[libcxx] Base MSVC autolinking on _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS

[mstorsjo]

Summary

Previously the decision of which library to try to autolink was
based on _DLL, however the _DLL define (which is set by the compiler)
is tied to whether using a dynamically linked CRT or not, and the choice
of dynamic or static CRT is entirely orthogonal to whether libc++ is
linked dynamically or statically.

If _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS isn't defined, then all
declarations are decorated with dllimport, and there's no doubt that
the DLL version of the library is what must be linked.

_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS is defined if building with
LIBCXX_ENABLE_SHARED disabled, and thus the static library is what
should be linked.

If defining _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS manually but wanting
to link against the DLL version of the library, that's not a canonical
configuration, and then it's probably reasonable to manually define
_LIBCPP_NO_AUTO_LINK too, and manually link against the desired
library.

This fixes, among other issues, running tests for the library if
built with LIBCXX_ENABLE_STATIC disabled.

This issue was brought up when autolinking was introduced in
https://reviews.llvm.org/D40660, but was shrugged off back then,
but it is causing real issues, and instead of avoiding the issue
by disabling the autolinking feature, we should fix it to actually
match how the library was built.

[rnk]

lgtm

I agree this makes sense, it's reasonable to statically link libc++ and dynamically link the CRT.

I think this is Windows-specific, so my approval is enough, but I could be wrong.

---

In Chrome, we define _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS in all libc++ static build configs:
https://source.chromium.org/chromium/chromium/src/+/master:build/config/c++/BUILD.gn;l=57?q=_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS&ss=chromium
I see that libc++ appears to support building both static and dynamic libraries in the same build:
https://github.com/llvm/llvm-project/blob/280678122d3175943e41310d56a17924ea38ffc1/libcxx/CMakeLists.txt#L89-L90
That makes it impossible to bake this visibility setting into __config_site. Otherwise, I would suggest that we do that, and make the Windows behavior the cross-platform behavior. Then we would rename this macro to something like _LIBCPP_STATIC_LIBRARY or something like that.

[mstorsjo]

In D100539#2692434, @rnk wrote:

I see that libc++ appears to support building both static and dynamic libraries in the same build:
https://github.com/llvm/llvm-project/blob/280678122d3175943e41310d56a17924ea38ffc1/libcxx/CMakeLists.txt#L89-L90
That makes it impossible to bake this visibility setting into __config_site. Otherwise, I would suggest that we do that, and make the Windows behavior the cross-platform behavior. Then we would rename this macro to something like _LIBCPP_STATIC_LIBRARY or something like that.

libcxx actually already does bake _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS into __config_site if LIBCXX_ENABLE_SHARED isn't set.

And yes, while one _can_ build both static and shared at the same time, the static library can't really be used practically in such configs. But that's an orthogonal issue (that I'm not planning on doing anything about really, as there's no really neat solutions that I see).

[mstorsjo]

Rebase to rerun CI

[mstorsjo]

Rerunning CI yet again after another fix on main for clang-format

[smeenai]

LGTM as well.

[curdeius]

LGTM.

[mstorsjo]

Ping for second approval

[Mordante]

LGTM.

libcxx/include/__config

 #    define _LIBCPP_POP_MACROS         \
        _Pragma("pop_macro(\"min\")")   \
        _Pragma("pop_macro(\"max\")")
 #  endif
 #endif // defined(_LIBCPP_HAS_NO_PRAGMA_PUSH_POP_MACRO)
 
 #ifndef _LIBCPP_NO_AUTO_LINK
 #  if defined(_LIBCPP_ABI_MICROSOFT) && !defined(_LIBCPP_BUILDING_LIBRARY)
-#    if defined(_DLL)
+#    if !defined(_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS)
 #      pragma comment(lib, "c++.lib")
 #    else
 #      pragma comment(lib, "libc++.lib")
 #    endif
 #  endif // defined(_LIBCPP_ABI_MICROSOFT) && !defined(_LIBCPP_BUILDING_LIBRARY)
 #endif // _LIBCPP_NO_AUTO_LINK
 
 #define _LIBCPP_UNUSED_VAR(x) ((void)(x))