Re: [rfa:breakpoint] Correctly count watchpoints

[Andrew Cagney <ac131313@redhat.com>]

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[I had an unscheduled e-mail outage]  My reply is attached.

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From: Andrew Cagney <ac131313@redhat.com>
To: Eli Zaretskii <eliz@is.elta.co.il>
Cc: gdb-patches@sources.redhat.com
Subject: Re: [rfa:breakpoint] Correctly count watchpoints
Date: Mon, 30 Sep 2002 15:18:43 -0400
Message-ID: <3D98A393.6010801@redhat.com>

<div class="moz-text-flowed" style="font-family: -moz-fixed">>> Date: Mon, 30 Sep 2002 12:34:53 -0400
>> From: Andrew Cagney <ac131313@redhat.com>
>> 
>> Each watch 
>> element / location / value in the watchpoint expression is assumed to 
>> consume one watch resource.
> 
> 
> Given that this assumption doesn't hold on at least one very popular
> architecture, is it a useful assumption?

I think the model holds.  It just leads to an inefficient allocation of 
watchpoint resources.  On the i386, one watch resource is two registers.

>> 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.
> 
> 
> But this is very hard or even impossible to do in practice.  For
> example, on a i386, if there are two watchpoint that watch the same
> 4-byte aligned int variable, you need only one debug register to watch
> them both, so counting each one as taking one resource is incorrect.

That is a bug.  A further change would be to accumulate all the regions 
and eliminate any overlap from the count.  I don't know how often this 
happens in real life.

> But you cannot return the correct result unless you are presented with
> the entire list of watchpoints GDB would like to set.  Alas, GDB's
> application code examines the watchpoints one by one and queries the
> target vector about each one of them in order.  Thus, the target
> vector doesn't see the whole picture and therefore cannot give the
> right answer.

For an architecture to try and optimally allocate watchpoint resources, 
I don't think (cf opencore code) a list of ADDR:LEN pairs is sufficient. 
  Instead it should be provided with all the watchpoint expressions.

> What is the value of the result you get if we _know_ in advance that
> it will be incorrect, sometimes grossly incorrect, in some not very
> rare cases?

I think the model is sufficient for the common case - a few independant 
variables and no complex expressions.   To follow through the opencore's 
code, an extension would be to let an architecture define its own more 
complex model, overriding this default.

For instance, the hw_resources_used_count() function in my other patch 
could be made part of the architecture vector so that architectures, 
such as the i386, could override the default model using some other type 
of allocation scheme.

>> 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.
> 
> 
> If this is for maintainers, the count should be accurate.  The i386
> native debugging implements a maintainer-mode command to do that, but
> it manipulates target-side data, and only works after all watchpoints
> have been inserted.

True, there are several pieces of information:
- how many resources GDB thinks it is consuming
- how efficiently GDB is assigning those resources to hardware.
Sounds like the information is watchpoint model dependant.

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

I'll revise the counts.

Andrew


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