[WIP][libc++] Link c++experimental.a into libc++.dylib when enabled

[ldionne]

Summary

When libc++ is built with experimental features enabled, we link the
c++experimental.a static library directly into libc++.dylib, which
allows users to avoid using -lc++experimental in order to use
experimental features.

It makes sense to me that when libc++ is configured with experimental
features enabled, it should just work. Of course, vendors are free to
follow a different course of action, since they may not want to ship
experimental features in their dylib (in which case they can simply
turn off the experimental library in their build).

[ldionne]

This isn't a complete patch, but mostly a RFC to see what folks would think about including libc++experimental.a into the static/shared library when it is enabled. My view of things is that -lc++ should always be sufficient to link against libc++, and on Darwin it is (libc++abi symbols are re-exported, so -lc++abi isn't required). With that view of things, it makes sense to include c++experimental.a in the dylib when it is enabled.

Note that vendors are free not to enable c++experimental.a in the dylib they ship because they need to ensure ABI compatibility and some amount of stability.

@jwakely , how does libstdc++ ship experimental features? Does it ship them in its shared library?

[jwakely]

In D78154#1981905, @ldionne wrote:

@jwakely , how does libstdc++ ship experimental features? Does it ship them in its shared library?

No. Everything is header-only, except the Filesystem TS stuff which is in a separate static lib (libstdc++fs.a) and not in the shared library. Generally we try to avoid adding things to the shared library until they are non-experimental (that wasn't the case in the past, and we have been burned by adding immature C++0x features to the shared library too soon).

[ldionne]

In D78154#1982221, @jwakely wrote:

In D78154#1981905, @ldionne wrote:

@jwakely , how does libstdc++ ship experimental features? Does it ship them in its shared library?

No. Everything is header-only, except the Filesystem TS stuff which is in a separate static lib (libstdc++fs.a) and not in the shared library. Generally we try to avoid adding things to the shared library until they are non-experimental (that wasn't the case in the past, and we have been burned by adding immature C++0x features to the shared library too soon).

Thanks for the input.

Revisiting this, I think it makes sense to have at least some explicit switch for enabling experimental features. I think the best would be a compiler flag that does whatever's required -- something like -fexperimental, which would automatically add -lc++experimental (and possibly other necessary things). Abandoning this.

libcxx/CMakeLists.txt

 #===============================================================================
 include(CMakeDependentOption)
 include(HandleCompilerRT)
 
 # Basic options ---------------------------------------------------------------
 option(LIBCXX_ENABLE_ASSERTIONS "Enable assertions independent of build mode." OFF)
 option(LIBCXX_ENABLE_SHARED "Build libc++ as a shared library." ON)
 option(LIBCXX_ENABLE_STATIC "Build libc++ as a static library." ON)
-option(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY "Build libc++experimental.a" ON)
+option(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY "Build libc++experimental.a, and link it into libc++" ON)
 set(ENABLE_FILESYSTEM_DEFAULT ON)
 if (WIN32)
   set(ENABLE_FILESYSTEM_DEFAULT OFF)
 endif()
 option(LIBCXX_ENABLE_FILESYSTEM "Build filesystem as part of the main libc++ library"
     ${ENABLE_FILESYSTEM_DEFAULT})
 option(LIBCXX_INCLUDE_TESTS "Build the libc++ tests." ${LLVM_INCLUDE_TESTS})
 option(LIBCXX_ENABLE_PARALLEL_ALGORITHMS "Enable the parallel algorithms library. This requires the PSTL to be available." OFF)

libcxx/src/CMakeLists.txt

 
 # Build the shared library.
 if (LIBCXX_ENABLE_SHARED)
   add_library(cxx_shared SHARED ${exclude_from_all} ${LIBCXX_SOURCES} ${LIBCXX_HEADERS})
   if(COMMAND llvm_setup_rpath)
     llvm_setup_rpath(cxx_shared)
   endif()
   target_link_libraries(cxx_shared PRIVATE ${LIBCXX_LIBRARIES})
+  if (LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY)
+    target_link_libraries(cxx_shared PRIVATE cxx_experimental)
+  endif()
   set_target_properties(cxx_shared
     PROPERTIES
       COMPILE_FLAGS "${LIBCXX_COMPILE_FLAGS}"
       LINK_FLAGS    "${LIBCXX_LINK_FLAGS}"
       OUTPUT_NAME   "c++"
       VERSION       "${LIBCXX_ABI_VERSION}.0"
       SOVERSION     "${LIBCXX_ABI_VERSION}"
       DEFINE_SYMBOL ""
set_target_properties(cxx_shared PROPERTIES
                           APPEND_STRING PROPERTY LINK_FLAGS " /MANIFEST:NO")
   endif()
 endif()
 
 # Build the static library.
 if (LIBCXX_ENABLE_STATIC)
   add_library(cxx_static STATIC ${exclude_from_all} ${LIBCXX_SOURCES} ${LIBCXX_HEADERS})
   target_link_libraries(cxx_static PRIVATE ${LIBCXX_LIBRARIES})
+  if (LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY)
+    target_link_libraries(cxx_static PRIVATE cxx_experimental)
+  endif()
   set(CMAKE_STATIC_LIBRARY_PREFIX "lib")
   set_target_properties(cxx_static
     PROPERTIES
       COMPILE_FLAGS "${LIBCXX_COMPILE_FLAGS}"
       LINK_FLAGS    "${LIBCXX_LINK_FLAGS}"
       OUTPUT_NAME   "c++"
   )
   cxx_add_common_build_flags(cxx_static)

 add_custom_target(cxx DEPENDS cxx-headers ${LIBCXX_BUILD_TARGETS})
 
 if (LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY)
   set(LIBCXX_EXPERIMENTAL_SOURCES
     experimental/memory_resource.cpp
     )
   add_library(cxx_experimental STATIC ${LIBCXX_EXPERIMENTAL_SOURCES})
   if (LIBCXX_ENABLE_SHARED)
-    target_link_libraries(cxx_experimental PRIVATE cxx_shared)
+    target_link_libraries(cxx_experimental INTERFACE cxx_shared)
   else()
-    target_link_libraries(cxx_experimental PRIVATE cxx_static)
+    target_link_libraries(cxx_experimental INTERFACE cxx_static)
   endif()
 
   set_target_properties(cxx_experimental
     PROPERTIES
       COMPILE_FLAGS "${LIBCXX_COMPILE_FLAGS}"
       OUTPUT_NAME   "c++experimental"
   )
   cxx_add_common_build_flags(cxx_experimental)