[libc++] Always generate a __config_site header

[ldionne]

Summary

Before this patch, the config_site header was only generated when at
least one config_site macro needed to be defined. This lead to two
different code paths in how libc++ is configured, depending on whether
a config_site header was generated or not. After this patch, the
config_site is always generated, but it can be empty in case there
are no macros to define in it.

More context on why this change is important

In addition to being confusing, this double-code-path situation lead to
broken code being checked in undetected in 2405bd689815, which introduced
the LIBCXX_HAS_MERGED_TYPEINFO_NAMES_DEFAULT CMake setting. Specifically,
the _LIBCPP_HAS_MERGED_TYPEINFO_NAMES_DEFAULT <config_site> macro was
supposed NOT to be defined unless LIBCXX_HAS_MERGED_TYPEINFO_NAMES_DEFAULT
was specified explicitly on the CMake command line. Instead, what happened
is that it was defined to 0 if it wasn't specified explicitly and a
<config_site> header was generated. And defining that macro to 0 had
the important effect of using the non-unique RTTI comparison implementation,
which changes the ABI.

This change in behavior wasn't noticed because the <config_site> header
is not generated by default. However, the Apple configuration does cause
a <config_site> header to be generated, which lead to the wrong RTTI
implementation being used, and to https://llvm.org/PR45549. We came close
to an ABI break in the dylib, but were saved due to a downstream-only
change that overrode the decision of the <__config_site> for the purpose
of RTTI comparisons in libc++abi. This is an incredible luck that we should
not rely on ever again.

While the problem itself was fixed with 2464d8135e2a by setting
LIBCXX_HAS_MERGED_TYPEINFO_NAMES_DEFAULT explicitly in the Apple
CMake cache and then in d0fcdcd28f95 by making the setting less
brittle, the point still is that we should have had a single code
path from the beginning. Unlike most normal libraries, the macros
that configure libc++ are really complex, there's a lot of them and
they control important properties of the C++ runtime. There must be
a single code path for that, and it must be simple and robust.

[ldionne]

Got LGTM from EricWF offline. We've been talking about this for a while.

libcxx/CMakeLists.txt

 # the headers to use DLL import/export semantics. However when building a
 # static library only we modify the headers to disable DLL import/export.
 if (DEFINED WIN32 AND LIBCXX_ENABLE_STATIC AND NOT LIBCXX_ENABLE_SHARED)
   message(STATUS "Generating custom __config for non-DLL Windows build")
   config_define(ON _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS)
 endif()
 
 set(site_config_path "${LIBCXX_BINARY_DIR}/__config_site")
-if (LIBCXX_NEEDS_SITE_CONFIG)
-  configure_file("include/__config_site.in"
-                 "${site_config_path}"
-                 @ONLY)
-elseif(EXISTS "${site_config_path}")
-  message(STATUS "Removing stale site configuration ${site_config_path}")
-  file(REMOVE "${site_config_path}")
-endif()
 
 function(cxx_add_config_site target)
-  if (LIBCXX_NEEDS_SITE_CONFIG)
-    if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC" OR "${CMAKE_CXX_SIMULATE_ID}" STREQUAL "MSVC")
-      target_compile_options(${target} PUBLIC /FI "${site_config_path}")
-    else()
-      target_compile_options(${target} PUBLIC -include "${site_config_path}")
-    endif()
-  endif()
 endfunction()
 
 # Setup all common build flags =================================================
 function(cxx_add_common_build_flags target)
   cxx_add_basic_build_flags(${target})
   cxx_add_warning_flags(${target})
   cxx_add_windows_flags(${target})
   cxx_add_config_site(${target})

libcxx/benchmarks/CMakeLists.txt

set(BENCHMARK_LIBCXX_COMPILE_FLAGS
     -Wl,-rpath,${LIBCXX_LIBRARY_DIR}
     ${SANITIZER_FLAGS}
     )
 if (DEFINED LIBCXX_CXX_ABI_LIBRARY_PATH)
   list(APPEND BENCHMARK_LIBCXX_COMPILE_FLAGS
           -L${LIBCXX_CXX_ABI_LIBRARY_PATH}
           -Wl,-rpath,${LIBCXX_CXX_ABI_LIBRARY_PATH})
 endif()
-if (LIBCXX_NEEDS_SITE_CONFIG)
-  list(APPEND BENCHMARK_LIBCXX_COMPILE_FLAGS -include "${LIBCXX_BINARY_DIR}/__config_site")
-endif()
 split_list(BENCHMARK_LIBCXX_COMPILE_FLAGS)
 
 ExternalProject_Add(google-benchmark-libcxx
         EXCLUDE_FROM_ALL ON
         DEPENDS cxx cxx-headers
         PREFIX benchmark-libcxx
         SOURCE_DIR ${LIBCXX_SOURCE_DIR}/utils/google-benchmark
         INSTALL_DIR ${CMAKE_CURRENT_BINARY_DIR}/benchmark-libcxx

libcxx/cmake/Modules/HandleLibcxxFlags.cmake

 endmacro()
 
 # Add a macro definition to the __config_site file if the specified condition
 # is 'true'. Note that '-D${def}' is not added. Instead it is expected that
 # the build include the '__config_site' header.
 macro(config_define_if condition def)
   if (${condition})
     set(${def} ON)
-    set(LIBCXX_NEEDS_SITE_CONFIG ON)
   endif()
 endmacro()
 
 macro(config_define_if_not condition def)
   if (NOT ${condition})
     set(${def} ON)
-    set(LIBCXX_NEEDS_SITE_CONFIG ON)
   endif()
 endmacro()
 
 macro(config_define value def)
   set(${def} ${value})
-  set(LIBCXX_NEEDS_SITE_CONFIG ON)
 endmacro()
 
 # Add a list of flags to all of 'CMAKE_CXX_FLAGS', 'CMAKE_C_FLAGS',
 # 'LIBCXX_COMPILE_FLAGS' and 'LIBCXX_LINK_FLAGS'.
 macro(add_target_flags)
   foreach(value ${ARGN})
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${value}")
     set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${value}")

libcxx/include/CMakeLists.txt

set(files
     support/win32/limits_msvc_win32.h
     support/win32/locale_win32.h
     support/xlocale/__nop_locale_mgmt.h
     support/xlocale/__posix_l_fallback.h
     support/xlocale/__strtonum_fallback.h
     )
 endif()
 
-if (LIBCXX_NEEDS_SITE_CONFIG)
-  # Generate a custom __config header. The new header is created
-  # by prepending __config_site to the current __config header.
-  add_custom_command(OUTPUT ${LIBCXX_BINARY_DIR}/__generated_config
-    COMMAND ${Python3_EXECUTABLE} ${LIBCXX_SOURCE_DIR}/utils/cat_files.py
-      ${LIBCXX_BINARY_DIR}/__config_site
-      ${LIBCXX_SOURCE_DIR}/include/__config
-      -o ${LIBCXX_BINARY_DIR}/__generated_config
-    DEPENDS ${LIBCXX_SOURCE_DIR}/include/__config
-            ${LIBCXX_BINARY_DIR}/__config_site
-  )
-  # Add a target that executes the generation commands.
-  add_custom_target(cxx-generated-config ALL
-    DEPENDS ${LIBCXX_BINARY_DIR}/__generated_config)
-  set(generated_config_deps cxx-generated-config)
-else()
-  set(files
-    ${files}
-    __config
-    )
-endif()
 
 # In some build configurations (like bootstrapping clang), we need to be able to
 # install the libcxx headers before the CMake configuration for libcxx runs. Making
 # the name of this target configurable allows LLVM/runtimes/CMakeLists.txt to
 # add this subdirectory to the LLVM build to put libcxx's headers in place
 # before libcxx's build configuration is run.
 if (NOT CXX_HEADER_TARGET)
   set(CXX_HEADER_TARGET cxx-headers)
 endif()
 if(NOT LIBCXX_USING_INSTALLED_LLVM AND LIBCXX_HEADER_DIR)
   set(output_dir ${LIBCXX_HEADER_DIR}/include/c++/v1)
 
   set(out_files)
   foreach(f ${files})
     set(src ${CMAKE_CURRENT_SOURCE_DIR}/${f})
     set(dst ${output_dir}/${f})
     add_custom_command(OUTPUT ${dst}
       DEPENDS ${src}
       COMMAND ${CMAKE_COMMAND} -E copy_if_different ${src} ${dst}
       COMMENT "Copying CXX header ${f}")
     list(APPEND out_files ${dst})
   endforeach()
 
-  if (LIBCXX_NEEDS_SITE_CONFIG)
-    # Copy the generated header as __config into build directory.
-    set(src ${LIBCXX_BINARY_DIR}/__generated_config)
-    set(dst ${output_dir}/__config)
-    add_custom_command(OUTPUT ${dst}
-        DEPENDS ${src} ${generated_config_deps}
-        COMMAND ${CMAKE_COMMAND} -E copy_if_different ${src} ${dst}
-        COMMENT "Copying CXX __config")
-    list(APPEND out_files ${dst})
-  endif()
 
   add_custom_target(${CXX_HEADER_TARGET} ALL DEPENDS ${out_files} ${LIBCXX_CXX_ABI_HEADER_TARGET})
 else()
   add_custom_target(${CXX_HEADER_TARGET})
 endif()
 set_target_properties(${CXX_HEADER_TARGET} PROPERTIES FOLDER "Misc")
 
 if (LIBCXX_INSTALL_HEADERS)
   foreach(file ${files})
     get_filename_component(dir ${file} DIRECTORY)
     install(FILES ${file}
       DESTINATION ${LIBCXX_INSTALL_HEADER_PREFIX}include/c++/v1/${dir}
       COMPONENT ${CXX_HEADER_TARGET}
       PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ
     )
   endforeach()
 
-  if (LIBCXX_NEEDS_SITE_CONFIG)
-    # Install the generated header as __config.
-    install(FILES ${LIBCXX_BINARY_DIR}/__generated_config
-      DESTINATION ${LIBCXX_INSTALL_HEADER_PREFIX}include/c++/v1
-      PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ
-      RENAME __config
-      COMPONENT ${CXX_HEADER_TARGET})
-  endif()
 
   if (NOT CMAKE_CONFIGURATION_TYPES)
     add_custom_target(install-${CXX_HEADER_TARGET}
                       DEPENDS ${CXX_HEADER_TARGET} ${generated_config_deps}
                       COMMAND "${CMAKE_COMMAND}"
                               -DCMAKE_INSTALL_COMPONENT=${CXX_HEADER_TARGET}
                               -P "${CMAKE_BINARY_DIR}/cmake_install.cmake")
     # Stripping is a no-op for headers
     add_custom_target(install-${CXX_HEADER_TARGET}-stripped DEPENDS install-${CXX_HEADER_TARGET})
   endif()
 endif()