Re: some questions about ranged breakpoints

[Pedro Alves <pedro at codesourcery dot com>]

On Monday 10 October 2011 21:03:47, Tom Tromey wrote:
> While implementing the ambiguous linespec proposal, I've tripped across
> an oddity involving ranged breakpoints.  I don't think I have access to
> a machine that implements these, so I thought I'd ask here.

You can always hack the backend to report support.

> update_breakpoint_locations has:
> 
>   /* Ranged breakpoints have only one start location and one end location.  */
>   gdb_assert (sals_end.nelts == 0 || (sals.nelts == 1 && sals_end.nelts == 1));
> 
> But breakpoint_re_set_default has:
> 
> 	  expanded_end = expand_line_sal_maybe (sals_end.sals[0]);
> 
> expand_line_sal_maybe can return a symtabs_and_lines that has multiple
> results.  This will crash gdb.
> 
> I think you could construct a case using a function which is inlined.
> Then set a breakpoint like "break-range foo.h:7, +5".
> If breakpoint re-setting discovers multiple locations for the function
> (e.g., the inferior loads a .so that has inlined it), then it should
> crash.
> 
> What should actually happen here?

I think we should remove the assertion, and have each location map to a
hardware accelerated ranged breakpoint, instead of assuming there can
be only one.  This isn't much different from creating a regular
(non-range) hardware breakpoint that ends up mapping to more than
one location.

> I think the simplest approach would be to make such breakpoints not be
> resettable.  Failing that I suppose they could deactivate if resetting
> introduces ambiguity.  Any other ideas?  Any preferences?
> 
> Also if someone has a use-case for ranged breakpoints I would like to
> know what it is.  I couldn't think of a situation where I'd use them.

Maybe Thiago's original submission casts some more light.  I think this
is more useful for systems programming than regular app development.
E.g., it could be used toe.g., break if anything calls into anywhere
within the shared library mapped at "FOO, +BAR", or if anything does
a wild jump into this memory mapped area, cause I can't figure out
where the wild pointer is.

-- 
Pedro Alves