[Bug default/28642] New: vtable offset position seems wrong

["woodard at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=28642

            Bug ID: 28642
           Summary: vtable offset position seems wrong
           Product: libabigail
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: default
          Assignee: dodji at redhat dot com
          Reporter: woodard at redhat dot com
                CC: libabigail at sourceware dot org
  Target Milestone: ---

Build libabigail with both gcc and clang then compare the resulting library
with abidiff

$ abidiff g5/lib/libabigail.so.0.0.0 l5/lib/libabigail.so.0.0.0 

the gcc object can be found:
http://ssh.bencoyote.net/~ben/libabigail.so.0.0.0.gcc.bz2 while the clang
object can be found: https://sourceware.org/bugzilla/attachment.cgi?id=13812

There seems to be something wrong with the vtable DWARF parsing or comparison.
it kind of looks like it may be a latent bug in gcc's vtable position numbering
which may not have been detected because it was encoded the same way and
therefore compared as equivalent.

There are many differences reported along like this:

 [C] 'method virtual abigail::ini::config::section::~section(int)' at
abg-ini.h:386:1 has some indirect sub-type changes:
    the vtable offset of method virtual
abigail::ini::config::section::~section(int) changed from 18446744073709551615
to 0
      note that this is an ABI incompatible change to the vtable of class
abigail::ini::config::section
    parameter 1 of type 'int' was removed

  [C] 'method virtual abigail::ini::config::~config(int)' at abg-ini.h:342:1
has some indirect sub-type changes:
    the vtable offset of method virtual abigail::ini::config::~config(int)
changed from 18446744073709551615 to 0
      note that this is an ABI incompatible change to the vtable of class
abigail::ini::config
    parameter 1 of type 'int' was removed

Note that the vtable offset for GCC is 18446744073709551615 which happens to
be: 0xFFFF FFFF FFFF FFFF so it looks like a decoding error.

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