Re: [RFC][gdb/cli] Add convenience vars _wp_old_val and _wp_val

[Pedro Alves <pedro@palves.net>]

On 2022-09-30 10:16 a.m., Tom de Vries via Gdb-patches wrote:
> Hi,
> 
> Add convenience variables _wp_old_val and _wp_val, that match the reported
> values of a stopped watchpoint:
> ...
> Hardware watchpoint 2: v
> 
> Old value = 3
> New value = 2
> main () at watchpoint-convenience-vars.c:12
> 12        return 0;
> (gdb) print $_wp_old_val
> $1 = 3
> (gdb) print $_wp_val
> $2 = 2
> ...
> 
> These can be used in a watchpoint condition, for instance to only stop when
> changing from one value to another value:
> ...
> (gdb) cond 2 $_wp_old_val == 3 && $_wp_val == 2
> ...

Cool.  I like it.  Can also be used in the watchpoint's commands.

It can't be safely used in regular CLI after the watchpoint hit is process, though,
given another watchpoint may trigger meanwhile.  I mean, say, in non-stop, you do:

(gdb) c -a &

Hardware watchpoint 2: v

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

# time passes, you type other gdb commands.

(gdb) print $_wp_old_val # you want to look at wp's 2 value, but just as you're
                         # typing this, another watchpoint triggers:

Hardware watchpoint 3: o

Old value = 4
New value = 5
main () at foo.c:14
14        return 2;

(gdb) print $_wp_old_val
$123 = 4

# whoops


A way to make this safer would be to make it a convenience function instead, that
takes as argument the number of the watchpoint, like:

(gdb) p $_wp_old_val(2)

For the condition example, you'd write:

  (gdb) cond 2 $_wp_old_val($bpnum) == 3 && $_wp_val($bpnum) == 2

I guess that isn't as convenient.  Unless...  I guess the convenient functions
could default to $bpnum if no argument is passed?  That'd reduce it to:

  (gdb) cond 2 $_wp_old_val() == 3 && $_wp_val() == 2

If that's still too much, I guess we could have both separately named convenience variables
and convenience functions.

> RFC for now, so no docs yet.
> 
> What about naming?  Maybe _wp_old_value and _wp_new_value are more intuitive
> because they match the "Old value = _/ New value = _" message?

I'm fine with the shorter names as you proposed.