smeenai added a comment. In https://reviews.llvm.org/D26657#597523, @hans wrote:
> In https://reviews.llvm.org/D26657#596897, @smeenai wrote: > > > 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? > > > Actually maybe I was being overly cautius. I think > https://reviews.llvm.org/rL225570 might just make this work. > > > 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@?$s@H@@QAEHXZ`. If I hoist the dllimport > > explicit instantiation definition above `g`, `t.f()` instead assembles to > > `__imp_?f@?$s@H@@QAEHXZ`. > > Yeah, dllimport is interesting here. If you turn on `-O1`, it will call the > `__imp` one. Fun times :-) Huh. Weird stuff. > > >> With cl 19.00.24210, `t.f()` always assembles to `__imp_?f@?$s@H@@QAEHXZ`, >> regardless of the placement of the explicit instantiation definition. > > Presumably they parse and analyze the whole translation unit before emitting > any code, which makes changing things around later easier for them than for > us. > >> 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. > > Yeah, I think https://reviews.llvm.org/rL225570 makes this work actually. So is the conclusion that this is probably safe as is, or should I limit it to `dllexport` specifically? >> 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. > > Dont' we need `dllimport` to work analogously though? When using libc++, > don't these template members need to be dllimported? If not, why are they > being exported in the first place? Ah. We put `dllimport` on the explicit template instantiation declarations in the header <https://reviews.llvm.org/diffusion/L/browse/libcxx/trunk/include/locale> (all the instances of `_LIBCPP_EXTERN_TEMPLATE2` in that file). Those get paired with the `dllexport` on the corresponding instantiation definitions. If you're compiling the library, the declarations have no DLL attribute, and the definitions have `dllexport`. If you're referencing the headers from outside the library, the declarations get the `dllimport`. https://reviews.llvm.org/D26657 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
