[Dwarf-Discuss] More on DW_AT_str_offset_base debug_str_offsets.dwo confusion

[David Blaikie]

On Mon, Aug 31, 2020 at 10:33 AM David Anderson via Dwarf-Discuss <
dwarf-discuss at lists.dwarfstd.org> wrote:

> I has occurred to me that simply restricting skeleton CUs
> to use DW_FORM_string or DW_FORM_strp
> would restore the unique meaning of DW_AT_str_offsets_base
> to apply to the dwp  (letting non-skeleton CUs use
> DW_FORM_strx1 etc ).  With seemingly little impact on
> overall size.
>

Seems a pity for orthogonality (and for a non-standard/extension use that
LLVM has, where skeleton units carry some DIEs (essentially "gmlt"-like
data, enough to symbolize with inline stack frames) in the skeleton CU -
not being able to used indexed strings would be an object size penalty due
to potentially needing to use more relocations)

I think the DWARFv5 spec is a bit conflicted, but does have some wording
that supports LLVM's existing usage:

".debug_info.dwo to .debug_str_offsets.dwo: Attribute values of class
string may have one of the forms DW_FORM_strx, 3 DW_FORM_strx1,
DW_FORM_strx2, DW_FORM_strx3 or 4 DW_FORM_strx4, whose value is an index
into the .debug_str_offsets.dwo section for the corresponding string"
"The string table section in .debug_str.dwo contains all the strings
referenced from DWARF attributes using any of the forms DW_FORM_strx,
DW_FORM_strx1, DW_FORM_strx2, DW_FORM_strx3 or DW_FORM_strx4. Any attribute
in a compilation unit or a type unit using this form refers to an entry in
that unit?s contribution to the .debug_str_offsets.dwo section, which in
turn provides the offset of a string in the .debug_str.dwo section."

 (& some that contradicts it):

"The DW_AT_addr_base and DW_AT_str_offsets_base attributes provide context
that may be necessary to interpret the contents of the corresponding split
DWARF object file."
"The following attributes are not part of a split full compilation unit
entry but instead are inherited (if present) from the corresponding
skeleton compilation unit: DW_AT_low_pc, DW_AT_high_pc, DW_AT_ranges,
DW_AT_stmt_list, DW_AT_comp_dir, DW_AT_str_offsets_base, DW_AT_addr_base
and DW_AT_rnglists_base."

I'd rather go with LLVM's existing interpretation - that strx encodings
used in .dwo do not attempt to use str_offsets in the skeleton.
But I wouldn't mind adding a str_offsets_base to the split full unit to
make it clear - this would be consistent with rnglists, I think? (I think,
in theory a rnglistx in a .dwo with a split full unit without a
rnglists_base would use the rnglists_base (and .debug_rnglists non-dwo) in
the executable, but if the split full unit has a rnglists_base, then the
rnglistx in the split full unit use that base to find rnglists in
debug_rnglists.dwo - arguably I'd say we might as well say the same thing
about loclists, too, for consistency, though I don't have any use for
skeleton location lists right now)
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