Re: frame->unwind->this_base()

[Andrew Cagney <ac131313@redhat.com>]

>> True dwarf2 debug info or that .eh_frame stuff (i'm curious)?
> 
> 
> Hmm, I thought it would write out .debug_frame without DWARF-2 but
> peering at the GCC source I seem to be wrong again.  So just .eh_frame.

So using .eh_frame is along the same lines as using ia64's libunwind. 
.eh_frame just happens to be implemented using something very like 
dwarf2's unwind code.

> In any case, we'll parse both, so I stand by my statement.  We'll have
> .eh_frame even without normal debug info.

I think there needs to be separate eh-frame and cfi-frame but with a 
common implementation.  That way the subtle, but important, differences 
are clear.

>> For stabs to work, it needs FRAME_LOCALS_ADDRESS(); and 
>> FRAME_LOCALS_ADDRESS() relies on the prologue analyzer (since frame ID 
>> won't correspond to `frame-base') for the computation of the correct 
>> value; and that means unwinding the same frame two ways.  Outch.
> 
> 
> Yeah...
>  - if we have CFI use it to find the frame address.  Does this
> become the frame ID?
>  - if we have dwarf2 debug and CFI, then we don't need to do prologue
> analysis; CFI should give us everything we need
>  - if we have stabs debug and CFI, then we do need to do prologue
> analysis to get FRAME_LOCALS_ADDRESS
>  - if we have either kind of debug info and no CFI then we need to do
> prologue analysis; for dwarf2 we'll also need to calculate the frame
> base from DW_AT_frame_base in order to use it to find locals
> 
> Is that about right?

Yes.  Try the following higher-level view of the problem:

On the left is the unwinder.  It exports methods to obtain the frame's 
ID and the registers.  It can be implemented using CFA, EH, libunwind, 
prologue analysis, ...; and the implementation is selected based on the 
low-level unwind information, or lack there of.

On the right is the local variable code and that needs a frame-base / 
frame-locals-address / ....  It uses high-level debug info and unwound 
register values to compute that base.  It can be implemented using 
dwarf2's frame_base, or prologue analysis (stabs), or ...; and the 
implementation is selected based on the frame's high-level debug info.

The mess occures when the high-level RHS frame-locals-address starts 
assuming the flavour of the low-level LHS unwinder and, consequently, 
tries to directly exploit that knowledge.  For instance, a RHS prologue 
based frame-locals-address assuming that the LHS is also prologue based, 
and hence, can directly access the LHS's prologue analysis cache.

It can be `fixed' two ways:

- refusing to allow that sharing of data, forcing the RHS 
frame-locals-address to re-analyze the prologue.

- make it possible to tease out the prologue analysis object so that 
both the LHS and RHS can share it.

I guess the second is it.

Andrew