[libcxx] Make it drastically simpler to link libc++.

[EricWF]

Summary

Currently on most platforms you have to manually link the c++ abi library used with libc++ whenever you use libc++. So your typical libc++ command like invocation might look like:

clang++ -stdlib=libc++ foo.cpp -lc++abi

Having to manually link libc++abi.so makes it harder for libc++ to be used generically. This patch fixes that by generating a linker script for libc++.so that correctly links the ABI library. On linux the linker script for libc++abi would look like:

# libc++.so
INPUT(libc++.so.1 -lc++abi)

With the linker script you can now use libc++ using only -stdlib=libc++. This is the technique that is used on FreeBSD in ordered to link cxxrt and I think it's the best approach to make our users lives simpler.

The CMake option used to enable this is LIBCXX_ENABLE_ABI_LINKER_SCRIPT. In future I would like to enable this by default on all platforms except for Darwin.

[jroelofs]

utils/gen_link_script/gen_link_script.py

2

Do you foresee these linker scripts being significantly more complicated for other platforms? ISTM that it would be much simpler to have CMake generate these directly.

[EricWF]

utils/gen_link_script/gen_link_script.py

2

The linker scripts will get no more complicated. The will always just be INPUT(libc++.so.x.x -l<abilib>). However generating these in CMake is a pain. I've tried before and I promise the python approach is a lot cleaner.

The main reason is that most CMake functions such as FILE(...) and CONFIG_FILE(...) are run at CMake configuration time and they don't allow generator expressions in them. I also don't know how to read the value of a symlink using CMake which is needed to generate the linker script.

If you really want me to rewrite this is CMake I will, but I promise that you probably don't.

[jroelofs]

I trust your judgement.

[EricWF]

@mclow.lists ping. Do you think this is the right approach to fix linking the ABI library?
@rsmith: I think you asked for this functionality a while ago. Any thoughts?

[rsmith]

I think this is a good direction. I used to link libc++abi into libc++, and switched to using a hand-rolled linker script a while back. It's been working great for me.

[EricWF]

Updating diff so it merges cleanly against master.

[EricWF]

I'm accepting this revision because the feedback about this patch has been positive. I will submit another patch that makes this behavior default as suggested on cfe-dev.

CMakeLists.txt

 # 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})
 
 option(LIBCXX_ENABLE_STATIC_ABI_LIBRARY "Statically link the ABI library" OFF)
 
+# Generate and install a linker script inplace of libc++.so. The linker script
+# will link libc++ to the correct ABI library.
+option(LIBCXX_ENABLE_ABI_LINKER_SCRIPT
+      "Use and install a linker script for the given ABI library" OFF)
+
 # Build libc++abi with libunwind. We need this option to determine whether to
 # link with libunwind or libgcc_s while running the test cases.
 option(LIBCXXABI_USE_LLVM_UNWINDER "Build and use the LLVM unwinder." OFF)
 
 # Target options --------------------------------------------------------------
 option(LIBCXX_BUILD_32_BITS "Build 32 bit libc++." ${LLVM_BUILD_32_BITS})
 set(LIBCXX_SYSROOT "" CACHE STRING "Use alternate sysroot.")
 set(LIBCXX_GCC_TOOLCHAIN "" CACHE STRING "Use alternate GCC toolchain.")

 if (LIBCXX_ENABLE_STATIC_ABI_LIBRARY)
   if (APPLE)
     message(FATAL_ERROR "LIBCXX_ENABLE_STATIC_ABI_LIBRARY is not supported on OS X")
   else()
     message(WARNING "LIBCXX_ENABLE_STATIC_ABI_LIBRARY is an experimental option")
   endif()
 endif()
 
+if (LIBCXX_ENABLE_ABI_LINKER_SCRIPT)
+    if (APPLE)
+      message(FATAL_ERROR "LIBCXX_ENABLE_ABI_LINKER_SCRIPT cannot be used on APPLE targets")
+    endif()
+    if (NOT PYTHONINTERP_FOUND)
+      message(FATAL_ERROR "LIBCXX_ENABLE_ABI_LINKER_SCRIPT requires python but it was not found.")
+    endif()
+endif()
+
+if (LIBCXX_ENABLE_STATIC_ABI_LIBRARY AND LIBCXX_ENABLE_ABI_LINKER_SCRIPT)
+    message(FATAL_ERROR "Conflicting options given.
+        LIBCXX_ENABLE_STATIC_ABI_LIBRARY cannot be specified with
+        LIBCXX_ENABLE_ABI_LINKER_SCRIPT")
+endif()
+
 #===============================================================================
 # Configure System
 #===============================================================================
 
 set(LIBCXX_COMPILER    ${CMAKE_CXX_COMPILER})
 set(LIBCXX_SOURCE_DIR  ${CMAKE_CURRENT_SOURCE_DIR})
 set(LIBCXX_BINARY_DIR  ${CMAKE_CURRENT_BINARY_DIR})
 set(LIBCXX_LIBRARY_DIR ${CMAKE_BINARY_DIR}/lib${LIBCXX_LIBDIR_SUFFIX})

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}"
   )
 
+# 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 "${CXXABI_LIBNAME}")
+  if (SCRIPT_ABI_LIBNAME STREQUAL "cxxabi_shared")
+    set("${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 ${LIBCXX_BUILD_DIR}
+  )
+endif()
+
+
 if (LIBCXX_INSTALL_LIBRARY)
   install(TARGETS cxx
     LIBRARY DESTINATION lib${LIBCXX_LIBDIR_SUFFIX} COMPONENT libcxx
     ARCHIVE DESTINATION lib${LIBCXX_LIBDIR_SUFFIX} COMPONENT libcxx
     )
+  # NOTE: This install command must go after the cxx install command otherwise
+  # it will not be executed after the library symlinks are installed.
+  if (LIBCXX_ENABLE_ABI_LINKER_SCRIPT)
+    install(FILES "$<TARGET_LINKER_FILE:cxx>"
+      DESTINATION lib${LIBCXX_LIBDIR_SUFFIX}
+      COMPONENT libcxx)
+  endif()
 endif()
 
 if (NOT CMAKE_CONFIGURATION_TYPES AND (LIBCXX_INSTALL_LIBRARY OR
                                        LIBCXX_INSTALL_HEADERS))
     add_custom_target(install-libcxx
                       DEPENDS cxx
                       COMMAND "${CMAKE_COMMAND}"
                       -DCMAKE_INSTALL_COMPONENT=libcxx
                       -P "${LIBCXX_BINARY_DIR}/cmake_install.cmake")
 endif()

test/CMakeLists.txt

 pythonize_bool(LIBCXX_ENABLE_EXCEPTIONS)
 pythonize_bool(LIBCXX_ENABLE_RTTI)
 pythonize_bool(LIBCXX_ENABLE_SHARED)
 pythonize_bool(LIBCXX_BUILD_32_BITS)
 pythonize_bool(LIBCXX_GENERATE_COVERAGE)
 pythonize_bool(LIBCXXABI_USE_LLVM_UNWINDER)
 
 # The tests shouldn't link to any ABI library when it has been linked into
-# libc++ statically.
-if (LIBCXX_ENABLE_STATIC_ABI_LIBRARY)
+# libc++ statically or via a linker script.
+if (LIBCXX_ENABLE_STATIC_ABI_LIBRARY OR LIBCXX_ENABLE_ABI_LINKER_SCRIPT)
   set(LIBCXX_CXX_ABI_LIBNAME "none")
 endif()
+
 set(LIBCXX_TARGET_INFO "libcxx.test.target_info.LocalTI" CACHE STRING
     "TargetInfo to use when setting up test environment.")
 set(LIBCXX_EXECUTOR "None" CACHE STRING
     "Executor to use when running tests.")
 
 set(AUTO_GEN_COMMENT "## Autogenerated by libcxx configuration.\n# Do not edit!")
 
 configure_file(

utils/gen_link_script/gen_link_script.py

+#!/usr/bin/env python
+import os
+import sys
+
+def print_and_exit(msg):
+    sys.stderr.write(msg + '\n')
+    sys.exit(1)
+
+def usage_and_exit():
+    print_and_exit("Usage: ./gen_link_script.py [--help] [--dryrun] <path/to/libcxx.so> <abi_libname>")
+
+def help_and_exit():
+    help_msg = \
+"""Usage
+
+  gen_link_script.py [--help] [--dryrun] <path/to/libcxx.so> <abi_libname>
+
+  Generate a linker script that links libc++ to the proper ABI library.
+  The script replaces the specified libc++ symlink.
+  An example script for c++abi would look like "INPUT(libc++.so.1 -lc++abi)".
+
+Arguments
+  <path/to/libcxx.so> - The top level symlink to the versioned libc++ shared
+                        library. This file is replaced with a linker script.
+  <abi_libname>       - The name of the ABI library to use in the linker script.
+                        The name must be one of [c++abi, stdc++, supc++, cxxrt].
+
+Exit Status:
+  0 if OK,
+  1 if the action failed.
+"""
+    print_and_exit(help_msg)
+
+def parse_args():
+    args = list(sys.argv)
+    del args[0]
+    if len(args) == 0:
+        usage_and_exit()
+    if args[0] == '--help':
+        help_and_exit()
+    dryrun = '--dryrun' == args[0]
+    if dryrun:
+        del args[0]
+    if len(args) != 2:
+        usage_and_exit()
+    symlink_file = args[0]
+    abi_libname = args[1]
+    return dryrun, symlink_file, abi_libname
+
+def main():
+    dryrun, symlink_file, abi_libname = parse_args()
+
+    # Check that the given libc++.so file is a valid symlink.
+    if not os.path.islink(symlink_file):
+        print_and_exit("symlink file %s is not a symlink" % symlink_file)
+
+    # Read the symlink so we know what libc++ to link to in the linker script.
+    linked_libcxx = os.readlink(symlink_file)
+
+    # Check that the abi_libname is one of the supported values.
+    supported_abi_list = ['c++abi', 'stdc++', 'supc++', 'cxxrt']
+    if abi_libname not in supported_abi_list:
+        print_and_exit("abi name '%s' is not supported: Use one of %r" %
+                        (abi_libname, supported_abi_list))
+
+    # Generate the linker script contents and print the script and destination
+    # information.
+    contents = "INPUT(%s -l%s)" % (linked_libcxx, abi_libname)
+    print("GENERATING SCRIPT: '%s' as file %s" % (contents, symlink_file))
+
+    # Remove the existing libc++ symlink and replace it with the script.
+    if not dryrun:
+        os.unlink(symlink_file)
+        with open(symlink_file, 'w') as f:
+            f.write(contents + "\n")
+
+
+if __name__ == '__main__':
+    main()