From: Pedro Alves <palves at redhat dot com>

To: gdb-patches at sourceware dot org

Subject: [PATCH] Fix GCC PR83906 - [8 Regression] Random FAIL: libstdc++-prettyprinters/80276.cc whatis p4

Date: Wed, 24 Jan 2018 17:27:22 +0000

Message-Id: <20180124172722.31553-1-palves@redhat.com>

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.

---

 gdb/gdbtypes.c | 41 +++++++++++++++++++++++++++++++++++++++--

 gdb/gdbtypes.h | 20 ++++++++++++++++++++

 2 files changed, 59 insertions(+), 2 deletions(-)

Index: gdb-8.0.1/gdb/gdbtypes.c

===================================================================

--- gdb-8.0.1.orig/gdb/gdbtypes.c

+++ gdb-8.0.1/gdb/gdbtypes.c

@@ -855,6 +855,44 @@ allocate_stub_method (struct type *type)

   return mtype;

 }

+/* See gdbtypes.h.  */

+

+bool

+operator== (const dynamic_prop &l, const dynamic_prop &r)

+{

+  if (l.kind != r.kind)

+    return false;

+

+  switch (l.kind)

+    {

+    case PROP_UNDEFINED:

+      return true;

+    case PROP_CONST:

+      return l.data.const_val == r.data.const_val;

+    case PROP_ADDR_OFFSET:

+    case PROP_LOCEXPR:

+    case PROP_LOCLIST:

+      return l.data.baton == r.data.baton;

+    }

+

+  gdb_assert_not_reached ("unhandled dynamic_prop kind");

+}

+

+/* See gdbtypes.h.  */

+

+bool

+operator== (const range_bounds &l, const range_bounds &r)

+{

+#define FIELD_EQ(FIELD) (l.FIELD == r.FIELD)

+

+  return (FIELD_EQ (low)

+	  && FIELD_EQ (high)

+	  && FIELD_EQ (flag_upper_bound_is_count)

+	  && FIELD_EQ (flag_bound_evaluated));

+

+#undef FIELD_EQ

+}

+

 /* Create a range type with a dynamic range from LOW_BOUND to

    HIGH_BOUND, inclusive.  See create_range_type for further details. */

@@ -3466,8 +3504,7 @@ check_types_equal (struct type *type1, s

   if (TYPE_CODE (type1) == TYPE_CODE_RANGE)

     {

-      if (memcmp (TYPE_RANGE_DATA (type1), TYPE_RANGE_DATA (type2),

-		  sizeof (*TYPE_RANGE_DATA (type1))) != 0)

+      if (*TYPE_RANGE_DATA (type1) != *TYPE_RANGE_DATA (type2))

 	return 0;

     }

   else

Index: gdb-8.0.1/gdb/gdbtypes.h

===================================================================

--- gdb-8.0.1.orig/gdb/gdbtypes.h

+++ gdb-8.0.1/gdb/gdbtypes.h

@@ -408,6 +408,16 @@ struct dynamic_prop

   union dynamic_prop_data data;

 };

+/* Compare two dynamic_prop objects for equality.  dynamic_prop

+   instances are equal iff they have the same type and storage.  */

+extern bool operator== (const dynamic_prop &l, const dynamic_prop &r);

+

+/* Compare two dynamic_prop objects for inequality.  */

+static inline bool operator!= (const dynamic_prop &l, const dynamic_prop &r)

+{

+  return !(l == r);

+}

+

 /* * Define a type's dynamic property node kind.  */

 enum dynamic_prop_node_kind

 {

@@ -568,6 +578,16 @@ struct range_bounds

   int flag_bound_evaluated : 1;

 };

+/* Compare two range_bounds objects for equality.  Simply does

+   memberwise comparison.  */

+extern bool operator== (const range_bounds &l, const range_bounds &r);

+

+/* Compare two range_bounds objects for inequality.  */

+static inline bool operator!= (const range_bounds &l, const range_bounds &r)

+{

+  return !(l == r);

+}

+

 union type_specific

 {

   /* * CPLUS_STUFF is for TYPE_CODE_STRUCT.  It is initialized to