Re: [PATCH 3/5] range stepping: gdb

Eli Zaretskii
2013-05-15T13:46:33Z

> Date: Wed, 15 May 2013 13:39:05 +0100
> From: Pedro Alves <palves@redhat.com>
> CC: gdb-patches@sourceware.org
>
> > Doesn't this mean that these two use cases are explicit exceptions
> > from the rule that END is excluded?
>
> Nope. There's no exception.
>
> With:
>
> vCont ;r START,END
>
> #1 - The stub single-steps the thread.
> #2 - Once the thread stops, the stub checks whether the thread
> stopped in the [START,END) range. If so, goto #1.
> It not, goto #3.
> #3 - The stub reports to gdb that the thread stopped stepping.
>
> If it happens that START and END are the same, then #2 always
> goes to #3.

I'm simulating a naive reader, while you are replying to someone you
consider an experienced code developer ;-) So we are talking past
each other.

When you say "END is the address of the first instruction beyond the
step range", that means, simply put, that execution will always stop
before it executes the instruction at END. IOW, the instruction at
END will _not_ be executed. With that interpretation, a range
[START,START) is _empty_ and will never execute any instructions at
all.

It is OK to use a different interpretation, but then we should either
(a) describe the semantics differently to begin with, or (b) explain
that [START,START) is an exception. You seem to object to (b), which
then brings us back at (a), meaning that this text:

> +@var{end} is the address of the first instruction beyond the step
> +range, and @strong{not} the address of the last instruction within it.

needs to be reworded, so as not to say that END is _beyond_ the range.

If you want a specific response for the algorithm you show above, then
I would ask why does GDB single-step the stub at all, when START and
END are equal? The fact that we implemented this is a 'do-until' loop
rather than a 'while' loop, i.e. test at the end instead of at the
beginning, is an important implementation detail which must be present
explicitly in the description of what this feature does. If you hide
it behind the [a,b) notation, you get a problem you will need to
explain, as we see.

> When I said:
>
> "(This has the property that @var{start} == @var{end} single-steps
> once, and only once, even if the instruction at @var{start} jumps to
> @var{start}.)"
>
> I was trying to clarify the case of the instruction at START being:
>
> jump START
>
> Then,
>
> vCont ;r START,START
>
> always single-steps once, and only once, instead of
> continuously single-stepping that instruction without
> reporting to GDB.

The very need you felt to explain this is already a clear sign that
the original description is wrong.