[CMake] Fix linker detection in AddLLVM.cmake

[light2yellow]

Summary

Fix linker not being correctly detected when a custom one is specified
through LLVM_USE_LINKER CMake variable.

In particular,

cmake -DCMAKE_BUILD_TYPE=Release -DLLVM_USE_LINKER=gold ../llvm

resulted into

• Linker detection: GNU ld

instead of

• Linker detection: GNU Gold

due to the construction not accounting for such variable. It led to the general
confusion and prevented setting linker-specific flags inside functions defined
in AddLLVM.cmake.

[lichray]

Tested on FreeBSD; works for me.

[ruiu]

cmake/modules/AddLLVM.cmake

153

Is this safe if a C compiler does not support the -fuse-ld option?

[lichray]

cmake/modules/AddLLVM.cmake

153

That may be too old to compile LLVM. I would say wait them come to us. After all, this option is to assist detection, and more users are specifying linkers directly.

[ruiu]

cmake/modules/AddLLVM.cmake

153

You can use compilers other than clang to build clang and llvm, and I believe only clang supports this -fuse-ld option.

[lichray]

cmake/modules/AddLLVM.cmake

153

No? gcc does https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55470 therefore Intel C++ does. Clang does, then so does XL on AIX and Oracle.

[ruiu]

I believe it only recognizes -fuse-ld=bfd and -fuse-ld=gold. You cannot pass any other linker name.

[lichray]

In D38665#905826, @ruiu wrote:

I believe it only recognizes -fuse-ld=bfd and -fuse-ld=gold. You cannot pass any other linker name.

I think that's enough. If you worry about lld, there is another option LLVM_USE_LLD=ON.

[ruiu]

I mean -DLLVM_USE_LINKER=lld is a valid option, so when it failed due to the lack compiler support of -fuse-ld=lld, it should be reported as a meaningful error, instead of a cryptic "CMake failed to compile this program blah blah." message. I think that is also true to the case when you pass -DLLVM_USE_LINKER=gold but gold was not installed to your system.

[lichray]

In D38665#905947, @ruiu wrote:

I mean -DLLVM_USE_LINKER=lld is a valid option, so when it failed due to the lack compiler support of -fuse-ld=lld, it should be reported as a meaningful error, instead of a cryptic "CMake failed to compile this program blah blah." message. I think that is also true to the case when you pass -DLLVM_USE_LINKER=gold but gold was not installed to your system.

Let me clarify this with refreshed information -- there is no LLVM_USE_LLD but LLVM_ENABLE_LLD, which builds lld and then set LLVM_USE_LINKER, and then in cmake/modules/HandleLLVMOptions.cmake line 197 detects -fuse-ld=. Which means, neither LLVM_USE_LINKER nor LLVM_ENABLE_LLD had magics to support compilers other than clang to use lld. That is to say, this patch doesn't make our support worse. The error message has room for improvement, but IMHO may not worth to work it out in this patch. The author can feel free to apply line 197's macro here, but unlikely we want to make even more errors from underlying compilers more meaningful.

[ruiu]

CMake inherently runs on a variety of systems, so if your change could result in producing a head-scratching mysterious error message on some system, I think you need to address that from the beginning, even if that system is not supported. Saying that "this flag is not supported/your system is too old" is one of essential roles of our cmakefiles and that is not "good to have".

That said, since we already have a check for a LLVM_USE_LINKER value in llvm/cmake/modules/HandleLLVMOptions.cmake, it seems that we don't need add the same check to this file. So LGTM.

(Actually, the existing check in HandleLLVMOptions.cmake is not working because of a bug, so when you pass a bogus value to -DLLVM_USE_LINKER, it prints out a bad error message. I'll fix it separately.)

[timshen]

Committed as r316956.

cmake/modules/AddLLVM.cmake

function(add_llvm_symbol_exports target_name export_file)
 
   # Add dependency to *_exports later -- CMake issue 14747
   list(APPEND LLVM_COMMON_DEPENDS ${target_name}_exports)
   set(LLVM_COMMON_DEPENDS ${LLVM_COMMON_DEPENDS} PARENT_SCOPE)
 endfunction(add_llvm_symbol_exports)
 
 if(NOT WIN32 AND NOT APPLE)
   # Detect what linker we have here
+  if( LLVM_USE_LINKER )
+    set(command ${CMAKE_C_COMPILER} -fuse-ld=${LLVM_USE_LINKER} -Wl,--version)
+  else()
+    set(command ${CMAKE_C_COMPILER} -Wl,--version)
+  endif()
   execute_process(
-    COMMAND ${CMAKE_C_COMPILER} -Wl,--version
+    COMMAND ${command}
     OUTPUT_VARIABLE stdout
     ERROR_VARIABLE stderr
     )
   set(LLVM_LINKER_DETECTED ON)
   if("${stdout}" MATCHES "GNU gold")
     set(LLVM_LINKER_IS_GOLD ON)
     message(STATUS "Linker detection: GNU Gold")
   elseif("${stdout}" MATCHES "^LLD")