Identifying a dummy frame using a frame id

[Andrew Cagney]

>> Now, there is a little detail missing - how GDB relocate the dummy frame 
>> object that contains those saved registers.  The problem is that GDB 
>> can't just take the most recent one as, due to long jumps and the like, 
>> it can be wrong.
> 
> 
> I can't comment on the internal mechanism, but I know that as a user of
> gdb, if something happens during a hand function call to marker2() like
> gdb hitting another breakpoint or me hitting ^C, then gdb starts acting
> a little drunk at that point.  So I bet that explicit save/restore of
> these frames would help.

FYI, that's a different problem.

This proposal just refines the mechanism used to identify a dummy frame. 
  For something like the i386, there would be no immediate change in 
behavior.

Andrew