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

[Simon Marchi]

On 2021-03-10 10:59 a.m., Jakub Jelinek via Dwarf-Discuss wrote:> Hi!
> 
> We got a report today that GCC even for -gdwarf-5 -gsplit-dwarf uses
> .debug_rnglists section + DW_AT_ranges + DW_AT_low_pc + DW_AT_rnglists_base
> attributes in the DW_TAG_skeleton_unit (and then some DW_AT_ranges in
> .debug_info.dwo that use DW_FORM_rnglistx, but no .debug_rnglists.dwo
> section).  That is the same spot where
> the DWARF 4 split DWARF extension was using, but the .debug_ranges
> section wasn't really placeable into dwo files.
> So, my first question is whether that is really invalid DWARF 5 or not.
> 
> LLVM seems to emit instead:
> .debug_rnglists section + DW_AT_ranges + DW_AT_low_pc + DW_AT_rnglists_base
> attributes in the DW_TAG_skeleton_unit (but .debug_rnglists only contains
> the ranges for the whole CU) and then
> .debug_rnglistsx.dwo section and DW_AT_ranges in .debug_info.dwo (not on the
> DW_TAG_compile_unit though) that refer to that.
> 
> Would it be ok to emit only DW_AT_low_pc in DW_TAG_skeleton_unit and
> .debug_rnglists.dwo section and DW_AT_ranges (but not DW_AT_rnglists_base)
> in DW_TAG_compile_unit?  I mean, is there any benefit in having the CU
> ranges on the skeleton over .debug_info.dwo?  Do split debug info consumers
> need to essentially merge the attributes from the skeleton unit and
> corresponding compile unit?

Just for the record, I think I stumbled on those differences earlier
when fixing GDB consuming clang's DWARF 5 range lists.  I asked on the
gdb-patches list:

  https://sourceware.org/pipermail/gdb-patches/2020-November/173053.html

Mark Wielaard's opinion was that both ways were valid.  I'll follow the
discussions here to see if the community settles on another
interpretation.

Simon