with patch,

> This adds sentinal-frame.[hc] and then modifies the unwind code in frame.c so that it can exploit the fact that there is alway an extra frame beyond the inner most frame.  This has a dramatic effect on the unwind code - so much simpler!
> 
> It doesn't do things like ensure that there is always a frame or create the inner most frame from the sentinel frame.
> 
> To confuse the patch slightly, it also adjusts the id_unwind, pc_unwind and register_unwind function types.
> 
> Andrew
> 
>