Re: [rfa:breakpoint] Correctly count watchpoints

[Andrew Cagney <ac131313@redhat.com>]

> On Sun, 29 Sep 2002, Andrew Cagney wrote:
> 
> 
>> (per earlier post) An expression like:
>> 
>> 	a + b
>> 
>> requires two watchpoint resources (&a and &b).
> 
> 
> What do you mean by ``watchpoint resources''?  On a i386, watching &a 
> might require much more than a single debug register, depending on a's 
> size and alignment.

At this point I'm trying desperatly to not define it :-) Each watch 
element / location / value in the watchpoint expression is assumed to 
consume one watch resource.

Anyway, the problem you refer to is why I was thinking of re-defining 
TARGET_REGION_OK_FOR_HW_WATCHPOINT() so that it returns the number of 
watchpoint resources required to watch addr/len.  If {&a, sizeof a} 
required two registers it could return two.

Such a model is obviously simple and will be sub-optimal in many 
situtations.  I think its sufficient though.

A more complete model would involve extending/generalizing what Marko 
Mlinar has implemented for the opencore:
http://sources.redhat.com/ml/gdb/2002-09/msg00308.html

>> When first creating the 
>> watchpoint, gdb correctly counts this as two.  However,  when GDB goes 
>> back to compute the number of watchpoints already used, it does a 
>> re-count and treats the above (and any watchpoint expression) as only one.
>> 
>> The attached, I belive, fixes this by saving the mem_cnt that was computed.
> 
> 
> I agree that the count should at least be consistent.
> 
> 
>> One thing I wonder about though, should ``info breakpoints'' or ``maint 
>> info breakpoints'' display this info?
> 
> 
> Only if it's useful.  Could you please make a concrete suggestion as to 
> what should be printed about this information?

I think it would be helpful if, at least in maintainer mode, the user 
could see how many resources have been allocated to a watchpoint.  That 
way (as well as letting me check its working :-) the user would be in a 
better position to figure out where, exactly, all their watchpoints have 
gone.

>> Ok?
> 
> 
> Fine with me.

(I've a sinking feeling that hardware breakpoints have the same problem 
...).

Andrew