cmake: simplify atomic checks

cmake/CheckAtomic.cmake

@@ -12,8 +12,12 @@ function(check_working_cxx_atomics varname)
   CHECK_CXX_SOURCE_COMPILES("
 #include <atomic>
 std::atomic<int> x;
+std::atomic<short> y;
+std::atomic<char> z;
 int main() {
-  return x;
+  ++z;
+  ++y;
+  return ++x;
 }
 " ${varname})
   set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
@@ -34,22 +38,6 @@ int main() {
   set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
 endfunction(check_working_cxx_atomics64)
 
-function(check_working_cxx_atomics_for_risc_v varname)
-  set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
-  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -std=c++11")
-  CHECK_CXX_SOURCE_COMPILES("
-#include <atomic>
-std::atomic<uint8_t> x;
-std::atomic<uint16_t> y;
-int main() {
-  x++;
-  y++;
-  return 0;
-}
-" ${varname})
-  set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
-endfunction(check_working_cxx_atomics_for_risc_v)
-
 # 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)
@@ -57,16 +45,11 @@ if (LLVM_COMPILER_IS_GCC_COMPATIBLE)
   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()
 
@@ -79,32 +62,11 @@ endif()
 
 # If not, check if the library exists, and atomics work with it.
 if(NOT HAVE_CXX_ATOMICS64_WITHOUT_LIB)
-  check_library_exists(atomic __atomic_load_8 "" HAVE_CXX_LIBATOMICS64)
-  if(HAVE_CXX_LIBATOMICS64)
   list(APPEND CMAKE_REQUIRED_LIBRARIES "atomic")
   check_working_cxx_atomics64(HAVE_CXX_ATOMICS64_WITH_LIB)
   if (NOT HAVE_CXX_ATOMICS64_WITH_LIB)
     message(FATAL_ERROR "Host compiler must support 64-bit std::atomic!")
   endif()
-  else()
-    message(FATAL_ERROR "Host compiler appears to require libatomic for 64-bit operations, but cannot find it.")
-  endif()
-endif()
-
-# Check for RISC-V atomic operations.
-if(MSVC)
-  set(HAVE_CXX_ATOMICS_RISC_V_WITHOUT_LIB True)
-else()
-  check_working_cxx_atomics_for_risc_v(HAVE_CXX_ATOMICS_RISC_V_WITHOUT_LIB)
-endif()
-
-# If not, check if the library exists, and atomics work with it.
-if(NOT HAVE_CXX_ATOMICS_RISC_V_WITHOUT_LIB)
-  list(APPEND CMAKE_REQUIRED_LIBRARIES "atomic")
-  check_working_cxx_atomics_for_risc_v(HAVE_CXX_ATOMICS_RISC_V_WITH_LIB)
-  if (NOT HAVE_CXX_ATOMICS_RISC_V_WITH_LIB)
-    message(FATAL_ERROR "Host compiler must support RISC-V std::atomic!")
-  endif()
 endif()
 
 ## TODO: This define is only used for the legacy atomic operations in