[Dwarf-discuss] rnglists and loclists and their base addresses

[David Anderson]

# Overview `.debug_loclists[.dwo]` and `.debug_rnglists[.dwo]`

A view from the consumer side.

Uses of `DW_AT_rnglistx` and `DW_AT_loclistx` and of the other
reated 'x' FORMS  is a bit complicated.
The issue is that one must find the correct part
of the target loclists/rnglists section.

We will focus on loclists here, but the
rangelists involve essentially identical
usages. With all the names shown below changed....

## Evaluating `DW_AT_location`

The addition of `DW_FORM_loclistx` in DWARF5
creates new tasks in DWARF readers to find
the applicable location list.

### `DW_AT_location` `DW_FORM_sec_offset`

With this FORM it is like DWARF2 to DWARF4
and the value of the form refers to a `.debug_loclists`
offset table which is local to a specific
set of loclists.  The offset table is used by
values of form `DW_FORM_loclistx`

### `DW_AT_location` `DW_FORM_loclistx`

With this FORM we do not know where the
appropriate loclistx is in .debug_loclists.
So we use additional data to find the correct
set of loclists.
The first two here
     base address
     `.debug_cu_index and .`debug_tu_index.dwo``
could be checked in either order.

### base address

`DW_AT_loclists_base`, if present in the CU DIE,
is the connector. Its value is the global offset,
in `.debug_loclists[.dwo]` of the offset table
of the relavent loclist.

If it is missing something else must be used.

### `.debug_cu_index.dwo` and .`debug_tu_index.dwo`

For `.debug_info.dwo` these can provide  the connection.

If the unit_type is `DW_UT_split_compile`
the reader does a lookup in `.debug_cu_index` based
on the compilation unit hash (signature).
If found and
if the `DW_SECT_LOCLISTS` table identifier does not represent an empty
value then we have a global offset into the `.debug_loclists`
section, The global offset is the base
address, equivalent to `DW_AT_loclists_base`.
[sec 7.3.5.3  Page 193]


If the unit_type is `DW_UT_split_type`
we look into `.debug_tu_index.dwo`.

### If Missing Base Address

Gcc, and Clang sometimes fail to emit either
a `DW_AT_loclists_base` or a `DW_cu_index.dwo` section
(with `DW_SECT_LOCLISTS` filled in)
the compiler simply assumes that there is
a base address of first-byte-after-initial-loclists-header

Similarly for rnglists

This is dwarfstd.org issue 240618.2