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dtk-template/docs/comment_section.md

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CodeWarrior .comment section

Files built with mwcc contain a .comment section:

$ powerpc-eabi-readelf -We object.o

Section Headers:
  [Nr] Name              Type            Addr     Off    Size   ES Flg Lk Inf Al
  [ 0]                   NULL            00000000 000000 000000 00      0   0  0
  [ 1] .text             PROGBITS        00000000 000034 000708 00  AX  0   0  4
  ...
  [16] .comment          PROGBITS        00000000 00153b 0001b4 01      0   0  1

The .comment section contains information that mwld uses during linking, primarily symbol alignment and a "force active" / export flag.

If missing, mwld will not adjust the alignment of symbols or remove any unused symbols.

This behavior is quite useful in some cases. When we split our program into objects, we're working from the final post-aligned, post-stripped result, and don't want the linker to make any changes. Most decompilation projects rely on this behavior unintentionally, since their generated objects don't contain a .comment section. (For example, objects built with powerpc-eabi-as.)

However, we need the .comment section for some purposes:

  • Reproducing the common BSS inflation bug requires the .comment section present, due to the above. The linker inflates the size of the first common BSS symbol in a TU, but won't actually move any data around unless the .comment section is present.
  • In newer versions of the linker, using common BSS at all without a valid .comment section will cause an internal linker error.

When the .comment section is generated, decomp-toolkit will mark all global symbols as "exported" to prevent any deadstripping, since the presence of the .comment section itself enables deadstripping.

Generating the .comment section and setting the "export" flag is also useful to prevent the linker from removing entire objects. A missing .comment section will prevent the removal of unused symbols inside of an object, but the linker will still remove the entire object itself if it thinks it's unused.

Contents

The contents of this section follow a very simple format:

Header

[0x0 size: 0xB] Magic: 43 6F 64 65 57 61 72 72 69 6F 72 ("CodeWarrior")

[0xB size: 0x1] Version(?): XX

It's not known whether this field actually affects mwld in any way, but it's configurable for completeness sake. (See mw_comment_version in config.example.yml.)

Known values:

  • 08 (8) - CodeWarrior for GameCube 1.0+
  • 0A (10) - CodeWarrior for GameCube 1.3.2+
  • 0B (11), 0C (12) - CodeWarrior for GameCube 2.7+ (difference unknown)
  • 0E (14), 0F (15) - CodeWarrior for GameCube 3.0a3+ (difference unknown)

[0xC size: 0x4] Compiler version: XX XX XX XX

First 3 bytes are major, minor, and patch version numbers.
4th byte is unknown, but is always 01.

Example: Version 2.3.3 build 144 -> 02 03 00 01
Often the .exe's properties (which --help reads from) and the internal version number (here) will differ.

[0x10 size: 1] Pool data: XX

  • 00 - Data pooling disabled
  • 01 - Data pooling enabled

[0x11 size: 1] Float type: XX

  • 00 - Floating point disabled
  • 01 - Software floating point
  • 02 - Hardware floating point

[0x12 size: 2] Processor type: 00 16 (Gekko)

[0x14 size: 1] Unknown, always 2C. Possibly the start of symbol entries.

[0x15 size: 1] "Quirk" flags: XX

Bitfield of miscellaneous flags. Known flags:

  • 01 - "Incompatible return small structs"
  • 02 - "Incompatible SFPE double params"
  • 04 - "Unsafe global reg vars"

[0x16 size: 22] Padding until 0x2C

Symbol entry

At 0x2C is the first symbol entry. There is one 8 byte entry per ELF symbol.

This includes the "null" ELF symbol, so the first entry will be all 0's.

[0x0 size: 4] Alignment: XX XX XX XX

[0x4 size: 1] Visibility flags(?): XX

Known values:

  • 00 - Default
  • 0D - Weak
  • 0E - Unknown, also weak?

[0x5 size: 1] Active flags(?): XX

Known values:

  • 00 - Default
  • 08 - Force active / export. Prevents the symbol from being deadstripped.
    When applied on a section symbol, the entire section is kept as-is. This is used by mwcc when data pooling is triggered (indicated by a symbol like ...data.0), likely to prevent the hard-coded section-relative offsets from breaking.
    Can also be set using #pragma force_active on or __declspec(export).
  • 10 - Unknown
  • 20 - Unknown

[0x6 size: 2] Padding(?): 00 00