[PATCH] D26657: [Sema] Respect DLL attributes more faithfully

[Shoaib Meenai via cfe-commits]

smeenai added a comment.

In https://reviews.llvm.org/D26657#596759, @hans wrote:

> > On MSVC, if an implicit instantiation already exists and an explicit
> >  instantiation definition with a DLL attribute is created, the DLL
> >  attribute still takes effect. Make clang match this behavior.
>
> This is scary territory, and behaviour I think might be hard for us to match.
>
> What if we already codegenned the implicit instantiation?


Hmm. Under what specific cases would that happen?

I can see odd behavior differences occurring for `dllimport`. For example, for the C++ source in https://reviews.llvm.org/P7936, if I compile with clang (even with this patch applied), the `t.f()` call in `g` gets assembled to a call to `?f@?$s at H@@QAEHXZ`. If I hoist the dllimport explicit instantiation definition above `g`, `t.f()` instead assembles to `__imp_?f@?$s at H@@QAEHXZ`. With cl 19.00.24210, `t.f()` always assembles to `__imp_?f@?$s at H@@QAEHXZ`, regardless of the placement of the explicit instantiation definition.

I'm having a harder time imagining things going awry for `dllexport`. I can limit this patch to only `dllexport`, if that's going to be safer.

To give some context, libc++ uses extern templates extensively, and I ran into an issue with dllexport explicit locale template instantiations <https://reviews.llvm.org/diffusion/L/browse/libcxx/trunk/src/locale.cpp;287072$6034-6094?color=1>. Some of those instantiations are also implicitly instantiated via sizeof calls <https://reviews.llvm.org/diffusion/L/browse/libcxx/trunk/src/locale.cpp;287072$67,77,87?color=1>. Therefore, all instances which make is called on <https://reviews.llvm.org/diffusion/L/browse/libcxx/trunk/src/locale.cpp;287072$177-204?color=1> end up having their `dllexport` attribute ignored.

We could work around this in libc++ by hoisting the affected instantiations to near the top of the file, but it seemed preferable to make clang match MSVC's behavior instead, at least for `dllexport`. I don't have any particular interest in making `dllimport` semantics match MSVC for this specific case.


https://reviews.llvm.org/D26657