[CodeView] Add missing record 0x1176

[aganea]

Linking most recent msvcrt libraries from Microsoft generates lld-link: ignoring unknown symbol record with kind 0x1176 when using /verbose.

@sivadeilra Would you by chance be able to add this record?

[llvmbot]

@llvm/issue-subscribers-debuginfo

Author: Alexandre Ganea (aganea)

Linking most recent msvcrt libraries from Microsoft generates `lld-link: ignoring unknown symbol record with kind 0x1176` when using `/verbose`.

@sivadeilra Would you by chance be able to add this record?

[sivadeilra]

It can be safely ignored and it has very little value. It relates to an old compiler feature, known as "mini PDBs" or "fast linking". It's obsolete now. I'm surprised that anything is producing that record.

[aganea]

You mean /DEBUG:FASTLINK? Yeah that's what I had in my notes, but I wasn't sure. Thanks for clarifying that. I guess something in the MSVC linker generates this info.

As for the value, it's mostly for documenting it in LLVM, since there are Microsoft-produced libraries that contain these records. As an example here's a simple commit that adds the S_UNAMESPACE record, just so we can document & dump it with llvm-pdbutil: ee8a720

[sivadeilra]

That's right. It was a feature that was intended to reduce link time (because generating the PDBs was quite expensive). It was a fragile feature that didn't really work reliably. DevDiv later made significant improvements in the normal linking, so the fast-link feature was deprecated.

For the purposes of documentation, I would just call it S_OBSOLETE_1176. Getting it documented publicly, for a deprecated feature, is probably an uphill battle for me.