[Bug breakpoints/29480] New: [gdb] watchpoint condition on specific value change

["vries at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=29480

            Bug ID: 29480
           Summary: [gdb] watchpoint condition on specific value change
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: breakpoints
          Assignee: unassigned at sourceware dot org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

Somebody asked on IRC:
...
is it possible to setup a watchpoint that fires off when the value of a memory
location changes from one specific value to another?
...

I wrote this test example:
...
$ cat test.c
int v = 0;

int
main (void)
{
  v = 1;
  v = 2;

  v = 3;
  v = 2;

  return 0;
}
$ gcc -g test.c
...
and decided I wanted to set a watchpoint on the change from 3 to 2.

I suppose it's possible to do this in python somehow, but I decided to try
something more simple.

I wrote this patch that introduces two new convenience variables:
...
diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
index 37f70a77721..90ec605aeeb 100644
--- a/gdb/breakpoint.c
+++ b/gdb/breakpoint.c
@@ -5062,7 +5062,10 @@ watchpoint_check (bpstat *bs)
                                                       new_val)))
        {
          bs->old_val = b->val;
+         set_internalvar (lookup_internalvar ("_wp_old_val"),
+                          bs->old_val.get ());
          b->val = release_value (new_val);
+         set_internalvar (lookup_internalvar ("_wp_val"), b->val.get ());
          b->val_valid = true;
          if (new_val != NULL)
            value_free_to_mark (mark);
...
and managed to do:
...
$ gdb -q -batch a.out -ex "watch v" -ex "cond 1 \$_wp_old_val == 3 && \$_wp_val
== 2" -ex run
Hardware watchpoint 1: v

Hardware watchpoint 1: v

Old value = 3
New value = 2
main () at test.c:12
12        return 0;
...

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