Revert "Introduce a cmake module to figure out whether we need to link with libatomic."

This reverts commit r260235. It breaks LLVM's bootstrap when building
with a -gcc-toolchain and the system's gcc installation does not provide
the libatomic library and its headers. We should check whether
LIBCXX_GCC_TOOLCHAIN is set and adjust the flags accordingly.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@260323 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/Modules/CheckLibcxxAtomic.cmake

@@ -1,38 +0,0 @@
-INCLUDE(CheckCXXSourceCompiles)
-
-# Sometimes linking against libatomic is required for atomic ops, if
-# the platform doesn't support lock-free atomics.
-#
-# We could modify LLVM's CheckAtomic module and have it check for 64-bit
-# atomics instead. However, we would like to avoid careless uses of 64-bit
-# atomics inside LLVM over time on 32-bit platforms.
-
-function(check_cxx_atomics varname)
-  set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
-  set(CMAKE_REQUIRED_FLAGS "-std=c++11")
-  check_cxx_source_compiles("
-#include <cstdint>
-#include <atomic>
-std::atomic<uintptr_t> x;
-std::atomic<uintmax_t> y;
-int main() {
-  return x + y;
-}
-" ${varname})
-  set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
-endfunction(check_cxx_atomics)
-
-check_cxx_atomics(HAVE_CXX_ATOMICS_WITHOUT_LIB)
-# If not, check if the library exists, and atomics work with it.
-if(NOT HAVE_CXX_ATOMICS_WITHOUT_LIB)
-  check_library_exists(atomic __atomic_fetch_add_8 "" HAVE_LIBATOMIC)
-  if(HAVE_LIBATOMIC)
-    list(APPEND CMAKE_REQUIRED_LIBRARIES "atomic")
-    check_cxx_atomics(HAVE_CXX_ATOMICS_WITH_LIB)
-    if (NOT HAVE_CXX_ATOMICS_WITH_LIB)
-      message(FATAL_ERROR "Host compiler must support std::atomic!")
-    endif()
-  else()
-    message(FATAL_ERROR "Host compiler appears to require libatomic, but cannot find it.")
-  endif()
-endif()

cmake/config-ix.cmake

@@ -1,6 +1,5 @@
 include(CheckLibraryExists)
 include(CheckCXXCompilerFlag)
-include(CheckLibcxxAtomic)
 
 # Check compiler flags
 
@@ -18,7 +17,3 @@ check_library_exists(c fopen "" LIBCXX_HAS_C_LIB)
 check_library_exists(m ccos "" LIBCXX_HAS_M_LIB)
 check_library_exists(rt clock_gettime "" LIBCXX_HAS_RT_LIB)
 check_library_exists(gcc_s __gcc_personality_v0 "" LIBCXX_HAS_GCC_S_LIB)
-
-if (NOT HAVE_CXX_ATOMICS_WITHOUT_LIB)
-  set(LIBCXX_HAS_ATOMIC_LIB True)
-endif()

lib/CMakeLists.txt

@@ -79,7 +79,6 @@ add_library_flags_if(LIBCXX_HAS_C_LIB c)
 add_library_flags_if(LIBCXX_HAS_M_LIB m)
 add_library_flags_if(LIBCXX_HAS_RT_LIB rt)
 add_library_flags_if(LIBCXX_HAS_GCC_S_LIB gcc_s)
-add_library_flags_if(LIBCXX_HAS_ATOMIC_LIB atomic)
 
 # Setup flags.
 add_flags_if_supported(-fPIC)

test/CMakeLists.txt

@@ -15,7 +15,6 @@ pythonize_bool(LIBCXX_ENABLE_SHARED)
 pythonize_bool(LIBCXX_BUILD_32_BITS)
 pythonize_bool(LIBCXX_GENERATE_COVERAGE)
 pythonize_bool(LIBCXXABI_USE_LLVM_UNWINDER)
-pythonize_bool(LIBCXX_HAS_ATOMIC_LIB)
 
 # The tests shouldn't link to any ABI library when it has been linked into
 # libc++ statically or via a linker script.

test/libcxx/test/target_info.py

@@ -172,9 +172,6 @@ class LinuxLocalTI(DefaultTargetInfo):
             flags += ['-lunwind', '-ldl']
         else:
             flags += ['-lgcc_s', '-lgcc']
-        use_libatomic = self.full_config.get_lit_bool('use_libatomic', False)
-        if use_libatomic:
-            flags += ['-latomic']
         san = self.full_config.get_lit_conf('use_sanitizer', '').strip()
         if san:
             # The libraries and their order are taken from the

test/lit.site.cfg.in

@@ -20,7 +20,6 @@ config.generate_coverage        = "@LIBCXX_GENERATE_COVERAGE@"
 config.target_info              = "@LIBCXX_TARGET_INFO@"
 config.executor                 = "@LIBCXX_EXECUTOR@"
 config.llvm_unwinder            = "@LIBCXXABI_USE_LLVM_UNWINDER@"
-config.use_libatomic            = "@LIBCXX_HAS_ATOMIC_LIB@"
 
 # Let the main config do the real work.
 lit_config.load_config(config, "@LIBCXX_SOURCE_DIR@/test/lit.cfg")