# Allow alternate encoding of DW_AT_object_pointer as a variable index

## 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][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][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][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.

[1]: https://godbolt.org/z/3TWjTfWon
[2]: https://github.com/llvm/llvm-project/pull/122742
[3]: https://github.com/llvm/llvm-project/pull/124790

---

2025-03-03: Accepted as written.
Alternative new form for DIE-relative offset could be a separate proposal
if deemed useful.