /* 4-byte int bitfield of 1-bit.  */
+  unsigned int field2 : 1;
+
+  /* 31-bit hole here.  */
+
+  /* 4-byte int.  */
+  signed int field3;
+
+  /* No hole here.  */
+
+  /* 1-byte char.  */
+  signed char field4;
+
+  /* 7-byte hole here.  */
+
+  /* 8-byte int.  */
+  uint64_t field5;
+
+  /* We just print the offset and size of a union, ignoring its
+     fields.  */
+  union
+  {
+    /* 8-byte address.  */
+    void *field6;
+
+    /* 4-byte int.  */
+    signed int field7;
+  } field8;
+
+  /* Empty constructor.  */
+  abc ()
+  {}
+
+  /* Typedef defined in-struct.  */
+  typedef int my_int_type;
+
+  my_int_type field9;
+};
+
+/* This struct will be nested inside 'struct xyz'.  */
+
+struct tuv
+{
+  signed int a1;
+
+  signed char *a2;
+
+  signed int a3;
+};
+
+/* This struct will be nested inside 'struct pqr'.  */
+
+struct xyz
+{
+  signed int f1;
+
+  signed char f2;
+
+  void *f3;
+
+  struct tuv f4;
+};
+
+/* A struct with a nested struct.  */
+
+struct pqr
+{
+  signed int ff1;
+
+  struct xyz ff2;
+
+  signed char ff3;
+};
+
+/* A union with two nested structs.  */
+
+union qwe
+{
+  struct tuv fff1;
+
+  struct xyz fff2;
+};
+
+/* A struct with an union.  */
+
+struct poi
+{
+  signed int f1;
+
+  union qwe f2;
+
+  uint16_t f3;
+
+  struct pqr f4;
+};
+
+/* A struct with bitfields.  */
+
+struct tyu
+{
+  signed int a1 : 1;
+
+  signed int a2 : 3;
+
+  signed int a3 : 23;
+
+  signed char a4 : 2;
+
+  int64_t a5;
+
+  signed int a6 : 5;
+
+  int64_t a7 : 3;
+};
+
+/* A struct with structs and unions.  */
+
+struct asd
+{
+  struct jkl
+  {
+      signed char *f1;
+      union
+      {
+	void *ff1;
+      } f2;
+      union
+      {
+	signed char *ff2;
+      } f3;
+      int f4 : 5;
+      unsigned int f5 : 1;
+      short f6;
+  } f7;
+  unsigned long f8;
+  signed char *f9;
+  int f10 : 4;
+  unsigned int f11 : 1;
+  unsigned int f12 : 1;
+  unsigned int f13 : 1;
+  unsigned int f14 : 1;
+  void *f15;
+  void *f16;
+};
+
+
+int
+main (int argc, char *argv[])
+{
+  struct abc foo;
+  struct pqr bar;
+  union qwe c;
+  struct poi d;
+  struct tyu e;
+  struct asd f;
+  uint8_t i;
+
+  return 0;
+}