[Bug default/26684] abidiff says that some bit field elements missing in version of binary with dwarf5 debuginfo.

["mark at klomp dot org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=26684

--- Comment #7 from Mark Wielaard <mark at klomp dot org> ---
There are a couple of issues in the libabigail code.

It has some code for die_unsigned_constant_attribute which relies on specific
FORMs which doesn't handle DW_FORM_implicit_const (and technically
DW_FORM_implicit_const represents a signed value). Ideally libabigail shouldn't
have to deal with forms directly but should be able to rely on the dwarf_attr
functions in libdw to get attribute values so it doesn't have to deal with any
particular data representation. In particular handling of 
DW_AT_byte_size and DW_AT_bit_size should probably simply use and dwarf_attr
plus dwarf_formudata.

Secondly in libabigail only handles DW_AT_data_member_location, but not
DW_AT_data_bit_offset. Technically DW_AT_data_bit_offset is already in DWARF4,
but gcc only outputs it for DWARF5 (from gcc/dwarf2out.c):

      /* While DW_AT_data_bit_offset has been added already in DWARF4,
         e.g. GDB only added support to it in November 2016.  For DWARF5
         we need newer debug info consumers anyway.  We might change this
         to dwarf_version >= 4 once most consumers catched up.  */

Note that if there is a DW_AT_data_bit_offset instead of a
DW_AT_data_member_location then there will be no DW_AT_byte_size and no
DW_AT_bit_offset.

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