linker debug info editing

[Alan Modra <amodra@bigpond.net.au>]

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This is a first pass at debug info editing to remove bogus entries for
link-once functions.  I'm not going to commit it just yet as the only
testing I've done is linking a current libstdc++ and examining the
results with readelf, and there are some as yet unresolved issues.

I'm posting now because I'd like some feedback/review, particularly on
the overall design assumptions as detailed in the following comment.
Another thing I'm unsure of at this point is whether I should remove
the CU header when all following info is removed in .debug_info,
.debug_line, and .debug_aranges sections.  I know one consumer (readelf)
became confused when .debug_aranges had an empty unit.  Perhaps other
consumers expect some relationship between the number of CUs in
different debug sections?

One known issue is that .debug_pubnames and .debug_pubtypes have a
debug_info_length field, and I guess I need to edit this when shrinking
.debug_info.

/* This file implements removal of DIEs for discarded link-once sections.
   Such DIEs are detected by parsing debug sections to find DIE boundaries,
   then examining relocations for each DIE.  Any DIE containing a field
   with a relocation against a symbol in a discarded section is marked for
   removal.  A removed DIE in .debug_info results in all of its children
   and associated data in .debug_loc and .debug_ranges being removed too.
   A removed CIE in .debug_frame results in all FDEs using that CIE
   being removed, but no attempt is made to remove usused CIEs (as can
   happen when all of a CIE's FDEs are removed).  Likewise, other unused
   shared info, ie. .debug_abbrev and .debug_str entries, are not removed.

   We assume that
   a) debug section relocations are sorted by r_offset,
   b) .debug_info location lists (references to .debug_loc) occur
      in increasing order of offset into .debug_loc,
   c) .debug_info ranges (references to .debug_ranges) occur
      in increasing order of offset into .debug_ranges,
   d) entries in .debug_loc and .debug_ranges are not shared,
   e) FDEs always occur at higher offsets than their associated CIEs
      in .debug_frame.

   ld -r using perverse linker scripts can break the first three
   assumptions.  */

-- 
Alan Modra
IBM OzLabs - Linux Technology Centre

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