[llvm] r239819 - Repair cmake libatomic check.

[Rafael Espíndola]

Looks like this check is failing in
http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-bootstrap/builds/7899/steps/annotate/logs/stdio

On 16 June 2015 at 10:00, James Y Knight <jyknight at google.com> wrote:
> Author: jyknight
> Date: Tue Jun 16 09:00:01 2015
> New Revision: 239819
>
> URL: http://llvm.org/viewvc/llvm-project?rev=239819&view=rev
> Log:
> Repair cmake libatomic check.
>
> The cmake check for whether libatomic could be used had been
> unconditionally setting the result to false. Which was somewhat
> fortunate, because the prerequisite check for whether it was *needed*
> was always claiming it was, even if it was not.
>
> However, this made platforms where libatomic is actually necessary
> fail to link.
>
> Differential Revision: http://reviews.llvm.org/D10453
>
> Modified:
>     llvm/trunk/cmake/modules/CheckAtomic.cmake
>
> Modified: llvm/trunk/cmake/modules/CheckAtomic.cmake
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/cmake/modules/CheckAtomic.cmake?rev=239819&r1=239818&r2=239819&view=diff
> ==============================================================================
> --- llvm/trunk/cmake/modules/CheckAtomic.cmake (original)
> +++ llvm/trunk/cmake/modules/CheckAtomic.cmake Tue Jun 16 09:00:01 2015
> @@ -2,15 +2,45 @@
>
>  INCLUDE(CheckCXXSourceCompiles)
>
> -check_function_exists(__atomic_fetch_add_4 HAVE___ATOMIC_FETCH_ADD_4)
> -if( NOT HAVE___ATOMIC_FETCH_ADD_4 )
> -  check_library_exists(atomic __atomic_fetch_add_4 "" HAVE_LIBATOMIC)
> -  set(HAVE_LIBATOMIC False)
> -  if( HAVE_LIBATOMIC )
> -    list(APPEND CMAKE_REQUIRED_LIBRARIES "atomic")
> +# Sometimes linking against libatomic is required for atomic ops, if
> +# the platform doesn't support lock-free atomics.
> +
> +function(check_working_cxx_atomics varname)
> +  set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
> +  set(CMAKE_REQUIRED_FLAGS "-std=c++11")
> +  CHECK_CXX_SOURCE_COMPILES("
> +#include <atomic>
> +std::atomic<int> x;
> +int main() {
> +  return x;
> +}
> +" ${varname})
> +  set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
> +endfunction(check_working_cxx_atomics)
> +
> +# This isn't necessary on MSVC, so avoid command-line switch annoyance
> +# by only running on GCC-like hosts.
> +if (LLVM_COMPILER_IS_GCC_COMPATIBLE)
> +  # First check if atomics work without the library.
> +  check_working_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_4 "" HAVE_LIBATOMIC)
> +    if( HAVE_LIBATOMIC )
> +      list(APPEND CMAKE_REQUIRED_LIBRARIES "atomic")
> +      check_working_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()
>  endif()
>
> +## TODO: This define is only used for the legacy atomic operations in
> +## llvm's Atomic.h, which should be replaced.  Other code simply
> +## assumes C++11 <atomic> works.
>  CHECK_CXX_SOURCE_COMPILES("
>  #ifdef _MSC_VER
>  #include <Intrin.h> /* Workaround for PR19898. */
>
>
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