Re: [RFA] Reverse Debugging, 1/5

[Michael Snyder <msnyder@vmware.com>]

Joel Brobecker wrote:
>> 2008-09-30  Michael Snyder  <msnyder@vmware.com>
>>       Target interface for reverse debugging.
>>       * target.h (enum target_waitkind):
>>       Add new wait event, TARGET_WAITKIND_NO_HISTORY.
>>       (enum exec_direction_kind): New enum.
>>       (struct target_ops): New methods to_set_execdir, to_get_execdir.
>>       * target.c (target_get_execdir): New generic method.
>>       (target_set_execdir): Ditto.
> 
> One of the questions I'm asking myself is why having the get_execdir
> method? It seems that, once we have called "target_sets_execdir" and
> it hasn't returned ERROR, core GDB should know what the execution
> direction is, no? Is there a situation where a round-trip to the
> target would be necessary?  Otherwise, we'll end up with the target
> code all doing the same thing, which is caching the current value
> of the same thing.
> 
> One thing that crossed my mind while thinking about it is whether
> we want to make this property global to all inferiors or specific
> to each inferior. Ahem, shall we say global?

It was a design choice.

There were two choices:
1) modify target_resume (ops->to_resume), to add a direction
parameter.
2) Add a to_set_direction target method.

The first would have required modifying all existing targets,
so I chose the second.

Once you have a "set" method, it seems logical to have a
"get" method.


Making no assumptions about HOW the target sets the direction,
it seems likely that at least *some* targets will have to
remember this state locally.  Whereas there is no reason
for core-gdb to have to remember the state locally, if it can
always get it from the target.

It seems a worse duplication to keep the same state information
simultaneously in the target and in the core, since now you
have to worry about them getting out of sync.

At worst, a target will need to maintain an int's worth of state
locally, and so long as we're never running two targets at the
same time, there's no synchronization issue.