reverse-next doing repeated reverse-stepi's ?

reverse-next doing repeated reverse-stepi's ?

[Julian Smith]

I've been trying out reverse-next with UndoDB as a remote debug server,
and it looks like reverse-next works by doing repeated
`bs' (reverse-stepi) commands.

I was expecting it to set breakpoints and do `bc' (reverse-continue)
commands, by analogy with normal forwards step etc.

Am i doing something wrong here ? Or is this the expected behaviour ?

Thanks,

- Julian

-- 
http://undo-software.com/

Re: reverse-next doing repeated reverse-stepi's ?

[Michael Snyder]

Julian Smith wrote:
> I've been trying out reverse-next with UndoDB as a remote debug server,
> and it looks like reverse-next works by doing repeated
> `bs' (reverse-stepi) commands.
> 
> I was expecting it to set breakpoints and do `bc' (reverse-continue)
> commands, by analogy with normal forwards step etc.
> 
> Am i doing something wrong here ? Or is this the expected behaviour ?

Short answer: this is expected behavior.

Forward-next works the same way, in general.
Both forward-next and reverse-next will set breakpoints
under some circumstances, but will often work by
singlestepping.

Re: reverse-next doing repeated reverse-stepi's ?

[Greg Law]

Michael Snyder wrote:
> Julian Smith wrote:
>> I've been trying out reverse-next with UndoDB as a remote debug server,
>> and it looks like reverse-next works by doing repeated
>> `bs' (reverse-stepi) commands.
>>
>> I was expecting it to set breakpoints and do `bc' (reverse-continue)
>> commands, by analogy with normal forwards step etc.
>>
>> Am i doing something wrong here ? Or is this the expected behaviour ?
> 
> Short answer: this is expected behavior.
> 
> Forward-next works the same way, in general.
> Both forward-next and reverse-next will set breakpoints
> under some circumstances, but will often work by
> singlestepping.

If I understand correctly, when doing a 'next' gdb does something like:

while (PC in current line) {
  ptrace (PTRACE_SINGLESTEP)
  if (PC outside current function) {
    plant breakpoint at return address
    ptrace (PTRACE_CONT)
    remove breakpoint
  }
}

however, when doing a reverse-next, we don't see this.  We just see 
repeated reverse single-steps.  If trying to do a reverse-next over a 
non-trivial function, this can take a very long time.  Is this expected?


Greg

-- 
Greg Law, Undo Software                       http://undo-software.com/

Re: reverse-next doing repeated reverse-stepi's ?

[Michael Snyder]

Greg Law wrote:
> Michael Snyder wrote:
>> Julian Smith wrote:
>>> I've been trying out reverse-next with UndoDB as a remote debug server,
>>> and it looks like reverse-next works by doing repeated
>>> `bs' (reverse-stepi) commands.
>>>
>>> I was expecting it to set breakpoints and do `bc' (reverse-continue)
>>> commands, by analogy with normal forwards step etc.
>>>
>>> Am i doing something wrong here ? Or is this the expected behaviour ?
>> Short answer: this is expected behavior.
>>
>> Forward-next works the same way, in general.
>> Both forward-next and reverse-next will set breakpoints
>> under some circumstances, but will often work by
>> singlestepping.
> 
> If I understand correctly, when doing a 'next' gdb does something like:
> 
> while (PC in current line) {
>   ptrace (PTRACE_SINGLESTEP)
>   if (PC outside current function) {
>     plant breakpoint at return address
>     ptrace (PTRACE_CONT)
>     remove breakpoint
>   }
> }
> 
> however, when doing a reverse-next, we don't see this.  We just see 
> repeated reverse single-steps.  If trying to do a reverse-next over a 
> non-trivial function, this can take a very long time.  Is this expected?

No, it should work similarly to forward next.
When you have stepped backward into a function, it should
place a breakpoint at the entry point, run backward to that,
and then step back into the caller.