[CMake][NFC] Clean up CheckAtomic.cmake

[luismarques]

Summary

CheckAtomic.cmake was skipping the test of whether atomics work in MSVC without an atomics library (they do), but not setting the value of HAVE_CXX_ATOMICS_WITHOUT_LIB. That caused build issues when trying to land D69869. I fixed that issue in f128f442a3d, by adding an elseif(MSVC), as was being done below in the 64-bit atomics check. That minimal fix did work, but it kept various inconsistencies between the original atomics check and the 64-bit one. This patch now makes the checks follow the same structure, cleaning them up.

[luismarques]

Ping.

[lenary]

I am happy that this is a simple logic refactoring of the original, given LLVM_COMPILER_IS_GCC_COMPATIBLE is currently just true/false based on the MSVC variable.

I am glad both blocks now have the same structure, which helps when debugging build issues.

LGTM (modulo nits below).

llvm/cmake/modules/CheckAtomic.cmake

45

Nit: whitespace before ).

66–67

Nit: whitespace before ).

[luismarques]

Fix whitespace.

llvm/cmake/modules/CheckAtomic.cmake

int main() {
   uint64_t i = x.load(std::memory_order_relaxed);
   return 0;
 }
 " ${varname})
   set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
 endfunction(check_working_cxx_atomics64)
 
 
-# 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)
+# Check for (non-64-bit) atomic operations.
+if(MSVC)
+  set(HAVE_CXX_ATOMICS_WITHOUT_LIB True)
+elseif(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 )
+    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!")
+        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()
-elseif(MSVC)
-  set(HAVE_CXX_ATOMICS_WITHOUT_LIB True)
 endif()
 
 # Check for 64 bit atomic operations.
 if(MSVC)
   set(HAVE_CXX_ATOMICS64_WITHOUT_LIB True)
-else()
+elseif(LLVM_COMPILER_IS_GCC_COMPATIBLE)
+  # First check if atomics work without the library.
   check_working_cxx_atomics64(HAVE_CXX_ATOMICS64_WITHOUT_LIB)
-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()
+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 <windows.h>
 #endif