[rfc] Add per frame frame_saved_pc()

[Andrew Cagney]

> On Sep 19, 11:30pm, Andrew Cagney wrote:
> 
> 
>> Back to (trumpet noises) CFI and more fall-out from the MIPS and generic 
>> dummy frames.
>> 
>> - The CFI doco describes a ``return_address_register'' which ``[...] 
>> might not correspond to a machine register''.  This means a CFI specific 
>> frame saved PC method.
> 
> 
> I'm not certain that this should (necessarily) drive our decisions.

There are currently three (er 4) frame_saved_pc() functions:

frame_saved_pc(dummy)
frame_saved_pc(CFI)
frame_saved_pc(``old style with frame init saved regs'')
frame_saved_pc(NULL) === read_pc() (but only sort of)

>> - Given a generic dummy frame, it knows exactly where the callers frame 
>> PC is.  No need to ask the target code.
>> 
>> With this in mind, I'd like to propose making frame_saved_pc() a 
>> per-frame method.
> 
> 
> I'd be willing to look at a patch.  In particular, what I'm curious
> about it to see the places where the frame_saved_pc field is
> initialized in the frame.  I can picture how the generic dummy
> frame code would look, but what I'm less clear about is what it'd
> look like for normal frames.  It seems to me that you have two choices,
> 1) have a generic method which falls back on the current machinery, or
> 2) have architecture specific methods.  I'm more interested in how (2)
> will be accomplished.

If I get the code right, I won't be changing any targets :-^  The target 
frame_saved_pc() code would, simply, not be called in certain cases.

It would be handled the same way as the unwind code - See 
set_unwind_by_pc().

Andrew