[Dwarf-Discuss] Split Dwarf vs. CU DW_AT_ranges / DW_AT_low_pc placement

[Jakub Jelinek]

On Thu, Mar 11, 2021 at 01:05:06AM -0800, David Blaikie wrote:
> What's your take on:
> 
> 1) Fixing GDB to handle GCC's current output.

I don't know what GDB will do, it is up to the GDB people.

> 2) Fixing GCC to produce something maybe more standards conforming (to my
> mind, ideally: ranges on the skeleton CU (using either
> rnglists_base+rnglistx (like LLVM), or sec_offset (actually more
> compact/better than LLVM anyway, and avoids the ambiguous situation), and
> rnglistx in child DIEs the split full unit using using .debug_rnglists.dwo)

Given the
3.1.3 "The following attributes are not part of a split full compilation
unit entry but instead are 18 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."
sentence, at least for DWARF5 putting DW_AT_ranges into the full unit rather
than skeleton unit for split DWARF seems like non-conforming, so I'll
probably adjust my patch, but see below.
Now, for DW_AT_addr_base and DW_AT_rnglists_base the spec talks about
it affecting just .debug_addr or .debug_rnglists section, doesn't mention
the .debug_rnglists.dwo section, while for DW_AT_str_offsets_base it talks
about .debug_str_offsets or .debug_str_offsets.dwo.
So, maybe one reason why DW_AT_rnglists_base might be ok on the skeleton
unit.  On the other side, e.g. in Table F.1 I see there for Skeleton and Split:
DW_AT_low_pc - Skeleton only
DW_AT_ranges - Split Full only (so, in contradition of 3.1.3)
DW_AT_rnglists_base - not present
So, DWARF5 is inconsistent.  But appendix F is informative and so I think
the normative 3.1.3 wins.

So, I think I'll go with DW_AT_ranges and DW_AT_low_pc in
DW_TAG_skeleton_unit, but the former using DW_FORM_sec_offset rather than
DW_FORM_rnglistx and no DW_AT_rnglists_base (I really don't see a benefit
of that, there is one relocation either way, either on the DW_AT_ranges
with DW_FORM_sec_offset or on DW_AT_rnglists_base with DW_FORM_sec_offset,
but for the latter one needs one byte for the DW_FORM_rnglistx too and
two extra bytes in .debug_abbrev.  DW_FORM_rnglistx can be beneficial if there
is more than one range, which is not the case for the skeleton.
But .debug_rnglists I'll probably use the *x suffixed DW_RLE_* opcodes when
DW_RLE_offset_pair can't be used even in the skeleton .debug_rnglists.

	Jakub