Re: frame->unwind->this_base()

[Andrew Cagney <ac131313@redhat.com>]

> On Sun, Mar 16, 2003 at 05:04:36PM -0500, Andrew Cagney wrote:
> 
>> 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.
> 
> 
> There's no guarantee right now that the DW_AT_frame_base agrees with
> the frame's base.  I don't even think it's necessary that they be the
> same.

There is certainly no guarentee that the DW_AT_frame_base and `struct 
frame_id.base' match.

However, shouldn't the only thing needing the `virtual frame pointer' / 
get_frame_base() be the code that needs a virtual base pointer when 
computing the value of a local variable?

Andrew