Re: [rfa:breakpoint] Correctly count watchpoints

[Eli Zaretskii <eliz@is.elta.co.il>]

On Mon, 30 Sep 2002, Andrew Cagney wrote:

> On the i386, one watch resource is two registers.

Why two?  Some expressions might need 3 registers.  If you use this 
worst-case scenario, GDB will think it cannot watch more than a single 
expression, and that some data types, such as double's, and complex 
aggregates, such as struct's, cannot be watched at all.  It's hardly a 
Good Thing to refuse to set watchpoints based on inaccurate decisions 
like this.

> > 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.

This requires a significant change in the high-level code of GDB: it 
needs to pass all the information about all the ``active'' watchpoints to 
the function that tells how many watchpoint resources are required for 
the next watchpoint.

> 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.

So that means an architecture should know about GDB's expression-parsing 
code.  In effect, we are going to have the arch-specific code be tightly 
coupled with arch-independent code in breakpoint.c and friends.

> 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.

As I write above, overriding the default model is not enough, since the 
application-level code doesn't feed the architecture with enough info.