[llvm] [CMake] Set LLVM_ATOMIC_LIB variable for convenient linking against libatomic

[thesamesam]

Summary

• 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 <[e-mail]>
Tested-by: [e-mail] <[e-mail]>

[mgorny]

I haven't tested it but at a first glance, it makes sense.

[MaskRay]

Is -latomic only needed on some 32-bit architectures? Does this change link against -latomic for major 64-bit architectures?

[Arfrever]

In D136280#3870090, @MaskRay wrote:

Is -latomic only needed on some 32-bit architectures? Does this change link against -latomic for major 64-bit architectures?

Earlier unchanged code in CheckAtomic.cmake tries to detect if libatomic exists and if linking against libatomic is possibly needed (see variables HAVE_LIBATOMIC, HAVE_CXX_LIBATOMICS64).
In such case, HAVE_CXX_ATOMICS_WITH_LIB and/or HAVE_CXX_ATOMICS64_WITH_LIB would be set.
LLVM_ATOMIC_LIB is set to non-empty value when HAVE_CXX_ATOMICS_WITH_LIB or HAVE_CXX_ATOMICS64_WITH_LIB are set.

When there was no linking against -latomic, there should still be none.
When there was linking against -latomic, there will be now (with all modern linkers) -Wl,--push-state,--as-needed,-latomic,--pop-state or (with archaic linkers) -latomic.
Due to --as-needed, on some architectures, some particular libraries or executables will no longer be unnecessarily linked against libatomic.

llvm/cmake/modules/AddLLVM.cmake

elseif("${stderr}" MATCHES "^LLD" OR
       message(STATUS "Linker detection: lld")
     else()
       set(LLVM_LINKER_DETECTED NO CACHE INTERNAL "")
       message(STATUS "Linker detection: unknown")
     endif()
   else()
     if("${stdout}" MATCHES "^mold")
       set(LLVM_LINKER_DETECTED YES CACHE INTERNAL "")
+      set(LLVM_LINKER_IS_MOLD YES CACHE INTERNAL "")
       message(STATUS "Linker detection: mold")
     elseif("${stdout}" MATCHES "GNU gold")
       set(LLVM_LINKER_DETECTED YES CACHE INTERNAL "")
       set(LLVM_LINKER_IS_GOLD YES CACHE INTERNAL "")
       message(STATUS "Linker detection: GNU Gold")
     elseif("${stdout}" MATCHES "^LLD")
       set(LLVM_LINKER_DETECTED YES CACHE INTERNAL "")
       set(LLVM_LINKER_IS_LLD YES CACHE INTERNAL "")

llvm/cmake/modules/CheckAtomic.cmake

if(HAVE_CXX_LIBATOMICS64)
         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()
 
+# 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.
 CHECK_CXX_SOURCE_COMPILES("
 #ifdef _MSC_VER
 #include <windows.h>
 #endif
 int main() {

llvm/lib/Support/CMakeLists.txt

if( HAVE_BACKTRACE AND NOT "${Backtrace_LIBRARIES}" STREQUAL "" )
     # regular short library name, suitable for appending to a -l link flag.
     get_filename_component(Backtrace_LIBFILE ${Backtrace_LIBRARIES} NAME_WE)
     STRING(REGEX REPLACE "^lib" "" Backtrace_LIBFILE ${Backtrace_LIBFILE})
     set(system_libs ${system_libs} ${Backtrace_LIBFILE})
   endif()
   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)
   endif()
   if( UNIX AND ${CMAKE_SYSTEM_NAME} MATCHES "SunOS" )
     set(system_libs ${system_libs} kstat)
   endif()
   if( FUCHSIA )

llvm/tools/dsymutil/CMakeLists.txt

add_llvm_tool(dsymutil
   DsymutilTableGen
   GENERATE_DRIVER
   )
 
 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})