Revert "Reapply [runtimes] Fix building initial libunwind+libcxxabi+l…

…ibcxx with compiler implied -lunwind"

This reverts commit 317dc31.

After that change, OpenMP doesn't find dependencies in the host
system (it fails do find e.g. /usr/lib/x86_64-linux-gnu/libelf.so
which it found before), which causes some OpenMP target offloading
plugins to not be found. This doesn't break the build, but just
causes the AMDGPU OpenMP target plugin to be omitted. See
https://reviews.llvm.org/D113253#3181934 for the report of this
issue.

libcxx/CMakeLists.txt

@@ -765,13 +765,6 @@ function(cxx_link_system_libraries target)
     target_add_link_flags_if_supported(${target} PRIVATE "/nodefaultlib")
   endif()
 
-  if (LIBCXX_SUPPORTS_UNWINDLIB_NONE_FLAG AND LIBCXXABI_USE_LLVM_UNWINDER)
-    # If we're linking directly against the libunwind that we're building
-    # in the same invocation, don't try to link in the toolchain's
-    # default libunwind (which may be missing still).
-    target_add_link_flags_if_supported(${target} PRIVATE "--unwindlib=none")
-  endif()
-
   if (LIBCXX_HAS_SYSTEM_LIB)
     target_link_libraries(${target} PRIVATE System)
   endif()

libcxx/cmake/config-ix.cmake

@@ -1,22 +1,9 @@
 include(CMakePushCheckState)
 include(CheckLibraryExists)
-include(CheckLinkerFlag)
 include(CheckCCompilerFlag)
 include(CheckCXXCompilerFlag)
 include(CheckCSourceCompiles)
 
-# The compiler driver may be implicitly trying to link against libunwind.
-# This is normally ok (libcxx relies on an unwinder), but if libunwind is
-# built in the same cmake invocation as libcxx and we've got
-# LIBCXXABI_USE_LLVM_UNWINDER set, we'd be linking against the just-built
-# libunwind (and the compiler implicit -lunwind wouldn't succeed as the newly
-# built libunwind isn't installed yet). For those cases, it'd be good to
-# link with --uwnindlib=none. Check if that option works.
-llvm_check_linker_flag("--unwindlib=none" LIBCXX_SUPPORTS_UNWINDLIB_NONE_FLAG)
-if (LIBCXX_SUPPORTS_UNWINDLIB_NONE_FLAG)
-  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} --unwindlib=none")
-endif()
-
 if(WIN32 AND NOT MINGW)
   # NOTE(compnerd) this is technically a lie, there is msvcrt, but for now, lets
   # let the default linking take care of that.

runtimes/CMakeLists.txt

@@ -1,21 +1,6 @@
 # This file handles building LLVM runtime sub-projects.
 cmake_minimum_required(VERSION 3.13.4)
-
-# 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)
+project(Runtimes C CXX ASM)
 
 set(LLVM_ALL_RUNTIMES "compiler-rt;libc;libcxx;libcxxabi;libunwind;openmp")
 set(LLVM_ENABLE_RUNTIMES "" CACHE STRING
@@ -43,6 +28,14 @@ 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"
+)
+
 set(LLVM_THIRD_PARTY_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../third-party")
 
 function(get_compiler_rt_path path)
@@ -91,29 +84,14 @@ set(LLVM_CMAKE_DIR ${LLVM_MAIN_SRC_DIR}/cmake/modules)
 set(LLVM_PATH ${CMAKE_CURRENT_SOURCE_DIR}/../llvm)
 
 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 "${CMAKE_REQUIRED_FLAGS} --unwindlib=none")
-  endif()
-endif()
-
 # Disable use of the installed C++ standard library when building runtimes.
 # Check for -nostdlib++ first; if there's no C++ standard library yet,
 # all check_cxx_compiler_flag commands will fail until we add -nostdlib++
 # (or -nodefaultlibs).
-llvm_check_linker_flag(-nostdlib++ LLVM_RUNTIMES_SUPPORT_NOSTDLIBXX_FLAG)
+check_c_compiler_flag(-nostdlib++ LLVM_RUNTIMES_SUPPORT_NOSTDLIBXX_FLAG)
 if (LLVM_RUNTIMES_SUPPORT_NOSTDLIBXX_FLAG)
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdlib++")
 endif()