Watching expressions that don't involve memory (e.g., watch $regfoo)

[Pedro Alves <pedro@codesourcery.com>]

Anyone else things this is useful?  Here's a 5 minute hack at it.

It allows for e.g.:

(top-gdb) watch $rax
Watchpoint 4: $rax
(top-gdb) c
Continuing.
Watchpoint 4: $rax

Old value = 11802104
New value = 140737488347216
0x0000000000456647 in main (argc=1, argv=0x7fffffffe158) at ../../src/gdb/gdb.c:28
28        memset (&args, 0, sizeof args);
(top-gdb)

Current GDB will successfuly create the watchpoint in the
breakpoint list, but it never triggers, because since the
watchpoint isn't watching any memory, it ends up with
no bp_location associated.


For extra fun, even watching $_siginfo works.

-- 
Pedro Alves

2010-03-04  Pedro Alves  <pedro@codesourcery.com>

	* breakpoint.c (update_watchpoint): Create a sentinel location if
	the software watchpoint isn't watching any memory.
	(breakpoint_address_bits): Skip dummy software watchpoint locations.

	* NEWS: Mention support for watching expressions that don't
	involve memory.

---
 gdb/NEWS         |    6 ++++++
 gdb/breakpoint.c |   20 +++++++++++++++++++-
 2 files changed, 25 insertions(+), 1 deletion(-)

Index: src/gdb/breakpoint.c
===================================================================
--- src.orig/gdb/breakpoint.c	2010-03-04 01:17:24.000000000 +0000
+++ src/gdb/breakpoint.c	2010-03-04 01:50:42.000000000 +0000
@@ -1237,6 +1237,19 @@ update_watchpoint (struct breakpoint *b,
 	    value_free (v);
 	}
 
+      /* If a software watchpoint is not watching any memory, then it
+	 will not have any location set up yet.  But,
+	 bpstat_stop_status requires a location to be able to report
+	 stops, so add a sentinel one now.  */
+      if (b->type == bp_watchpoint && b->loc == NULL)
+	{
+	  b->loc = allocate_bp_location (b);
+	  b->loc->pspace = frame_pspace;
+	  b->loc->address = -1;
+	  b->loc->length = -1;
+	  b->loc->watchpoint_type = -1;
+	}
+
       /* We just regenerated the list of breakpoint locations.
          The new location does not have its condition field set to anything
          and therefore, we must always reparse the cond_string, independently
@@ -4516,7 +4529,12 @@ breakpoint_address_bits (struct breakpoi
 
   for (loc = b->loc; loc; loc = loc->next)
     {
-      int addr_bit = gdbarch_addr_bit (loc->gdbarch);
+      int addr_bit;
+
+      if (b->type == bp_watchpoint && loc->watchpoint_type == -1)
+	continue;
+
+      addr_bit = gdbarch_addr_bit (loc->gdbarch);
       if (addr_bit > print_address_bits)
 	print_address_bits = addr_bit;
     }
Index: src/gdb/NEWS
===================================================================
--- src.orig/gdb/NEWS	2010-03-04 01:44:06.000000000 +0000
+++ src/gdb/NEWS	2010-03-04 01:45:37.000000000 +0000
@@ -3,6 +3,12 @@
 
 *** Changes since GDB 7.1
 
+* Watchpoints on expressions not involving memory
+
+  GDB now supports watching expressions that don't involve memory.
+  This allows, for example, watching for register changes.
+  E.g. "watch $pc" will do the right thing.
+
 * X86 general purpose registers
 
   GDB now supports reading/writing byte, word and double-word x86