Re: [discuss] semantics, "replay debugging" vs. "reverse debugging"

teawater
2008-10-23T03:46:21Z

I heard some non-opensource implement of reverse. Do you have some
message about them?

On Thu, Oct 23, 2008 at 03:37, Jakob Engblom <jakob@virtutech.com> wrote:
>> > Sorry I am wrong.
>> > In ARM, Just adds set cpsr reg. So:
>> > add r0,r0,#10
>> > Can reverse without record too.
>>
>> OK, well, I was really speaking in the abstract.
>> I only meant "it's possible to imagine a target or
>> architecture in which reverse execution can be done
>> by some means other than record/replay".
>>
>> Didn't necessarily mean that it could be done on any
>> real, existing architecture.
>
> Just to add some more to this:
>
> 1. Simics is such a target, as is really any deterministic simulator that is
> running without any asynchronous (from the perspective of the simulator program)
> inputs. Typical cases involve booting a machine, which tends to be
> non-interactive. Or running a program that has all its input data compiled into
> it or read from some place that is already on the target. And you could say
> that this is a degenerate form of record, since you DO have to "record" the
> initial state in some way, even if the initial state is something that you write
> yourself as a simulator configuration. It is recorded in a form that can be
> brought back identically.
>
> 2. There are some odd-ball ideas in computer architecture close to the thinking
> behing transactional memory where you do checkpoint and restore and reexecute
> inside an actual physical CPU core. These might also for limited scopes support
> reversing without replay.
>
> But in general, the only way to get back to an earlier state is to record how to
> get there, usually from some even earlier point. Alternatively, use a
> tape-recorder analogy and just have a limited buffer with interesting data.
>
>
> Best regards,
>
> /jakob
>
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