[lld] [CMake] Link liblldCOFF against libatomic when necessary

[thesamesam]

Summary

Also simplify code for liblldCommon using the new LLVM_ATOMIC_LIB variable.

Depends on D136280.

Bug: https://bugs.gentoo.org/832675
Thanks-to: Arfrever Frehtes Taifersar Arahesis <[e-mail]>
Tested-by: [e-mail] <[e-mail]>

[mgorny]

std::atomic seems to be used in ELF and MachO libs as well; though maybe libatomic isn't needed for these types?

[Arfrever]

In D136281#3870058, @mgorny wrote:

std::atomic seems to be used in ELF and MachO libs as well; though maybe libatomic isn't needed for these types?

lld-15.0.2:20221005-094634.log (from https://bugs.gentoo.org/832675) says that this is the place where linking is needed:

/usr/lib/gcc/powerpc-unknown-linux-gnu/11.3.1/../../../../powerpc-unknown-linux-gnu/bin/ld: COFF/CMakeFiles/lldCOFF.dir/DebugTypes.cpp.o: in function `void llvm::function_ref<void (unsigned int)>::callback_fn<lld::coff::TypeMerger::mergeTypesWithGHash()::{lambda(unsigned int)#3}>(int, unsigned int)':
DebugTypes.cpp:(.text._ZN4llvm12function_refIFvjEE11callback_fnIZN3lld4coff10TypeMerger19mergeTypesWithGHashEvEUljE1_EEvij+0x250): undefined reference to `__atomic_load_8'
/usr/lib/gcc/powerpc-unknown-linux-gnu/11.3.1/../../../../powerpc-unknown-linux-gnu/bin/ld: DebugTypes.cpp:(.text._ZN4llvm12function_refIFvjEE11callback_fnIZN3lld4coff10TypeMerger19mergeTypesWithGHashEvEUljE1_EEvij+0x284): undefined reference to `__atomic_compare_exchange_8'
/usr/lib/gcc/powerpc-unknown-linux-gnu/11.3.1/../../../../powerpc-unknown-linux-gnu/bin/ld: DebugTypes.cpp:(.text._ZN4llvm12function_refIFvjEE11callback_fnIZN3lld4coff10TypeMerger19mergeTypesWithGHashEvEUljE1_EEvij+0x364): undefined reference to `__atomic_compare_exchange_8'
collect2: error: ld returned 1 exit status

After fixing of liblldCOFF, there were no other errors during building of LLD.

[mgorny]

I suppose this makes sense. Worst case, we can add it elsewhere later.

[MaskRay]

@efriedma @mstorsjo

[efriedma]

I don't think 32-bit PPC has 64-bit atomics at all, so calling libatomic is expected. (Compare with D123872, where we were using libatomic when it shouldn't have been necessary.)

If the performance is important, we might want to look at changing the code to avoid 64-bit atomics. I see 64-bit std::atomic types in lld/COFF/DebugTypes.cpp, lld/MachO/ICF.cpp, and lld/include/lld/Common/Timer.h .

lld/COFF/CMakeLists.txt

add_lld_library(lldCOFF
   Passes
   Support
   WindowsDriver
   WindowsManifest
 
   LINK_LIBS
   lldCommon
   ${LLVM_PTHREAD_LIB}
+  ${LLVM_ATOMIC_LIB}
 
   DEPENDS
   COFFOptionsTableGen
   intrinsics_gen
   )

lld/Common/CMakeLists.txt

-set(LLD_SYSTEM_LIBS ${LLVM_PTHREAD_LIB})
-
-if(NOT HAVE_CXX_ATOMICS64_WITHOUT_LIB)
-  list(APPEND LLD_SYSTEM_LIBS atomic)
-endif()
-
 find_first_existing_vc_file("${LLVM_MAIN_SRC_DIR}" llvm_vc)
 find_first_existing_vc_file("${LLD_SOURCE_DIR}" lld_vc)
 
 set(version_inc "${CMAKE_CURRENT_BINARY_DIR}/VCSVersion.inc")
 set(generate_vcs_version_script "${LLVM_CMAKE_DIR}/GenerateVersionFromVCS.cmake")
 
 if(lld_vc AND LLVM_APPEND_VC_REV)
   set(lld_source_dir ${LLD_SOURCE_DIR})
add_lld_library(lldCommon
   DebugInfoDWARF
   Demangle
   MC
   Option
   Support
   Target
 
   LINK_LIBS
-  ${LLD_SYSTEM_LIBS}
+  ${LLVM_PTHREAD_LIB}
+  ${LLVM_ATOMIC_LIB}
 
   DEPENDS
   intrinsics_gen
   )