Reapply #2 of [runtimes] Fix building initial libunwind+libcxxabi+libcxx with compiler implied -lunwind

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 --unwindlib=none if
supported, if we are going to link explicitly against a newly built
unwinder anyway.

Since the previous attempt, this no longer uses
llvm_enable_language_nolink (and thus doesn't set
CMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY during the compiler
sanity checks). Setting CMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY
during compiler sanity checks makes cmake not learn about some
aspects of the compiler, which can make further find_library or
find_package fail. This caused OpenMP to not detect libelf and libffi,
disabling some OpenMP target plugins.

Instead, require the caller to set CMAKE_{C,CXX}_COMPILER_WORKS=YES
when building in a configuration with an incomplete toolchain.

Differential Revision: https://reviews.llvm.org/D113253

1 files changed, 18 insertions, 1 deletions

diff --git a/runtimes/CMakeLists.txt b/runtimes/CMakeLists.txt
index eeaff6c43184..8f37d0e59feb 100644
--- a/runtimes/CMakeLists.txt
+++ b/runtimes/CMakeLists.txt
@@ -32,6 +32,8 @@ find_package(Clang PATHS "${LLVM_BINARY_DIR}" NO_DEFAULT_PATH NO_CMAKE_FIND_ROOT
 list(INSERT CMAKE_MODULE_PATH 0
   "${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"
 )
@@ -84,14 +86,29 @@ 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 "${CMAKE_REQUIRED_FLAGS} --unwindlib=none")
+  endif()
+endif()
+
 # Disable use of the installed C++ standard library when building runtimes.
 # Check for -nostdlib++ first; if there's no C++ standard library yet,
 # all check_cxx_compiler_flag commands will fail until we add -nostdlib++
 # (or -nodefaultlibs).
-check_c_compiler_flag(-nostdlib++ LLVM_RUNTIMES_SUPPORT_NOSTDLIBXX_FLAG)
+llvm_check_linker_flag(-nostdlib++ LLVM_RUNTIMES_SUPPORT_NOSTDLIBXX_FLAG)
 if (LLVM_RUNTIMES_SUPPORT_NOSTDLIBXX_FLAG)
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdlib++")
 endif()