[Dwarf-discuss] New Issue: Tombstoning TU entries in .debug_names

[David Blaikie]

(derived from:
https://lists.dwarfstd.org/mailman/private/dwarf-workgroup/2023-October/002444.html
)

# Tombstoning TU entries in `.debug_names`

## Background

Local type unit entries in `.debug_names` reference type units via their
offset in the `.debug_info` section.

Assuming a non-DWARF-aware linker, per 6.1.1.3: "When linking object files
containing per-CU indexes, the linker may choose to concatenate the indexes
as ordinary sections ..."

If a linker does this, it will need to choose what value to resolve a
relocation for the TU offset to, if that TU was discarded. (if the TUs are
bit identical, the linker might resolve all relocations to identical copies
to refer to the remaining copy - but TUs aren't guaranteed to be byte
identical - and since the offsets in the `.debug_names` table refer to
specific byte offset in the TU, these entries can't be used if a
different-but-equivalent copy of the TU is the one that's preserved by the
linker)

To deal with this, the linker would use a tombstone value of some kind to
indicate that the offset couldn't be resolved. By default, this offset
might be zero - which is a valid offset within the `.debug_info` section,
and would not be possible for the DWARF consumer to differentiate the
"ignore this index entry" from a valid index entry pointing to a TU at
offset zero.

With https://dwarfstd.org/issues/200609.1.html we chose a tombstone value
for other cases (.debug_info referencing into code - in the case of
discarded functions) of "the largest representable value". This seems like
a good foundation for addressing this new, similar case.

## Proposed Changes

Add a paragraph between the first and second/last in 6.1.1.4.3 that reads:

"Any local TU entry with a maximum representable value is considered not
present. Any index entry referencing such a local TU entry should be
ignored."
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