Fix watching structs in C++

Some of the watchpoint logic depends on the fact that the head of the
value chain represents the user-specified value to watch.  Thus no
additional values should be added to the value chain after that.  However,
if a watchpoint is defined for a C++ structure/class object, then run-time
type information (RTTI) may be present.  Thus, while constructing the
value chain for the watchpoint, the dynamic type is fetched by
gnuv3_rrti_type, which invokes value_addr, which then adds a new value to
the head of the value chain.  This new value represents the pointer to the
structure instead of the structure itself.

With such a "polluted" value chain the watchpoint logic does not recognize
when the user intended to watch a struct, and can_use_hardware_watchpoint
returns zero.  Instead of a hardware watchpoint, a software watchpoint
will then be set for no apparent reason.

This is fixed by adding an early exit to gnuv3_rtti_type when the input
value is not a dynamic class object.

gdb/testsuite/ChangeLog:

	* gdb.cp/watch-cp.cc: New test.
	* gdb.cp/watch-cp.exp: New file.

gdb/ChangeLog:

	* gnu-v3-abi.c (gnuv3_rtti_type): Add early exit if the given
	value is not a dynamic class object.

1 files changed, 28 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.cp/watch-cp.cc b/gdb/testsuite/gdb.cp/watch-cp.cc
new file mode 100644
index 0000000..6954e85
--- /dev/null
+++ b/gdb/testsuite/gdb.cp/watch-cp.cc
@@ -0,0 +1,28 @@
+/* Copyright 2018 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/>.  */
+
+/* A class that does not have RTTI and is small enough to fit in a
+   hardware watchpoint on all targets.  */
+class smallstuff { public: int n; };
+
+smallstuff watchme[5];
+
+int
+main ()
+{
+  for (int i = 0; i < sizeof (watchme) / sizeof (watchme[0]); i++)
+    watchme[i].n = i;
+  return 0;
+}