[AIX][cmake] Set atomics related macros when build with xlclang

[lkail]

Summary

Set HAVE_CXX_ATOMICS_WITHOUT_LIB or HAVE_LIBATOMIC when build LLVM with xlclang. With these macros set, libraries like libLLVMSupport are able to know whether it's necessary to add -latomic to dependent system libs. If HAVE_LIBATOMIC is set, llvm-config --system-libs appends -latomic to its output.

[jsji]

Can we set LLVM_COMPILER_IS_GCC_COMPATIBLE in ./llvm/cmake/modules/DetermineGCCCompatible.cmake instead?

[lkail]

In D111782#3064012, @jsji wrote:

Can we set LLVM_COMPILER_IS_GCC_COMPATIBLE in ./llvm/cmake/modules/DetermineGCCCompatible.cmake instead?

xlclang is in theory compatible with gcc's command line syntax as clang, but there are some unexpected behaviors exposed if enable it in current LLVM cmake system. For example, when LLVM_ENABLE_WERROR is set which is our default configuration, current cmake config only add -Werror when LLVM_COMPILER_IS_GCC_COMPATIBLE

# Warnings-as-errors handling for GCC-compatible compilers:                                                                                                                                   
if ( LLVM_COMPILER_IS_GCC_COMPATIBLE )                                                                                                                                                        
  append_if(LLVM_ENABLE_WERROR "-Werror" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)                                                                                                                       
  append_if(LLVM_ENABLE_WERROR "-Wno-error" CMAKE_REQUIRED_FLAGS)                                                                                                                             
endif( LLVM_COMPILER_IS_GCC_COMPATIBLE )

If -Werror is added to xlclang's compile flags, some files are unable to compile due to treating warning as error

/home/lkail/llvm/dev/llvm-project/llvm/lib/Support/BuryPointer.cpp:22:3: error: 1540-2990 The attribute " __attribute__((used))" is not supported. The attribute is ignored.
  LLVM_ATTRIBUTE_USED static const void *GraveYard[kGraveYardMaxSize];

[jsji]

LGTM. Let us deal with the LLVM_COMPILER_IS_GCC_COMPATIBLE in a follow up patch later.

llvm/cmake/modules/CheckAtomic.cmake

 " ${varname})
   set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
 endfunction(check_working_cxx_atomics64)
 
 
 # Check for (non-64-bit) atomic operations.
 if(MSVC)
   set(HAVE_CXX_ATOMICS_WITHOUT_LIB True)
-elseif(LLVM_COMPILER_IS_GCC_COMPATIBLE)
+elseif(LLVM_COMPILER_IS_GCC_COMPATIBLE OR CMAKE_CXX_COMPILER_ID MATCHES "XL")
   # 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()
 
 # Check for 64 bit atomic operations.
 if(MSVC)
   set(HAVE_CXX_ATOMICS64_WITHOUT_LIB True)
-elseif(LLVM_COMPILER_IS_GCC_COMPATIBLE)
+elseif(LLVM_COMPILER_IS_GCC_COMPATIBLE OR CMAKE_CXX_COMPILER_ID MATCHES "XL")
   # First check if atomics work without the library.
   check_working_cxx_atomics64(HAVE_CXX_ATOMICS64_WITHOUT_LIB)
   # 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)