[PATCH] ld: Allow section groups to be resolved as part of a relocatable link

[Andrew Burgess]

Alan,

Any followup to the below?

Thanks,
Andrew

* Andrew Burgess <andrew.burgess@embecosm.com> [2017-04-13 23:07:13 +0100]:

> * Alan Modra <amodra@gmail.com> [2017-04-04 19:31:21 +0930]:
> 
> > On Fri, Mar 24, 2017 at 03:11:29PM +0000, Andrew Burgess wrote:
> > > This commit adds a new linker feature: the ability to resolve section
> > > groups as part of a relocatable link.
> > 
> > What happens with dwarf debug or .eh_frame info for discarded
> > sections?
> 
> Alan,
> 
> Thanks for the feedback, sorry for the slow reply.
> 
> I wasn't totally sure what you're asking for, as far as I'm aware
> debug and eh_frame sections will work largely as before.  However,
> given your question I suspect you've spotted something that I'm still
> not getting.
> 
> Below I've written down how I _think_ things work.  I'm guessing that
> there's more complexity in some aspect that I'm missing (and that
> you've spotted).
> 
> Currently, if we do a relocatable link of object_a and object_b into
> object_ab, then fully link this into exe_ab I believe that we expect
> the following to happen:
> 
>  1. object_a and object_b contain section groups for some .text*
>  sections.
> 
>  2. object_a and object_b contain .debug* that reference symbols in
>  .text* sections, some these sections will be in section groups, and
>  some will not.
> 
>  3. During the partial link we merge the section groups, discarding
>  all but one copy of each.  References to symbols in discarded
>  sections are updated to reference the symbols in the kept copy.
> 
>  4. During the final link the section group is discarded, and the
>  members become "regular" sections, to be placed in the normal way.
>  Symbol references in the .debug* are resolved to the single copy of
>  each symbol we now have.
> 
> If the new feature I propose is not used (by the user) then I expect
> nothing to change.
> 
> If the user requests that groups be resolve at partial link time then
> I expect:
> 
>   1. As above.
> 
>   2. As above.
> 
>   3. During partial link we discard all section groups, and all but
>   one copy of each member of the section groups.  The one remaining
>   section group members become "regular" sections and are placed in
>   the normal way.  References to symbols in the discarded section
>   groups are updated to reference the symbols in the kept sections.
> 
>   4. During the final link there are now no section groups.  All
>   sections are placed in the normal way and relocations are patched as
>   normal.
> 
> If the user requests that groups NOT be resolved during the final link
> then I expect to see this behaviour:
> 
>   1. As above.
> 
>   2. As above.
> 
>   3. Back to the original behaviour; During the partial link we merge
>   the section groups, discarding all but one copy of each.  References
>   to symbols in discarded sections are updated to reference the
>   symbols in the kept copy.
> 
>   4. During final link section groups are merged.  All but one copy of
>   each of the group members is discarded, references to symbols in
>   discarded sections are updated to reference the symbols in the kept
>   copy.  The group members will most likely be placed as orphan
>   sections with the linker picking a location for the section to
>   live.  As this is a final like relocations in the debug* sections
>   will be resolved in the normal way.
> 
> I'd expect eh_frame sections to be handled in exactly the same way.
> 
> Any guidance you could offer would be gratefully received.
> 
> 
> Thanks,
> Andrew