[patch] Warn on constant value watchpoints

[Jan Kratochvil <jan dot kratochvil at redhat dot com>]

Hi,

idea came from a discussion with the Firefox maintainer Martin Stransky who had
disfunctional watchpoint on the address of a variable:

(gdb) watch 0x4343548
Watchpoint 1: 70530376


Regards,
Jan

2008-06-08  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* breakpoint.c (watch_command_1): New variable VAL_RESULT.  Fill in
	VAL_RESULT by the existing FETCH_WATCHPOINT_VALUE call.  Ask user for
	constant list VAL_RESULT.

2008-06-08  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* gdb.texinfo (Set Watchpoints): Document constant value watchpoints.

2008-06-08  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* gdb.base/watchpoint.exp: New test for constant value watchpoints.

--- gdb/breakpoint.c	6 Jun 2008 20:58:08 -0000	1.324
+++ gdb/breakpoint.c	8 Jun 2008 15:18:38 -0000
@@ -5818,7 +5818,7 @@ watch_command_1 (char *arg, int accessfl
   struct symtab_and_line sal;
   struct expression *exp;
   struct block *exp_valid_block;
-  struct value *val, *mark;
+  struct value *val, *mark, *val_result;
   struct frame_info *frame;
   struct frame_info *prev_frame = NULL;
   char *exp_start = NULL;
@@ -5903,10 +5903,23 @@ watch_command_1 (char *arg, int accessfl
   exp_end = arg;
   exp_valid_block = innermost_block;
   mark = value_mark ();
-  fetch_watchpoint_value (exp, &val, NULL, NULL);
+  fetch_watchpoint_value (exp, &val, &val_result, NULL);
   if (val != NULL)
     release_value (val);
 
+  if (from_tty)
+    {
+      struct value *v;
+
+      /* VAL may be unset for unreachable final values.  */
+      for (v = val_result; v; v = value_next (v))
+	if (VALUE_LVAL (v) == lval_memory || VALUE_LVAL (v) == lval_register)
+	  break;
+      if (v == NULL && !query
+		(_("Do you insist on this watchpoint with a constant value? ")))
+	error (_("Watchpoint with a constant value was cancelled."));
+    }
+
   tok = arg;
   while (*tok == ' ' || *tok == '\t')
     tok++;
--- gdb/doc/gdb.texinfo	6 Jun 2008 20:58:08 -0000	1.503
+++ gdb/doc/gdb.texinfo	8 Jun 2008 15:19:53 -0000
@@ -3375,6 +3426,18 @@ This command prints a list of watchpoint
 it is the same as @code{info break} (@pxref{Set Breaks}).
 @end table
 
+If you watch for a change in a numerically entered address you need to
+dereference it as the address itself is just a constant number which will never
+change:
+
+@smallexample
+(@value{GDBP}) watch 0x600850
+Do you insist on this watchpoint with a constant value? (y or n) n
+Watchpoint with a constant value was cancelled.
+(@value{GDBP}) watch *(int *) 0x600850
+Watchpoint 1: *(int *) 6293584
+@end smallexample
+
 @value{GDBN} sets a @dfn{hardware watchpoint} if possible.  Hardware
 watchpoints execute very quickly, and the debugger reports a change in
 value at the exact instruction where the change occurs.  If @value{GDBN}
--- gdb/testsuite/gdb.base/watchpoint.exp	15 Apr 2008 14:33:54 -0000	1.18
+++ gdb/testsuite/gdb.base/watchpoint.exp	8 Jun 2008 15:19:55 -0000
@@ -679,6 +679,18 @@ set prev_timeout $timeout
 set timeout 600	
 verbose "Timeout now 600 sec.\n"
 
+# Test constant-value watchpoints.
+gdb_test "watch 123" "Watchpoint with a constant value was cancelled."	      \
+    "constant watchpoint"						      \
+    "Do you insist on this watchpoint with a constant value\\? \\(y or n\\) " \
+    "n"
+# For unsupported constant-value watchpoints catching we need to reset the
+# breakpoints counter.
+gdb_exit
+gdb_start
+gdb_reinitialize_dir $srcdir/$subdir
+gdb_load $binfile
+
 if [initialize] then {
 
     test_simple_watchpoint