`exclude_from_explicit_instantiation` doesn't seem to exclude virtual methods, causing problems after 84216d1

[zmodem]

Consider:

C:\src\llvm-project>cat \src\temp\a.cc
template <typename>
struct Fruit {
  __attribute((exclude_from_explicit_instantiation)) Fruit() {}

  virtual void __attribute((exclude_from_explicit_instantiation)) draw() {}
};

extern template struct Fruit<int>;

int main() {
  Fruit<int> f;
  return 0;
}

C:\src\llvm-project>build\bin\clang-cl \src\temp\a.cc -fuse-ld=lld
lld-link: error: undefined symbol: public: virtual void __cdecl Fruit<int>::draw(void)
>>> referenced by c:\src\temp\a-6ec007.obj:(const Fruit<int>::`vftable')
clang-cl: error: linker command failed with exit code 1 (use -v to see invocation)

It seems that despite the exclude_from_explicit_instantiation attribute on Fruit<>::draw, the explicit instantiation decl prevents the definition from being emitted when referenced by the vtable. Instead the definition of Fruit<>::draw will only get emitted once there's an explicit instantiation definition.

However, after 84216d1 that stopped working across DLL boundaries, since the method will no longer be dllimport/export.

It seems the exclusion from explicit instantiation is not working completely.

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Consider:

C:\src\llvm-project>cat \src\temp\a.cc
template <typename>
struct Fruit {
  __attribute((exclude_from_explicit_instantiation)) Fruit() {}

  virtual void __attribute((exclude_from_explicit_instantiation)) draw() {}
};

extern template struct Fruit<int>;

int main() {
  Fruit<int> f;
  return 0;
}

C:\src\llvm-project>build\bin\clang-cl \src\temp\a.cc -fuse-ld=lld
lld-link: error: undefined symbol: public: virtual void __cdecl Fruit<int>::draw(void)
>>> referenced by c:\src\temp\a-6ec007.obj:(const Fruit<int>::`vftable')
clang-cl: error: linker command failed with exit code 1 (use -v to see invocation)

It seems that despite the exclude_from_explicit_instantiation attribute on Fruit<>::draw, the explicit instantiation decl prevents the definition from being emitted when referenced by the vtable. Instead the definition of Fruit<>::draw will only get emitted once there's an explicit instantiation definition.

However, after 84216d1 that stopped working across DLL boundaries, since the method will no longer be dllimport/export.

It seems the exclusion from explicit instantiation is not working completely.

[zmodem]

I think the crux of the problem is in Sema::DefineUsedVTables() (which in our case gets called after InstantiateFunctionDefinition of Fruit<int>::Fruit):

      // If we have a class with no key function that is the subject
      // of an explicit instantiation declaration, suppress the
      // vtable; it will live with the explicit instantiation
      // definition.

That does not hold in our case: because the Fruit constructor is marked exclude_from_explicit_instantiation we will emit that constructor, and it will try to emit the vtable.

But because Sema::DefineUsedVTables() decided not to emit the vtable, it has not marked Fruit<int>::draw() as referenced as it normally would (MarkVirtualMembersReferenced), and so the vtable ends up referencing an undefined function.