# Allow alternate encoding of DW_AT_object_pointer as a variable index instead of DIE reference
## Background `DW_AT_object_pointer` is used by LLDB to conveniently determine the CV-qualifiers and storage class of C++ member functions when reconstructing types from DWARF. GCC currently emits `DW_AT_object_pointer` on both declaration and definition DIEs [1]. Clang does not emit them on declarations, making the LLDB heuristics to find the object parameter fragile. We tried attaching `DW_AT_object_pointer` to declarations in Clang too [2], but that came at the cost of a ~5-10% increase in the `.debug_info` section size for some users, so we reverted it. This proposal describes an alternate encoding of the `DW_AT_object_pointer` which allows us to add it to declaration DIEs without incurring such size overheads. ## Overview The idea is to encode the index of the `DW_TAG_formal_parameter` that is the object parameter instead of a DIE reference. This index could then be of form `DW_FORM_implicit_const`, so we don't pay the 4 bytes for each reference, but instead pay for it once in the abbreviation. The implementation in Clang for this is currently being discussed in [3]. The DWARF spec currently only mentions `reference` as the attribute class of `DW_AT_object_pointer`. So consumers may be surprised by this alternate encoding. Hence we thought it'd be good to run this past the committee. An alternative solution could be a new attribute describing the object parameter index (e.g., `DW_AT_object_pointer_index` with a `constant` attribute class). ## Proposed Changes In chapter "7.5.4 Attribute Encodings", change the "Table 7.5: Attribute encodings" table as follows: [ORIGINAL TEXT] >>>>>>>>>>>>>>> Attribute Name | Value | Classes ----------------------------------------------------------------------- ... DW_AT_object_pointer | 0x64 | reference ... [NEW TEXT] ========== Attribute Name | Value | Classes ----------------------------------------------------------------------- ... DW_AT_object_pointer | 0x64 | reference, constant ... <<<<<<<<<<<<<<< In chapter "5.7.8 Member Function Entries", extend the attribute class recommendation as follows: [ORIGINAL TEXT] >>>>>>>>>>>>>>> If the member function entry describes a non-static member function, then that entry has a DW_AT_object_pointer attribute whose value is a reference to the formal parameter entry that corresponds to the object for which the function is called. [NEW TEXT] ========== If the member function entry describes a non-static member function, then that entry has a DW_AT_object_pointer attribute whose value is a reference to the formal parameter entry that corresponds to the object for which the function is called. A producer may also choose to represent it as a constant whose value is the zero-based index of the formal parameter that corresponds to the object parameter. <<<<<<<<<<<<<<< ## References * [1]: https://godbolt.org/z/3TWjTfWon * [2]: https://github.com/llvm/llvm-project/pull/122742 * [3]: https://github.com/llvm/llvm-project/pull/124790 -- Dwarf-discuss mailing list Dwarf-discuss@lists.dwarfstd.org https://lists.dwarfstd.org/mailman/listinfo/dwarf-discuss
