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.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2018-03-07  Andreas Arnez  <arnez@linux.vnet.ibm.com>
+
+	* gnu-v3-abi.c (gnuv3_rtti_type): Add early exit if the given
+	value is not a dynamic class object.
+
 2018-03-06  Tom Tromey  <tom@tromey.com>
 
 	* rust-exp.y: Formatting fixes.

gdb/gnu-v3-abi.c

@@ -299,8 +299,9 @@ gnuv3_rtti_type (struct value *value,
   LONGEST offset_to_top;
   const char *atsign;
 
-  /* We only have RTTI for class objects.  */
-  if (TYPE_CODE (values_type) != TYPE_CODE_STRUCT)
+  /* We only have RTTI for dynamic class objects.  */
+  if (TYPE_CODE (values_type) != TYPE_CODE_STRUCT
+      || !gnuv3_dynamic_class (values_type))
     return NULL;
 
   /* Determine architecture.  */

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2018-03-07  Andreas Arnez  <arnez@linux.vnet.ibm.com>
+
+	* gdb.cp/watch-cp.cc: New test.
+	* gdb.cp/watch-cp.exp: New file.
+
 2018-03-06  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* gdb.base/infcall-nested-structs.exp: New file.

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;
+}

gdb/testsuite/gdb.cp/watch-cp.exp

@@ -0,0 +1,35 @@
+# 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/>.
+
+if { [skip_cplus_tests] || [skip_hw_watchpoint_tests]} { continue }
+
+standard_testfile .cc
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile {debug c++}]} {
+    return -1
+}
+
+if ![runto_main] then {
+    perror "couldn't run to main"
+    continue
+}
+
+gdb_test "watch watchme\[3\]" "atchpoint .*: watchme.*" "set watchpoint"
+
+# Verify that a hardware watchpoint is used.
+gdb_test "info watchpoints" " hw watchpoint .* watchme.*"
+
+# Now see whether it actually works.
+gdb_test "continue" "Hardware watchpoint .*New value = \\{n = 3\\}.*"