frame->unwind->this_base()

[Andrew Cagney <ac131313@redhat.com>]

At present there is a per-frame ID method since different frames 
determine their ID using different techniques.  The ID (which identifies 
a given frame instance) includes a base and pc/func value.

GDB's frame code also makes available the get_frame_base() method. 
While the default implementation returns get_frame_id().base, I think 
there is going to need to be a per-frame frame->unwind->this_base method.

For dwarf2 frames, it would return, DW_AT_frame_base.  For prologue 
frames, it would return an attempt at an equivalent value.  Hopefully it 
wouldn't be called for other frame types :-).

It might even be reasonable for a prologue based unwinder to error out 
when asked for the frame's base before the stack frame has been created.

Thoughts?

I should note that dwarf2expr.c contains code that tries to 
locally/directly evaluate the frame base.  I think that should instead 
do a get_frame_base() call.

Andrew