Create new library 'libc++experimental.a' for packaging TS symbols.

[EricWF]

Summary

Out-of-line symbols for <experimental/...> headers are not ABI or API stable and cannot live in the 'libc++.dylib'. Currently they have nowhere to live. I would like to add a new library target libc++experimental.a to fix this.

Previously I had suggested different libraries for different TS's (libc++filesystem.a, 'libc++LFTS.a`, ect). I no longer think this is the right approach.
Instead c++experimental will hold *all* TS implementations as a single monolithic library. I see two main benefits to this:

1. Users only have to know about and manually link one library.
2. It makes it easy to implement TS's with one or two out-of-line symbols. (Ex. PMRs)

c++experimental provides NO ABI compatibility. Symbols can freely be added/removed/changed without concern for ABI stability.
I will add documentation for this after landing this patch (but before adding anything to it).

c++experimental only builds as a static library. By default CMake will build/test this library but will *NOT* install it.

This patch adds the CMake and LIT logic needed to build/test the new library. Once this lands I plan on using it to implement parts of <experimental/memory_resource>.

[EricWF]

Adding cfe-commits to the subscribers.

[bcraig]

LGTM

[mclow.lists]

I like this approach. LGTM.

[mclow.lists]

Q: Should the .a file get installed into /lib when you do a make install?

That will make it easier to link against.

[EricWF]

In D19856#420366, @mclow.lists wrote:

Q: Should the .a file get installed into /lib when you do a make install?

That will make it easier to link against.

For now the install rule is off by default. -DLIBCXX_INSTALL_EXPERIMENTAL_LIBRARY=ON will install it alongside libc++.so.
We can turn it ON by default once the library actually contains some code.

CMakeLists.txt

 
 #===============================================================================
 # Setup CMake Options
 #===============================================================================
 
 # Basic options ---------------------------------------------------------------
 option(LIBCXX_ENABLE_ASSERTIONS "Enable assertions independent of build mode." ON)
 option(LIBCXX_ENABLE_SHARED "Build libc++ as a shared library." ON)
-
+option(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY "Build libc++experimental.a" ON)
 option(LIBCXX_INCLUDE_TESTS "Build the libc++ tests." ${LLVM_INCLUDE_TESTS})
 option(LIBCXX_INCLUDE_DOCS "Build the libc++ documentation." ${LLVM_INCLUDE_DOCS})
 set(LIBCXX_LIBDIR_SUFFIX "${LLVM_LIBDIR_SUFFIX}" CACHE STRING
     "Define suffix of library directory name (32/64)")
 option(LIBCXX_INSTALL_HEADERS "Install the libc++ headers." ON)
 option(LIBCXX_INSTALL_LIBRARY "Install the libc++ library." ON)
 option(LIBCXX_INSTALL_SUPPORT_HEADERS "Install libc++ support headers." ON)
+option(LIBCXX_INSTALL_EXPERIMENTAL_LIBRARY "Install libc++experimental.a" OFF)
 set(LIBCXX_ABI_VERSION 1 CACHE STRING "ABI version of libc++.")
 option(LIBCXX_ABI_UNSTABLE "Unstable ABI of libc++." OFF)
 
 # ABI Library options ---------------------------------------------------------
 set(LIBCXX_CXX_ABI "${LIBCXX_CXX_ABI}" CACHE STRING
     "Specify C++ ABI library to use." FORCE)
 set(CXXABIS none libcxxabi libcxxrt libstdc++ libsupc++)
 set_property(CACHE LIBCXX_CXX_ABI PROPERTY STRINGS ;${CXXABIS})

lib/CMakeLists.txt

set_target_properties(cxx
   PROPERTIES
     COMPILE_FLAGS "${LIBCXX_COMPILE_FLAGS}"
     LINK_FLAGS    "${LIBCXX_LINK_FLAGS}"
     OUTPUT_NAME   "c++"
     VERSION       "${LIBCXX_ABI_VERSION}.0"
     SOVERSION     "${LIBCXX_ABI_VERSION}"
   )
 
+if (LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY)
+  file(GLOB LIBCXX_EXPERIMENTAL_SOURCES ../src/experimental/*.cpp)
+  add_library(cxx_experimental STATIC ${LIBCXX_EXPERIMENTAL_SOURCES})
+  target_link_libraries(cxx_experimental cxx)
+  set_target_properties(cxx_experimental
+    PROPERTIES
+      COMPILE_FLAGS "${LIBCXX_COMPILE_FLAGS}"
+      OUTPUT_NAME   "c++experimental"
+  )
+endif()
+
 # Generate a linker script inplace of a libc++.so symlink. Rerun this command
 # after cxx builds.
 if (LIBCXX_ENABLE_ABI_LINKER_SCRIPT)
   # Get the name of the ABI library and handle the case where CXXABI_LIBNAME
   # is a target name and not a library. Ex cxxabi_shared.
   set(SCRIPT_ABI_LIBNAME "${LIBCXX_CXX_ABI_LIBRARY}")
   if (SCRIPT_ABI_LIBNAME STREQUAL "cxxabi_shared")
     set(SCRIPT_ABI_LIBNAME "c++abi")
   endif()
   # Generate a linker script inplace of a libc++.so symlink. Rerun this command
   # after cxx builds.
   add_custom_command(TARGET cxx POST_BUILD
     COMMAND
       ${PYTHON_EXECUTABLE} ${LIBCXX_SOURCE_DIR}/utils/gen_link_script/gen_link_script.py
     ARGS
       "$<TARGET_LINKER_FILE:cxx>"
       "${SCRIPT_ABI_LIBNAME}"
     WORKING_DIRECTORY ${LIBCXX_BUILD_DIR}
   )
 endif()
 
 if (LIBCXX_INSTALL_LIBRARY)
-  install(TARGETS cxx
+  if (LIBCXX_INSTALL_EXPERIMENTAL_LIBRARY)
+    set(experimental_lib cxx_experimental)
+  endif()
+  install(TARGETS cxx ${experimental_lib}
     LIBRARY DESTINATION lib${LIBCXX_LIBDIR_SUFFIX} COMPONENT libcxx
     ARCHIVE DESTINATION lib${LIBCXX_LIBDIR_SUFFIX} COMPONENT libcxx
     )
   # NOTE: This install command must go after the cxx install command otherwise
   # it will not be executed after the library symlinks are installed.
   if (LIBCXX_ENABLE_ABI_LINKER_SCRIPT)
     # Replace the libc++ filename with $<TARGET_LINKER_FILE:cxx>
     # after we required CMake 3.0.
     install(FILES "${LIBCXX_LIBRARY_DIR}/libc++${CMAKE_SHARED_LIBRARY_SUFFIX}"
       DESTINATION lib${LIBCXX_LIBDIR_SUFFIX}
       COMPONENT libcxx)
   endif()
 endif()
 
 if (NOT CMAKE_CONFIGURATION_TYPES AND (LIBCXX_INSTALL_LIBRARY OR
                                        LIBCXX_INSTALL_HEADERS))
     if(LIBCXX_INSTALL_LIBRARY)
       set(lib_install_target cxx)
     endif()
+    if (LIBCXX_INSTALL_EXPERIMENTAL_LIBRARY)
+      set(experimental_lib_install_target cxx_experimental)
+    endif()
     if(LIBCXX_INSTALL_HEADERS)
       set(header_install_target install-libcxx-headers)
     endif()
     add_custom_target(install-libcxx
-                      DEPENDS ${lib_install_target} ${header_install_target}
+                      DEPENDS ${lib_install_target}
+                              ${experimental_lib_install_target}
+                              ${header_install_target}
                       COMMAND "${CMAKE_COMMAND}"
                       -DCMAKE_INSTALL_COMPONENT=libcxx
                       -P "${LIBCXX_BINARY_DIR}/cmake_install.cmake")
 endif()

src/experimental/placeholder.cpp

+//===--------------------------- TODO.cpp ---------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "experimental/__config"
+
+_LIBCPP_BEGIN_NAMESPACE_LFTS
+
+_LIBCPP_END_NAMESPACE_LFTS

test/CMakeLists.txt

 macro(pythonize_bool var)
   if (${var})
     set(${var} True)
   else()
     set(${var} False)
   endif()
 endmacro()
 
 set(LIBCXX_LIT_VARIANT "libcxx" CACHE STRING
     "Configuration variant to use for LIT.")
 
 pythonize_bool(LIBCXX_ENABLE_EXCEPTIONS)
+pythonize_bool(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY)
 pythonize_bool(LIBCXX_ENABLE_RTTI)
 pythonize_bool(LIBCXX_ENABLE_SHARED)
 pythonize_bool(LIBCXX_BUILD_32_BITS)
 pythonize_bool(LIBCXX_GENERATE_COVERAGE)
 pythonize_bool(LIBCXXABI_ENABLE_SHARED)
 pythonize_bool(LIBCXXABI_USE_LLVM_UNWINDER)
 pythonize_bool(LIBCXX_HAS_ATOMIC_LIB)
 

test/libcxx/test/config.py

class Configuration(object):
     def configure_link_flags_abi_library_path(self):
         # Configure ABI library paths.
         self.abi_library_root = self.get_lit_conf('abi_library_path')
         if self.abi_library_root:
             self.cxx.link_flags += ['-L' + self.abi_library_root,
                                     '-Wl,-rpath,' + self.abi_library_root]
 
     def configure_link_flags_cxx_library(self):
+        libcxx_experimental = self.get_lit_bool('enable_experimental', default=False)
+        if libcxx_experimental:
+            self.config.available_features.add('c++experimental')
+            self.cxx.link_flags += ['-lc++experimental']
         libcxx_shared = self.get_lit_bool('enable_shared', default=True)
         if libcxx_shared:
             self.cxx.link_flags += ['-lc++']
         else:
             cxx_library_root = self.get_lit_conf('cxx_library_root')
             if cxx_library_root:
                 abs_path = os.path.join(cxx_library_root, 'libc++.a')
                 self.cxx.link_flags += [abs_path]

test/lit.site.cfg.in

 @AUTO_GEN_COMMENT@
 config.cxx_under_test           = "@LIBCXX_COMPILER@"
 config.project_obj_root         = "@CMAKE_BINARY_DIR@"
 config.libcxx_src_root          = "@LIBCXX_SOURCE_DIR@"
 config.libcxx_obj_root          = "@LIBCXX_BINARY_DIR@"
 config.cxx_library_root         = "@LIBCXX_LIBRARY_DIR@"
 config.enable_exceptions        = "@LIBCXX_ENABLE_EXCEPTIONS@"
+config.enable_experimental      = "@LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY@"
 config.enable_rtti              = "@LIBCXX_ENABLE_RTTI@"
 config.enable_shared            = "@LIBCXX_ENABLE_SHARED@"
 config.enable_32bit             = "@LIBCXX_BUILD_32_BITS@"
 config.cxx_abi                  = "@LIBCXX_CXX_ABI_LIBNAME@"
 config.use_sanitizer            = "@LLVM_USE_SANITIZER@"
 config.sanitizer_library        = "@LIBCXX_SANITIZER_LIBRARY@"
 config.abi_library_path         = "@LIBCXX_CXX_ABI_LIBRARY_PATH@"
 config.configuration_variant    = "@LIBCXX_LIT_VARIANT@"