Fix random 'FAIL: libstdc++-prettyprinters/80276.cc whatis p4' (GCC PR83906) (Pedro Alves).
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From: Pedro Alves <palves at redhat dot com>
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To: gdb-patches at sourceware dot org
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Subject: [PATCH] Fix GCC PR83906 - [8 Regression] Random FAIL: libstdc++-prettyprinters/80276.cc whatis p4
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Date: Wed, 24 Jan 2018 17:27:22 +0000
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Message-Id: <20180124172722.31553-1-palves@redhat.com>
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GCC PR83906 [1] is about a GCC/libstdc++ GDB/Python type printer
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testcase failing randomly, as shown by running (in libstdc++'s
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testsuite):
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make check RUNTESTFLAGS=prettyprinters.exp=80276.cc
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in a loop. Sometimes you get this:
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FAIL: libstdc++-prettyprinters/80276.cc whatis p4
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I.e., this:
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type = std::unique_ptr<std::vector<std::unique_ptr<std::list<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >>[]>>[99]>
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instead of this:
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type = std::unique_ptr<std::vector<std::unique_ptr<std::list<std::string>[]>>[99]>
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Jonathan Wakely tracked it on the printer side to this bit in
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libstdc++'s type printer:
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if self.type_obj == type_obj:
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return strip_inline_namespaces(self.name)
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This assumes the two types resolve to the same gdb.Type but some times
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the comparison unexpectedly fails.
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Running the testcase manually under Valgrind finds the problem in GDB:
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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==6118== Conditional jump or move depends on uninitialised value(s)
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==6118== at 0x4C35CB0: bcmp (vg_replace_strmem.c:1100)
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==6118== by 0x6F773A: check_types_equal(type*, type*, VEC_type_equality_entry_d**) (gdbtypes.c:3515)
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==6118== by 0x6F7B00: check_types_worklist(VEC_type_equality_entry_d**, bcache*) (gdbtypes.c:3618)
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==6118== by 0x6F7C03: types_deeply_equal(type*, type*) (gdbtypes.c:3655)
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==6118== by 0x4D5B06: typy_richcompare(_object*, _object*, int) (py-type.c:1007)
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==6118== by 0x63D7E6C: PyObject_RichCompare (object.c:961)
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==6118== by 0x646EAEC: PyEval_EvalFrameEx (ceval.c:4960)
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==6118== by 0x646DC08: PyEval_EvalFrameEx (ceval.c:4519)
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==6118== by 0x646DC08: PyEval_EvalFrameEx (ceval.c:4519)
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==6118== by 0x646DC08: PyEval_EvalFrameEx (ceval.c:4519)
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==6118== by 0x646DC08: PyEval_EvalFrameEx (ceval.c:4519)
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==6118== by 0x646DC08: PyEval_EvalFrameEx (ceval.c:4519)
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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That "bcmp" call is really a memcmp call in check_types_equal. The
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problem is that gdb is memcmp'ing two objects that are equal in value:
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(top-gdb) p *TYPE_RANGE_DATA (type1)
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$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,
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flag_bound_evaluated = 0}
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(top-gdb) p *TYPE_RANGE_DATA (type2)
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$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,
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flag_bound_evaluated = 0}
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but differ in padding. Notice the 4-byte hole:
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(top-gdb) ptype /o range_bounds
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/* offset | size */ type = struct range_bounds {
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/* 0 | 16 */ struct dynamic_prop {
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/* 0 | 4 */ dynamic_prop_kind kind;
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/* XXX 4-byte hole */
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/* 8 | 8 */ union dynamic_prop_data {
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/* 8 */ LONGEST const_val;
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/* 8 */ void *baton;
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/* total size (bytes): 8 */
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} data;
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which is filled with garbage:
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(top-gdb) x /40bx TYPE_RANGE_DATA (type1)
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0x2fa7ea0: 0x01 0x00 0x00 0x00 0x43 0x01 0x00 0x00
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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0x2fa7ea8: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
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0x2fa7eb0: 0x01 0x00 0x00 0x00 0xfe 0x7f 0x00 0x00
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0x2fa7eb8: 0x0f 0x00 0x00 0x00 0x00 0x00 0x00 0x00
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0x2fa7ec0: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
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(top-gdb) x /40bx TYPE_RANGE_DATA (type2)
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0x20379b0: 0x01 0x00 0x00 0x00 0xfe 0x7f 0x00 0x00
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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0x20379b8: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
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0x20379c0: 0x01 0x00 0x00 0x00 0xfe 0x7f 0x00 0x00
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0x20379c8: 0x0f 0x00 0x00 0x00 0x00 0x00 0x00 0x00
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0x20379d0: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
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(top-gdb) p memcmp (TYPE_RANGE_DATA (type1), TYPE_RANGE_DATA (type2), sizeof (*TYPE_RANGE_DATA (type1)))
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$3 = -187
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In some cases objects of type range_bounds are memset when allocated,
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but then their dynamic_prop low/high fields are copied over from some
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template dynamic_prop object that wasn't memset. E.g.,
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create_static_range_type's low/high locals are left with garbage in
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the padding, and then that padding is copied over to the range_bounds
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object's low/high fields.
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At first, I considered making sure to always memset range_bounds
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objects, thinking that maybe type objects are being put in some bcache
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instance somewhere. But then I hacked bcache/bcache_full to poison
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non-pod types, and made dynamic_prop a non-pod, and GDB still
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compiled.
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So given that, it seems safest to not assume padding will always be
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memset, and instead treat them as regular value types, implementing
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(in)equality operators and using those instead of memcmp.
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This fixes the random FAILs in GCC's testcase.
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[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83906
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gdb/ChangeLog:
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2018-01-24 Pedro Alves <palves@redhat.com>
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GCC PR libstdc++/83906
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* gdbtypes.c (operator==(const dynamic_prop &,
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const dynamic_prop &)): New.
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(operator==(const range_bounds &, const range_bounds &)): New.
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(check_types_equal): Use them instead of memcmp.
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* gdbtypes.h (operator==(const dynamic_prop &,
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const dynamic_prop &)): Declare.
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(operator!=(const dynamic_prop &, const dynamic_prop &)): Declare.
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(operator==(const range_bounds &, const range_bounds &)): Declare.
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(operator!=(const range_bounds &, const range_bounds &)): Declare.
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---
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gdb/gdbtypes.c | 41 +++++++++++++++++++++++++++++++++++++++--
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gdb/gdbtypes.h | 20 ++++++++++++++++++++
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2 files changed, 59 insertions(+), 2 deletions(-)
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Index: gdb-8.0.1/gdb/gdbtypes.c
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===================================================================
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--- gdb-8.0.1.orig/gdb/gdbtypes.c
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+++ gdb-8.0.1/gdb/gdbtypes.c
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@@ -855,6 +855,44 @@ allocate_stub_method (struct type *type)
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return mtype;
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}
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+/* See gdbtypes.h. */
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+
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+bool
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+operator== (const dynamic_prop &l, const dynamic_prop &r)
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+{
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+ if (l.kind != r.kind)
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+ return false;
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+
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+ switch (l.kind)
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+ {
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+ case PROP_UNDEFINED:
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+ return true;
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+ case PROP_CONST:
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+ return l.data.const_val == r.data.const_val;
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+ case PROP_ADDR_OFFSET:
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+ case PROP_LOCEXPR:
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+ case PROP_LOCLIST:
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+ return l.data.baton == r.data.baton;
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+ }
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+
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+ gdb_assert_not_reached ("unhandled dynamic_prop kind");
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+}
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+
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+/* See gdbtypes.h. */
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+
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+bool
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+operator== (const range_bounds &l, const range_bounds &r)
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+{
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+#define FIELD_EQ(FIELD) (l.FIELD == r.FIELD)
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+
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+ return (FIELD_EQ (low)
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+ && FIELD_EQ (high)
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+ && FIELD_EQ (flag_upper_bound_is_count)
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+ && FIELD_EQ (flag_bound_evaluated));
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+
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+#undef FIELD_EQ
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+}
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+
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/* Create a range type with a dynamic range from LOW_BOUND to
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HIGH_BOUND, inclusive. See create_range_type for further details. */
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@@ -3466,8 +3504,7 @@ check_types_equal (struct type *type1, s
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if (TYPE_CODE (type1) == TYPE_CODE_RANGE)
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{
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- if (memcmp (TYPE_RANGE_DATA (type1), TYPE_RANGE_DATA (type2),
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- sizeof (*TYPE_RANGE_DATA (type1))) != 0)
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+ if (*TYPE_RANGE_DATA (type1) != *TYPE_RANGE_DATA (type2))
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return 0;
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}
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else
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Index: gdb-8.0.1/gdb/gdbtypes.h
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===================================================================
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--- gdb-8.0.1.orig/gdb/gdbtypes.h
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+++ gdb-8.0.1/gdb/gdbtypes.h
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@@ -408,6 +408,16 @@ struct dynamic_prop
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union dynamic_prop_data data;
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};
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+/* Compare two dynamic_prop objects for equality. dynamic_prop
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+ instances are equal iff they have the same type and storage. */
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+extern bool operator== (const dynamic_prop &l, const dynamic_prop &r);
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+
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+/* Compare two dynamic_prop objects for inequality. */
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+static inline bool operator!= (const dynamic_prop &l, const dynamic_prop &r)
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+{
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+ return !(l == r);
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+}
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+
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/* * Define a type's dynamic property node kind. */
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enum dynamic_prop_node_kind
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{
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@@ -568,6 +578,16 @@ struct range_bounds
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int flag_bound_evaluated : 1;
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};
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+/* Compare two range_bounds objects for equality. Simply does
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+ memberwise comparison. */
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+extern bool operator== (const range_bounds &l, const range_bounds &r);
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+
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+/* Compare two range_bounds objects for inequality. */
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+static inline bool operator!= (const range_bounds &l, const range_bounds &r)
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+{
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+ return !(l == r);
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+}
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+
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union type_specific
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{
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/* * CPLUS_STUFF is for TYPE_CODE_STRUCT. It is initialized to
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6
gdb.spec
6
gdb.spec
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@ -26,7 +26,7 @@ Version: 8.0.1
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# The release always contains a leading reserved number, start it at 1.
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# `upstream' is not a part of `name' to stay fully rpm dependencies compatible for the testing.
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Release: 33%{?dist}
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Release: 34%{?dist}
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License: GPLv3+ and GPLv3+ with exceptions and GPLv2+ and GPLv2+ with exceptions and GPL+ and LGPLv2+ and LGPLv3+ and BSD and Public Domain and GFDL
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Group: Development/Debuggers
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@ -1731,6 +1731,10 @@ then
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fi
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%changelog
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* Wed Jan 24 2018 Sergio Durigan Junior <sergiodj@fedoraproject.org> - 8.0.1-34
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- Fix random 'FAIL: libstdc++-prettyprinters/80276.cc whatis p4' (GCC PR83906)
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(Pedro Alves).
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* Wed Dec 6 2017 Jan Kratochvil <jan.kratochvil@redhat.com> - 8.0.1-33.fc26
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- [rhel7] Fix C++ compiler compatibility.
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