[runtimes] Fix building initial libunwind+libcxxabi+libcxx with compiler implied -lunwind

[mstorsjo]

Summary

This does mostly the same as D112126, but for the runtimes cmake files.
Most of that is straightforward, but the interdependency between
libcxx and libunwind is tricky:

Libunwind is built at the same time as libcxx, but libunwind is not
installed yet. LIBCXXABI_USE_LLVM_UNWINDER makes libcxx link directly
against the just-built libunwind, but the compiler implicit -lunwind
isn't found. This patch avoids that by adding the intermediate library
output path to link_directories(). I have a separate alternative
patch for the same issue that I'll post in tandem.

(There might be a similar interdependency between libcxxabi and
libunwind too - it doesn't show up in my builds targeting Windows,
as libcxxabi is always built statically though. @hvdijk, can you test
using the new build layout by targeting cmake at llvm-project/runtimes
instead of building libunwind/libcxxabi/libcxx individually in your
environment and see how it behaves?)

[mstorsjo]

Hmm, the CI failures show that --unwindlib=none does break cases when we're configuring with ASAN/TSAN, because those runtimes (which get implicitly linked everywhere during the cmake tests) do need the unwinder. Should we change the tests to first see if linking works without any flags added, and only try --unwindlib=none if linking otherwise fails?

Secondly, adding --unwindlib=none to CMAKE_REQUIRED_FLAGS does break a lot of other tests within HandleLLVMOptions (is that even needed when building runtimes, which are kinda disconnected from the things done in the main llvm dir?), as those tests check for options with -Werror (to avoid picking up options that do nothing but add warnings), because --unwindlib=none produces warnings when set while compiling (contrary to -nostdlib++ which doesn't cause warnings if set while compiling).

[mstorsjo]

(I guess we could make Clang not warn on unused --unwindlib options, but that doesn't help people building with the currently supported Clang 12 or 13.)

[mstorsjo]

Check if linking works before trying to add --unwindlib=none, this should fix the libcxx CI ASAN/TSAN tests.

Tests within HandleLLVMOptions (add_flag_if_supported and similar) still fail when --unwindlib=none ends up added though, as the option causes warnings, and that function tests with -Werror.

[mstorsjo]

Preferring D113253 over this one.

libcxx/src/CMakeLists.txt

function(cxx_set_common_defines name)
   endif()
 endfunction()
 
 split_list(LIBCXX_COMPILE_FLAGS)
 split_list(LIBCXX_LINK_FLAGS)
 
 # Build the shared library.
 if (LIBCXX_ENABLE_SHARED)
+  if (LIBCXXABI_USE_LLVM_UNWINDER AND (TARGET unwind OR HAVE_LIBUNWIND))
+    # If the compiler driver implicitly links -lunwind, and it's built as part
+    # of this same build but not installed in the compiler's default lib
+    # directories yet, -lunwind wouldn't be found. (I.e., after adding
+    # implicit options, the linker would be getting the arguments
+    # "<intermediate-lib-dir>/libunwind.<ext> [...] -lunwind".)
+    # Add the intermediate lib directory to the linker path, so that the
+    # linker is given the arguments
+    # "<intermediate-lib-dir>/libunwind.<ext> -L<intermediate-lib-dir>
+    #  [...] -lunwind". The -lunwind should resolve to the recently
+    # built library and nothing gets linked in from there anyway.
+    #
+    # LIBUNWIND_LIBRARY_DIR might not be set yet, but assume it's the same
+    # as LIBCXX_LIBRARY_DIR.
+    link_directories(${LIBCXX_LIBRARY_DIR})
+  endif()
   add_library(cxx_shared SHARED ${exclude_from_all} ${LIBCXX_SOURCES} ${LIBCXX_HEADERS})
   target_link_libraries(cxx_shared PUBLIC cxx-headers
                                    PRIVATE ${LIBCXX_LIBRARIES})
   set_target_properties(cxx_shared
     PROPERTIES
       COMPILE_FLAGS "${LIBCXX_COMPILE_FLAGS}"
       LINK_FLAGS    "${LIBCXX_LINK_FLAGS}"
       OUTPUT_NAME   "c++"

runtimes/CMakeLists.txt

 # This file handles building LLVM runtime sub-projects.
 cmake_minimum_required(VERSION 3.13.4)
-project(Runtimes C CXX ASM)
+
+# Add path for custom and the LLVM build's modules to the CMake module path.
+list(INSERT CMAKE_MODULE_PATH 0
+  "${CMAKE_CURRENT_SOURCE_DIR}/cmake"
+  "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules"
+  "${CMAKE_CURRENT_SOURCE_DIR}/../cmake"
+  "${CMAKE_CURRENT_SOURCE_DIR}/../cmake/Modules"
+  "${CMAKE_CURRENT_SOURCE_DIR}/../llvm/cmake"
+  "${CMAKE_CURRENT_SOURCE_DIR}/../llvm/cmake/modules"
+)
+
+# We may have an incomplete toolchain - do language support tests without
+# linking.
+include(EnableLanguageNolink)
+project(Runtimes LANGUAGES NONE)
+llvm_enable_language_nolink(C CXX ASM)
 
 set(LLVM_ALL_RUNTIMES "compiler-rt;libc;libcxx;libcxxabi;libunwind;openmp")
 set(LLVM_ENABLE_RUNTIMES "" CACHE STRING
   "Semicolon-separated list of runtimes to build (${LLVM_ALL_RUNTIMES}), or \"all\".")
 if(LLVM_ENABLE_RUNTIMES STREQUAL "all" )
   set(LLVM_ENABLE_RUNTIMES ${LLVM_ALL_RUNTIMES})
 endif()
 

 
 function(runtime_register_component name)
   set_property(GLOBAL APPEND PROPERTY SUB_COMPONENTS ${name})
 endfunction()
 
 find_package(LLVM PATHS "${LLVM_BINARY_DIR}" NO_DEFAULT_PATH NO_CMAKE_FIND_ROOT_PATH)
 find_package(Clang PATHS "${LLVM_BINARY_DIR}" NO_DEFAULT_PATH NO_CMAKE_FIND_ROOT_PATH)
 
-# Add path for custom and the LLVM build's modules to the CMake module path.
-list(INSERT CMAKE_MODULE_PATH 0
-  "${CMAKE_CURRENT_SOURCE_DIR}/cmake"
-  "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules"
-  "${CMAKE_CURRENT_SOURCE_DIR}/../llvm/cmake"
-  "${CMAKE_CURRENT_SOURCE_DIR}/../llvm/cmake/modules"
-)
-
 function(get_compiler_rt_path path)
   foreach(entry ${runtimes})
     get_filename_component(projName ${entry} NAME)
     if("${projName}" MATCHES "compiler-rt")
       set(${path} ${entry} PARENT_SCOPE)
       return()
     endif()
   endforeach()

 # This variable is used by individual runtimes to locate LLVM files.
 set(LLVM_PATH ${CMAKE_CURRENT_SOURCE_DIR}/../llvm)
 
 if(APPLE)
   set(LLVM_ENABLE_LIBCXX ON CACHE BOOL "")
 endif()
 
 include(CheckLibraryExists)
+include(CheckLinkerFlag)
 include(CheckCCompilerFlag)
 include(CheckCXXCompilerFlag)
 
+
+check_c_compiler_flag("" LLVM_RUNTIMES_LINKING_WORKS)
+if (NOT LLVM_RUNTIMES_LINKING_WORKS)
+  # The compiler driver may be implicitly trying to link against libunwind, which
+  # might not work if libunwind doesn't exist yet. Try to check if
+  # --unwindlib=none is supported, and use that if possible.
+  # Don't add this if not necessary to fix linking, as it can break using
+  # e.g. ASAN/TSAN.
+  llvm_check_linker_flag("--unwindlib=none" LLVM_RUNTIMES_SUPPORT_UNWINDLIB_NONE_FLAG)
+  if (LLVM_RUNTIMES_SUPPORT_UNWINDLIB_NONE_FLAG)
+    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} --unwindlib=none")
+  endif()
+endif()
+
 # Disable use of the installed C++ standard library when building runtimes.
 # Check for -nostdlib++ first; if there's no C++ standard library yet,
 # all check_cxx_compiler_flag commands will fail until we add -nostdlib++
 # (or -nodefaultlibs).
-check_c_compiler_flag(-nostdlib++ LLVM_RUNTIMES_SUPPORT_NOSTDLIBXX_FLAG)
+llvm_check_linker_flag(-nostdlib++ LLVM_RUNTIMES_SUPPORT_NOSTDLIBXX_FLAG)
 if (LLVM_RUNTIMES_SUPPORT_NOSTDLIBXX_FLAG)
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdlib++")
 endif()
 check_cxx_compiler_flag(-nostdinc++ LLVM_RUNTIMES_SUPPORT_NOSTDINCXX_FLAG)
 if (LLVM_RUNTIMES_SUPPORT_NOSTDINCXX_FLAG)
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdinc++")
 endif()