GCC PR83906 [1] is about a GCC/libstdc++ GDB/Python type printer
testcase failing randomly, as shown by running (in libstdc++'s
testsuite):

 make check RUNTESTFLAGS=prettyprinters.exp=80276.cc

in a loop.  Sometimes you get this:

 FAIL: libstdc++-prettyprinters/80276.cc whatis p4

I.e., this:
 type = std::unique_ptr<std::vector<std::unique_ptr<std::list<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >>[]>>[99]>

instead of this:
 type = std::unique_ptr<std::vector<std::unique_ptr<std::list<std::string>[]>>[99]>

Jonathan Wakely tracked it on the printer side to this bit in
libstdc++'s type printer:

            if self.type_obj == type_obj:
                return strip_inline_namespaces(self.name)

This assumes the two types resolve to the same gdb.Type but some times
the comparison unexpectedly fails.

Running the testcase manually under Valgrind finds the problem in GDB:

 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 ==6118== Conditional jump or move depends on uninitialised value(s)
 ==6118==    at 0x4C35CB0: bcmp (vg_replace_strmem.c:1100)
 ==6118==    by 0x6F773A: check_types_equal(type*, type*, VEC_type_equality_entry_d**) (gdbtypes.c:3515)
 ==6118==    by 0x6F7B00: check_types_worklist(VEC_type_equality_entry_d**, bcache*) (gdbtypes.c:3618)
 ==6118==    by 0x6F7C03: types_deeply_equal(type*, type*) (gdbtypes.c:3655)
 ==6118==    by 0x4D5B06: typy_richcompare(_object*, _object*, int) (py-type.c:1007)
 ==6118==    by 0x63D7E6C: PyObject_RichCompare (object.c:961)
 ==6118==    by 0x646EAEC: PyEval_EvalFrameEx (ceval.c:4960)
 ==6118==    by 0x646DC08: PyEval_EvalFrameEx (ceval.c:4519)
 ==6118==    by 0x646DC08: PyEval_EvalFrameEx (ceval.c:4519)
 ==6118==    by 0x646DC08: PyEval_EvalFrameEx (ceval.c:4519)
 ==6118==    by 0x646DC08: PyEval_EvalFrameEx (ceval.c:4519)
 ==6118==    by 0x646DC08: PyEval_EvalFrameEx (ceval.c:4519)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~

That "bcmp" call is really a memcmp call in check_types_equal.  The
problem is that gdb is memcmp'ing two objects that are equal in value:

 (top-gdb) p *TYPE_RANGE_DATA (type1)
 $1 = {low = {kind = PROP_CONST, data = {const_val = 0, baton = 0x0}}, high = {kind = PROP_CONST, data = {const_val = 15, baton = 0xf}}, flag_upper_bound_is_count = 0,
   flag_bound_evaluated = 0}
 (top-gdb) p *TYPE_RANGE_DATA (type2)
 $2 = {low = {kind = PROP_CONST, data = {const_val = 0, baton = 0x0}}, high = {kind = PROP_CONST, data = {const_val = 15, baton = 0xf}}, flag_upper_bound_is_count = 0,
   flag_bound_evaluated = 0}

but differ in padding.  Notice the 4-byte hole:

  (top-gdb) ptype /o range_bounds
  /* offset    |  size */  type = struct range_bounds {
  /*    0      |    16 */    struct dynamic_prop {
  /*    0      |     4 */        dynamic_prop_kind kind;
  /* XXX  4-byte hole  */
  /*    8      |     8 */        union dynamic_prop_data {
  /*                 8 */            LONGEST const_val;
  /*                 8 */            void *baton;

				     /* total size (bytes):    8 */
				 } data;

which is filled with garbage:

  (top-gdb) x /40bx TYPE_RANGE_DATA (type1)
  0x2fa7ea0:      0x01    0x00    0x00    0x00    0x43    0x01    0x00    0x00
						  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  0x2fa7ea8:      0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
  0x2fa7eb0:      0x01    0x00    0x00    0x00    0xfe    0x7f    0x00    0x00
  0x2fa7eb8:      0x0f    0x00    0x00    0x00    0x00    0x00    0x00    0x00
  0x2fa7ec0:      0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
  (top-gdb) x /40bx TYPE_RANGE_DATA (type2)
  0x20379b0:      0x01    0x00    0x00    0x00    0xfe    0x7f    0x00    0x00
						  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  0x20379b8:      0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
  0x20379c0:      0x01    0x00    0x00    0x00    0xfe    0x7f    0x00    0x00
  0x20379c8:      0x0f    0x00    0x00    0x00    0x00    0x00    0x00    0x00
  0x20379d0:      0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00

  (top-gdb) p memcmp (TYPE_RANGE_DATA (type1), TYPE_RANGE_DATA (type2), sizeof (*TYPE_RANGE_DATA (type1)))
  $3 = -187

In some cases objects of type range_bounds are memset when allocated,
but then their dynamic_prop low/high fields are copied over from some
template dynamic_prop object that wasn't memset.  E.g.,
create_static_range_type's low/high locals are left with garbage in
the padding, and then that padding is copied over to the range_bounds
object's low/high fields.

At first, I considered making sure to always memset range_bounds
objects, thinking that maybe type objects are being put in some bcache
instance somewhere.  But then I hacked bcache/bcache_full to poison
non-pod types, and made dynamic_prop a non-pod, and GDB still
compiled.

So given that, it seems safest to not assume padding will always be
memset, and instead treat them as regular value types, implementing
(in)equality operators and using those instead of memcmp.

This fixes the random FAILs in GCC's testcase.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83906

gdb/ChangeLog:
2018-01-24  Pedro Alves  <palves@redhat.com>

	GCC PR libstdc++/83906
	* gdbtypes.c (operator==(const dynamic_prop &,
	const dynamic_prop &)): New.
	(operator==(const range_bounds &, const range_bounds &)): New.
	(check_types_equal): Use them instead of memcmp.
	* gdbtypes.h (operator==(const dynamic_prop &,
	const dynamic_prop &)): Declare.
	(operator!=(const dynamic_prop &, const dynamic_prop &)): Declare.
	(operator==(const range_bounds &, const range_bounds &)): Declare.
	(operator!=(const range_bounds &, const range_bounds &)): Declare.