Re: [remote protocol] step range?

Jason Molenda
2008-09-06T00:17:10Z

On Sep 4, 2008, at 7:38 PM, Michael Snyder wrote:

> I don't think that's necessarily true -- the remote agent
> could just do what gdb does, single-step repeatedly and
> check the stop pc against the range.

Yeah, that'll work as long as you have some form of single-instruction-
step support in your target environment. If not, then you'll need a
disassembler to (a) determine the length of the current instruction so
you can overwrite the next instruction with a trap opcode, and (b)
determine if the current instruction branches/calls/jumps anywhere.
It quickly becomes Complicated. I'm assuming you have some form of
single-instruction-step in the target you're interested in, otherwise
I council against pursuing this. :)

For what it's worth we use the remote protocol for debugging
applications on the iPhone / iPod Touch devices. When we first got it
up and running, we saw command-line level "step" commands taking
multiple (4-5!) seconds to complete. We optimized it to no end and
got this down to something like .2 seconds without doing anything too
weird to the protocol. We didn't have any single-instruction-step
feature so we didn't even consider trying to push range-stepping down
to the device.

But I don't see any problems with adding this stepping capability for
environments that could make use of it.

> Well, if the remote can deal with threads at all (eg. gdbserver),
> then it could probably treat this just as gdb would. A preemptive
> stop in another thread would be outside the step range, therefore
> we would tell gdb that we stopped.

Since we've established that you must have single-instruction-step
capability in the target to do this, I think it's safe to assume that
only the current continue thread will execute. But as you say, if the
remote agent determines that it stopped in a different thread than it
began the step, it can give up and return control to gdb.

J