[libcxx-commits] [PATCH] D125924: [libcxx] Omit dllimport in public headers in MinGW mode

[Martin Storsjö via Phabricator via libcxx-commits]

mstorsjo created this revision.
mstorsjo added reviewers: ldionne, mati865, thieta.
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In MinGW environments, thanks to slightly different code generation
and linker tricks, it's possible to link against a DLL C++ standard
library without dllimport attributes.

This allows using one single set of headers for linking against
either the DLL or a static library, leaving the decision entirely
up to the linking stage (where it can be switched with options like
-static-libstdc++).

This matches how libstdc++ headers work; there's no dllimport attributes
by default (unless the user has defined _GLIBCXX_DLL when including
headers).

This allows using one single set of headers while linking against
either a DLL or a static library, just like on Unix platforms.

This matches how libc++ has been used in MinGW configurations for
years (by first building the DLL, then configuring a static-only
build and installing on top, overwriting the libc++ config file
with one for static linking) by multiple MinGW toolchains, making
the dllimport-less use the official, tested configuration. This
also allows building all of libc++ in one single CMake configuration,
instead of having to do two separate builds on top of each other.

(Linking against a DLL without dllimport can break if e.g. templates
use inconsistent visibility attributes - in cases where it still
works when using explicit dllimport; such a case was fixed in
948dd664c3ed30dd853df03cb931436f280bad4a <https://reviews.llvm.org/rG948dd664c3ed30dd853df03cb931436f280bad4a> / D99932 <https://reviews.llvm.org/D99932>. With this as the
default configuration, we can catch such issues in CI.)

This patch allows opting in back to dllimports by defining
_LIBCPP_DLLIMPORT. (Alternatively, one could consider a CMake build
time option.)


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D125924

Files:
  libcxx/include/__config


Index: libcxx/include/__config
===================================================================
--- libcxx/include/__config
+++ libcxx/include/__config
@@ -533,7 +533,9 @@
 #  define _LIBCPP_CRT_FUNC
 #endif
 
-#if defined(_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS)
+// Default to no DLL visibility attributes on MinGW, unless
+// requested by defining _LIBCPP_DLLIMPORT.
+#if defined(_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS) || (defined(__MINGW32__) && !defined(_LIBCPP_BUILDING_LIBRARY) && !defined(_LIBCPP_DLLIMPORT))
 #  define _LIBCPP_DLL_VIS
 #  define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS
 #  define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS


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