[Dwarf-discuss] Proposal: Allow padding in all tables

Wed Jan 31 14:54:19 GMT 2024

This proposal is guidance for the producer, not the linker. The producer needs this guidance specifically because linkers don’t pad/align contributions.

I believe padding is rarely a functional requirement, and when it is, it’s not for alignment IME. This is where the line-table padding came from, allowing elbow room to replace a function’s line table without having to update references to other contributions. (Motivating examples include JIT (re-)compilation and incremental linking.)

Padding for alignment, which is generally for performance or convenience and which I have run into in past years (pre-LLVM), must not confuse dumpers (which would be inclined to interpret padding bytes as the next header); therefore the padding bytes have to be interpretable.

I think if we’re going to mention padding (which we already do in six of the ten non-string-section cases described below) we should be complete about it, hence this proposal. I’m not especially excited about the .debug_macro case, but as we failed to give that section a header with a length, we have to live with the consequences.

If you think padding should never be mentioned (and so anyone who feels moved to provide padding has to re-invent the wheel), feel free to write a counter-proposal removing the existing mentions.
--paulr

From: David Blaikie <dblaikie@gmail.com>
Sent: Tuesday, January 30, 2024 6:01 PM
To: Robinson, Paul <paul.robinson@sony.com>
Cc: dwarf-discuss@lists.dwarfstd.org
Subject: Re: [Dwarf-discuss] Proposal: Allow padding in all tables

Is anyone actually using this? In my experience linkers are generally concatenating these sections together with no extra padding/alignment.

I'd rather not spec something that's not used/needed. I'm happy for consumers to be improved in the face of degenerate entries that might be created for padding if developers of such consumers feel so inclined (though I'd probably push back a bit on it in the consumers I work on - in the absence of any evidence of particular need/use case).

On Thu, Jan 18, 2024 at 11:08 AM Robinson, Paul via Dwarf-discuss <dwarf-discuss@lists.dwarfstd.org<mailto:dwarf-discuss@lists.dwarfstd.org>> wrote:
# Allow padding in all tables

Enhancement; multiple sections.

## Background

Issue 230329.1 requires all tables to be contiguous. During the discussion of that issue, the question came up of whether all tables allowed padding, so that contiguous concatenated contributions could be aligned reasonably. This is the result of my research.

## Overview

The set of tables (merging the two tables from 230329.1) is as follows:

- .debug_abbrev / .debug_abbrev.dwo (Section 7.5.3)
- .debug_aranges (Section 6.1.2)
- .debug_addr (Section 7.27)
- .debug_frame (Section 6.4.1)
- .debug_info / .debug_info.dwo (Section 7.5.1)
- .debug_line / .debug_line.dwo  (Section 6.2.4)
- .debug_line_str
- .debug_loclists / .debug_loclists.dwo (Section 7.29)
- .debug_macro / .debug_macro.dwo (Section 6.3.1)
- .debug_names (Section 6.1.1)
- .debug_rnglists / .debug_rnglists.dwo (Section 7.28)
- .debug_str / .debug_str.dwo
- .debug_str_offsets / .debug_str_offsets.dwo (Section 7.26)

### .debug_abbrev

Entries have arbitrary size. Can be padded by adding an unused abbrev entry. Proposing a non-normative paragraph describing this.

### .debug_aranges

Removed by 220724.1.

### .debug_addr

Entries have a size of (segment_selector_size + address_size) and don't explicitly provide a padding mechanism. Adding unused entries at the end of the table should suffice. Proposing a non-normative paragraph describing this.

### .debug_frame

Already permits padding by use of DW_CFA_nop.

### .debug_info

Already permits padding by use of the abbreviation code 0 (see Section 7.5.2).

### .debug_line

Already has DW_LNE_padding.

### .debug_line_str

This is a string section and does not need padding (typically would be merged, not concatenated).

### .debug_loclists

Already permits padding by use of repeated DW_LLE_end_of_list, with a non-normative comment to that effect.

### .debug_macro

This has no unit_length and no explicit provision for padding. One could insert unused opcodes into the opcode_operands_table but this seems like quite a hack. In keeping with other sections, I'm proposing a DW_MACRO_padding opcode.

### .debug_names

Components are mostly 4- or 8-byte multiples, except for the abbreviation table. The abbreviation table explicitly permits padding (Section 6.1.1.4.7).

### .debug_rnglists

Already permits padding by use of repeated DW_RLE_end_of_list, with a non-normative comment to that effect.

### .debug_str

This is a string section and does not need padding (typically would be merged, not concatenated).

### .debug_str_offsets

This has a header of 8 or 16 bytes, and entries of 4 or 8 bytes. This can still require padding if you want alignment greater than 4 bytes, and there is no explicit provision. Proposing a non-normative paragraph describing this.

### Conclusion

Everything is already covered except .debug_abbrev, .debug_addr, .debug_str_offsets, and .debug_macro. The first three need non-normative notes describing how to pad the sections, and .debug_macro requires a new opcode to introduce padding cleanly.

## Proposed Changes

I sorted these by affected section. In addition to the section-specific changes there is one general note.

### .debug_abbrev

In Section 7.5.3 "Abbreviations Tables" (p.207), at the end of the section, add a new non-normative paragraph:

*This table may be padded by adding an unused abbreviation entry. The minimum number of bytes in an abbreviation entry is four (abbreviation number, child flag, and two 0 bytes indicating the end of the attribute/form pairs). This can be expanded by choosing a large abbreviation number with a longer LEB128 encoding, or adding non-zero attribute/form pairs.*

### .debug_macro

Add new Section 6.3.4 "Other Entries" (~ p.170) as follows:

1. DW_MACRO_padding
   The DW_MACRO_padding opcode takes two operands, a byte count and a sequence
   of arbitrary bytes. The byte count is an unsigned LEB128 encoded number and
   does not include the size of the opcode or the byte count operand. The opcode
   and operands have no effect on the macro information.

   *This permits a producer to pad the macro information with a minimum of two bytes.*

### .debug_str_offsets

In Section 7.26 "String Offsets Table" (p.241), at the end of the section, add a new non-normative paragraph:

*This table may be padded with unused entries to fill out the table to some desired alignment. These entries should have all 1 bits as a hint that the entries are unused.*

### .debug_addr

In Section 7.27 "Address Table" (p.241), at the end of the section, add a new non-normative paragraph:

*This table may be padded with unused entries to fill out the table to some desired alignment. These entries should have all 1 bits as a hint that the entries are unused.*

### General

In Section 7.34 "Contiguous Tables" (added by issue 230329.1), at the end of the section, add a new non-normative paragraph:

*Every table of information has a way for the table as a whole to be padded to some desired alignment if the producer wishes to do so. Tables from multiple object files that are concatenated by a linker could then each be aligned, which may provide performance or other benefits. This padding is entirely optional, and does not relax any constraint specified in section 7.30.*

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