[all-commits] [llvm/llvm-project] 4a4a8a: [LLD] [COFF] Fix linking import libraries with -wh...

[Martin Storsjö via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 4a4a8a1476b1386b523dc5b292ba9a5a6748a9cf
      https://github.com/llvm/llvm-project/commit/4a4a8a1476b1386b523dc5b292ba9a5a6748a9cf
  Author: Martin Storsjö <martin at martin.st>
  Date:   2025-01-16 (Thu, 16 Jan 2025)

  Changed paths:
    M lld/COFF/InputFiles.cpp
    A lld/test/COFF/empty-section-decl.yaml
    A lld/test/COFF/wholearchive-implib.s
    M llvm/include/llvm/Object/COFF.h

  Log Message:
  -----------
  [LLD] [COFF] Fix linking import libraries with -wholearchive: (#122806)

When LLD links against an import library (for the regular, short import
libraries), it doesn't actually link in the header/trailer object files
at all, but synthesizes new corresponding data structures into the right
sections.

If the whole of such an import library is forced to be linked, e.g. with
the -wholearchive: option, we actually end up linking in those
header/trailer objects. The header objects contain a construct which LLD
fails to handle; previously we'd error out with the error ".idata$4
should not refer to special section 0".

Within the import library header object, in the import directory we have
relocations towards the IAT (.idata$4 and .idata$5), but the header
object itself doesn't contain any data for those sections.

In the case of GNU generated import libraries, the header objects
contain zero length sections .idata$4 and .idata$5, with relocations
against them. However in the case of LLVM generated import libraries,
the sections .idata$4 and .idata$5 are not included in the list of
sections. The symbol table does contain section symbols for these
sections, but without any actual associated section. This can probably
be seen as a declaration of an empty section.

If the header/trailer objects of a short import library are linked
forcibly and we also reference other functions in the library, we end up
with two import directory entries for this DLL, one that gets
synthesized by LLD, and one from the actual header object file. This is
inelegant, but should be acceptable.

While it would seem unusual to link import libraries with the
-wholearchive: option, this can happen in certain scenarios.

Rust builds libraries that contain relevant import libraries bundled
along with compiled Rust code as regular object files, all within one
single archive. Such an archive can then end up linked with the
-wholarchive: option, if build systems decide to use such an option for
including static libraries.

This should fix https://github.com/msys2/MINGW-packages/issues/21017.

This works for the header/trailer object files in import libraries
generated by LLVM; import libraries generated by MSVC are vaguely
different. ecb5ea6a266d5cc4e05252f6db4c73613b73cc3b did an attempt at
fixing the issue for MSVC generated libraries, but it's not entirely
correct, and isn't enough for making things work for that case.



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