lld-link LTO doesn't support comdat selection kinds

[rnk]

This code in LLD COFF doesn't ever look at the LLVM IR comdat selection kind, so it doesn't implement selection kinds other than "any". It simply boils comdats down to a boolean, rather than a selection kind.

This means that mixing RTTI code with no-RTTI code can crash at runtime because the static linker forgot to implement IMAGE_COMDAT_SELECT_LARGEST or the largest comdat selection kind (LangRef).

A Chrome-related user exercised these conditions and reported those symptoms, and I'm reasonably convinced I've spotted the bug by looking at the source code. The source does say "// FIXME: Check nodeduplicate" after all. =S

[llvmbot]

@llvm/issue-subscribers-lld-coff

Author: Reid Kleckner (rnk)

[This code in LLD COFF](https://github.com/llvm/llvm-project/blob/474234a09655e57b7a4270150f0926db77e864b4/lld/COFF/InputFiles.cpp#L1200) doesn't ever look at the LLVM IR comdat selection kind, so it doesn't implement selection kinds other than "any". It simply boils comdats down to a boolean, rather than a selection kind.

This means that mixing RTTI code with no-RTTI code can crash at runtime because the static linker forgot to implement IMAGE_COMDAT_SELECT_LARGEST or the largest comdat selection kind (LangRef).

A Chrome-related user exercised these conditions and reported those symptoms, and I'm reasonably convinced I've spotted the bug by looking at the source code. The source does say "// FIXME: Check nodeduplicate" after all. =S

[nico]

(48dc110 added support for non-LTO. Apparently it has a fixme about _LARGEST not always being handled correctly there either.)

Having a small repro would be nice.