Thanks! I've added this as Issue 250130.1: https://dwarfstd.org/issues/250130.1.html
-cary On Thu, Jan 30, 2025 at 4:01 AM Michael Buch via Dwarf-discuss < dwarf-discuss@lists.dwarfstd.org> wrote: > # 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 >
-- Dwarf-discuss mailing list Dwarf-discuss@lists.dwarfstd.org https://lists.dwarfstd.org/mailman/listinfo/dwarf-discuss
