[libcxx] Redo adding support for building and testing with an ABI library not along linker paths

[EricWF]

Summary

This is the second attempt at allowing for the use of libraries that the linker cannot find. The first attempt used CMAKE_LIBRARY_PATH and find_library to select which ABI library should be used. There were a number of problems with this approach:

• find_library didn't work with cmake targets (ie in-tree libcxxabi build)
• It wasn't always possible to determine where find_library actually found your library.
• target_link_libraries inserted the path of the ABI library into libc++'s RPATH when find_library was used.
• Linking libc++ and it's ABI library is a special case. It's a lot easier to keep it simple.

After discussion with @cbergstrum a new approach was decided upon.
This patch achieve the same ends by simply using LIBCXX_CXX_ABI_LIBRARY_PATH to specify where to find the library (if the linker won't find it). When this variable is defined it is simply added as a library search path when linking libc++. It is a lot easier to duplicate this behavior in LIT. It also prevents libc++ from being linked with an RPATH.

[chandlerc]

LGTM (I'm familiar with the CMake side of things, so happy to provide the review)

I think the real advantage of this is the improvement for an in-tree build. We could make find_library work for other cases even though its hard. But this is much nicer with the in-tree stuff.

[danalbert]

LGTM.

cmake/Modules/HandleLibCXXABI.cmake

 #
 # Parameters:
 #   abidefines: A list of defines needed to compile libc++ with the ABI library
-#   abilibs   : A list of libraries to link against
+#   abilib    : The ABI library to link against.
 #   abifiles  : A list of files (which may be relative paths) to copy into the
 #               libc++ build tree for the build.  These files will also be
 #               installed alongside the libc++ headers.
 #   abidirs   : A list of relative paths to create under an include directory
 #               in the libc++ build directory.
 #
-macro(setup_abi_lib abipathvar abidefines abilibs abifiles abidirs)
+macro(setup_abi_lib abipathvar abidefines abilib abifiles abidirs)
   list(APPEND LIBCXX_CXX_FEATURE_FLAGS ${abidefines})
   set(${abipathvar} "${${abipathvar}}"
     CACHE PATH
     "Paths to C++ ABI header directories separated by ';'." FORCE
     )
 
-  # To allow for libraries installed along non-default paths we use find_library
-  # to locate the ABI libraries we want. Making sure to clean the cache before
-  # each run of find_library.
-  set(LIBCXX_CXX_ABI_LIBRARIES "")
-  foreach(alib ${abilibs})
-    # cxxabi is a cmake target and not a library.
-    # Handle this special case explicitly.
-    # Otherwise use find_library to locate the correct binary.
-    if (alib STREQUAL "cxxabi")
-      list(APPEND LIBCXX_CXX_ABI_LIBRARIES cxxabi)
-    else()
-      unset(_Res CACHE)
-      find_library(_Res ${alib})
-      if (${_Res} STREQUAL "_Res-NOTFOUND")
-        message(FATAL_ERROR "Failed to find ABI library: ${alib}")
-      else()
-        message(STATUS "Adding ABI library: ${_Res}")
-        list(APPEND LIBCXX_CXX_ABI_LIBRARIES ${_Res})
-      endif()
-    endif()
-  endforeach()
+  set(LIBCXX_CXX_ABI_LIBRARY ${abilib})
 
   set(LIBCXX_ABILIB_FILES ${abifiles})
 

lib/CMakeLists.txt

 endif()
 
 # Generate library list.
-set(libraries ${LIBCXX_CXX_ABI_LIBRARIES})
+set(libraries ${LIBCXX_CXX_ABI_LIBRARY})
 append_if(libraries LIBCXX_HAS_PTHREAD_LIB pthread)
 append_if(libraries LIBCXX_HAS_C_LIB c)
 append_if(libraries LIBCXX_HAS_M_LIB m)
 append_if(libraries LIBCXX_HAS_RT_LIB rt)
 append_if(libraries LIBCXX_HAS_GCC_S_LIB gcc_s)
 
+#if LIBCXX_CXX_ABI_LIBRARY_PATH is defined we want to add it to the search path.
+if (DEFINED LIBCXX_CXX_ABI_LIBRARY_PATH)
+  target_link_libraries(cxx "-L${LIBCXX_CXX_ABI_LIBRARY_PATH}")
+endif()
 target_link_libraries(cxx ${libraries})
 
 # Setup flags.

test/lit.cfg

 
     def configure_link_flags(self):
         # Configure library search paths
-        lpaths = self.get_lit_conf('library_paths', '').split(';')
-        lpaths = [l for l in lpaths if l.strip()]
+        abi_library_path = self.get_lit_conf('abi_library_path', '')
         self.link_flags += ['-L' + self.obj_root + '/lib']
-        self.link_flags += ['-L' + l for l in lpaths]
+        if not self.use_system_lib:
+            self.link_flags += ['-Wl,-rpath', '-Wl,' + self.obj_root + '/lib']
+        if abi_library_path:
+            self.link_flags += ['-L' + abi_library_path,
+                                '-Wl,-rpath', '-Wl,' + abi_library_path]
         # Configure libraries
         self.link_flags += ['-lc++']
         link_flags_str = self.get_lit_conf('link_flags')

         if link_flags_str:
             self.link_flags += shlex.split(link_flags_str)
 
-        # Configure library runtime search paths
-        if not self.use_system_lib:
-            self.link_flags += ['-Wl,-rpath', '-Wl,' + self.obj_root + '/lib']
-        for l in lpaths:
-            self.link_flags += ['-Wl,-rpath', '-Wl,' + l]
 
     def configure_std_flag(self):
         # Try and get the std version from the command line. Fall back to

test/lit.site.cfg.in

 config.enable_shared         = @LIBCXX_ENABLE_SHARED@
 config.cxx_abi               = "@LIBCXX_CXX_ABI_LIBNAME@"
 config.llvm_use_sanitizer    = "@LLVM_USE_SANITIZER@"
-config.library_paths         = "@CMAKE_LIBRARY_PATH@"
+config.abi_library_path      = "@LIBCXX_CXX_ABI_LIBRARY_PATH@"
 
 # Let the main config do the real work.
 lit_config.load_config(config, "@LIBCXX_SOURCE_DIR@/test/lit.cfg")

www/index.html

     Normally you must link libc++ against a ABI shared library that the
     linker can find.  If you want to build and test libc++ against an ABI
     library not in the linker's path you need to set
-    <code>-DCMAKE_LIBRARY_PATH=/path/to/abi/lib</code> when configuring CMake.
+    <code>-DLIBCXX_CXX_ABI_LIBRARY_PATH=/path/to/abi/lib</code> when
+    configuring CMake.
   </p>
   <p>
     An example build using libc++abi would look like:

     <li><code>CC=clang CXX=clang++ cmake
               -DLIBCXX_CXX_ABI=libc++abi
               -DLIBCXX_LIBCXXABI_INCLUDE_PATHS="/path/to/libcxxabi/include"
-              -DCMAKE_LIBRARY_PATH="/path/to/libcxxabi-build/lib"
+              -DLIBCXX_CXX_ABI_LIBRARY_PATH="/path/to/libcxxabi-build/lib"
               path/to/libcxx</code></li>
     <li><code>make</code></li>
     </ul>