Re: [PATCH 2/4] Add a dwarf unit type to represent 24 bit values.

[John Darrington <john@darrington.wattle.id.au>]

On Thu, Sep 27, 2018 at 01:53:35PM -0400, Simon Marchi wrote:
     
     After a short discussion with John on IRC, what I understand is that the
     debug info he is dealing with encodes the 24-bit addresses on 32-bits,
     with the most significant byte equal to zero.  Therefore that revised
     patch looks ok to me.  I would just ask you to add a comment explaining
     that this "case" exists for producer XYZ on S12Z, which encodes 24 bit
     addresses on 32 bits.  Maybe that in the future somebody will stumble on
     a producer that encodes 24 bit addresses on 24 bits.  Having an
     explanation for why things are the way they are will help.
     

When I look at the situation in more detail a number of things are
apparent:

1.  The dwarf .frame_debug CFI information does indeed code these values
    as 32 bits.

2.  This would seem to contradict the dwarf 2.0 standard which says they are 
    "addressing-unit sized values".

3.  objdump expects these values to be 24 bits.

4.  However readelf expects them to be 32 bits.

5.  Notwithstanding the above, the produced dwarf which I have, contains
    lots of FDE entries, with lots of CFIs.  However the contents of all 
    CFIs are empty.   Consequently it's of little use.

Given the above, I think the best course of action is to do nothing, on 
this point.

So I'll do without a dwarf unwinder until there's a producer which
actually produces something useful.

J'