Set LLVM_ATOMIC_LIB variable for convenient linking against libatomic

* Set LLVM_ATOMIC_LIB to keep track of when we need to link against libatomic.
* Add detection of mold linker which is required for this.
* Use --as-needed when linking against libatomic as a bonus. On some platforms,
  libatomic may be required only sometimes.

Bug: https://bugs.gentoo.org/832675
Thanks-to: Arfrever Frehtes Taifersar Arahesis <Arfrever@Apache.Org>
Tested-by: erhard_f@mailbox.org <erhard_f@mailbox.org>

Differential Revision: https://reviews.llvm.org/D136280

(cherry picked from commit fa981b5)

Author: thesamesam

llvm/cmake/modules/AddLLVM.cmake

@@ -212,6 +212,7 @@ if (NOT DEFINED LLVM_LINKER_DETECTED AND NOT WIN32)
   else()
     if("${stdout}" MATCHES "^mold")
       set(LLVM_LINKER_DETECTED YES CACHE INTERNAL "")
+      set(LLVM_LINKER_IS_MOLD YES CACHE INTERNAL "")
       message(STATUS "Linker detection: mold")
     elseif("${stdout}" MATCHES "GNU gold")
       set(LLVM_LINKER_DETECTED YES CACHE INTERNAL "")

llvm/cmake/modules/CheckAtomic.cmake

@@ -82,6 +82,19 @@ elseif(LLVM_COMPILER_IS_GCC_COMPATIBLE OR CMAKE_CXX_COMPILER_ID MATCHES "XL")
   endif()
 endif()
 
+# Set variable LLVM_ATOMIC_LIB specifying flags for linking against libatomic.
+if(HAVE_CXX_ATOMICS_WITH_LIB OR HAVE_CXX_ATOMICS64_WITH_LIB)
+  # Use options --push-state, --as-needed and --pop-state if linker is known to support them.
+  # Use single option -Wl of compiler driver to avoid incorrect re-ordering of options by CMake.
+  if(LLVM_LINKER_IS_GNULD OR LLVM_LINKER_IS_GOLD OR LLVM_LINKER_IS_LLD OR LLVM_LINKER_IS_MOLD)
+    set(LLVM_ATOMIC_LIB "-Wl,--push-state,--as-needed,-latomic,--pop-state")
+  else()
+    set(LLVM_ATOMIC_LIB "-latomic")
+  endif()
+else()
+  set(LLVM_ATOMIC_LIB)
+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.

llvm/lib/Support/CMakeLists.txt

@@ -59,9 +59,7 @@ elseif( CMAKE_HOST_UNIX )
   if( LLVM_ENABLE_TERMINFO )
     set(imported_libs ${imported_libs} Terminfo::terminfo)
   endif()
-  if( LLVM_ENABLE_THREADS AND (HAVE_LIBATOMIC OR HAVE_CXX_LIBATOMICS64) )
-    set(system_libs ${system_libs} atomic)
-  endif()
+  set(system_libs ${system_libs} ${LLVM_ATOMIC_LIB})
   set(system_libs ${system_libs} ${LLVM_PTHREAD_LIB})
   if( UNIX AND NOT (BEOS OR HAIKU) )
     set(system_libs ${system_libs} m)

llvm/tools/dsymutil/CMakeLists.txt

@@ -40,6 +40,4 @@ if(APPLE)
   target_link_libraries(dsymutil PRIVATE "-framework CoreFoundation")
 endif(APPLE)
 
-if(HAVE_CXX_ATOMICS_WITH_LIB OR HAVE_CXX_ATOMICS64_WITH_LIB)
-  target_link_libraries(dsymutil PRIVATE atomic)
-endif()
+target_link_libraries(dsymutil PRIVATE ${LLVM_ATOMIC_LIB})