Issue 220724.1: Remove .debug_aranges and require unit-level ranges/high/low
| Author: | David Blaikie |
|---|---|
| Champion: | David Blaikie |
| Date submitted: | 2022-07-24 |
| Date revised: | 2024-10-14 |
| Date closed: | 2024-10-28 |
| Type: | Enhancement |
| Status: | Accepted |
| DWARF version: | 6 |
Section 6.1.2, pg 147
Given a discussion on dwarf-discuss (mailing lists down at the moment, so I
can't include a link) and Clang's experience with debug_aranges being disabled
by default for the better part of a decade at this point, I'd propose removing
them from the DWARF standard in favor of adding a guarantee that CU DIEs must
(rather than may) include a comprehensive address range (DW_AT_high/low_pc) or
list of ranges (DW_AT_ranges). This reduces redundancy in DWARF — reducing
debug info size and implementation complexity (having to support reading data
from multiple/varied locations — aranges or CU DIE ranges).
While .debug_aranges is specified to include data ranges (in addition to
code/instruction ranges), .debug_ranges does not. Practical experience
from major DWARF producers has shown this to be not
necessary/used/reliable - GCC doesn't include data in .debug_aranges,
and Clang doesn't produce .debug_aranges at all by default. It would be
novel to try to put data into .debug_ranges (it currently is only used
for instruction addresses) and in the absence of existing usage of the
functionality, seems not worth the design work to try to add such
functionality to .debug_ranges or otherwise address this regression in
specified functionality.
Proposed solution
Remove section 6.1.2 and 7.21 and all other references to .debug_aranges.
The index lists the following references to .debug_aranges: 147, 184, 186,
187, 197, 235, 274, 275, 278, 279, 366, 371, 393, 394, 399–401, 416 - these
seem to generally be easy to remove/an editorial matter of removing a
reference from a list of examples/descriptions. (I can enumerate them all
explicitly if desired.)
Edit 3.1.1, p61 from:
A full or partial compilation unit entry may have the following attributes:
1. Either a
DW_AT_low_pcandDW_AT_high_pcpair of attributes or aDW_AT_rangesattribute whose values encode the contiguous or non-contiguous address ranges, respectively, of the machine instructions generated for the compilation unit (see Section 2.17 on page 51).A
DW_AT_low_pcattribute may also be specified in ...
to:
A full or partial compilation unit entry must have either a
DW_AT_low_pcandDW_AT_high_pcpair of attributes or aDW_AT_rangesattribute whose values encode the contiguous or non-contiguous address ranges, respectively, of the machine instructions generated from the compilation unit (see Section 2.17 on page 61).A full or partial compilation unit entry may have the following attributes:
1. A
DW_AT_low_pcattribute may be specified in ...
On p67, bullet 2 should be moved above the immediately preceding non-bullet text, before line 13 (thereby making it required), and revised as follows:
2. Either a
DW_AT_low_pcandDW_AT_high_pcpair of attributes or aDW_AT_rangesattribute whose values encode the contiguous or non-contiguous address ranges, respectively, of the machine instructions and static data generated for the compilation unit (see Section 2.17 and page 61).
2022-07-24: Original proposal.
2023-04-18: Revised.
2023-05-15: Accepted pending revised wording.
2024-09-23: Reopened for discussion about whether static data could or
should or must be included in the newly mandated (rather than optional)
DW_AT_ranges.
2024-10-14: Revised with new 2nd paragraph in intro section addressing data ranges.
2024-10-28: Accepted.
