[LLVM] Add LLVM_ENABLE_LLD option to use LLD as C/C++ linker.

[Eugene.Zelenko]

Summary

Probably CMAKE_MODULE_LINKER_FLAGS should be changed too, but I'm not sure how to check it and how well LLD handles modules.

I checked this patch on my own build on RHEL 6. Regressions were OK.

[compnerd]

cmake/modules/HandleLLVMOptions.cmake

15

Why the whitespace removal?

112

Why the whitespace removal?

148

We need CMAKE_MODULE_LINKER_FLAGS and CMAKE_SHARED_LINKER_FLAGS. This probably works well enough right now due to the fact that the default configuration does not use shared libraries.

[Eugene.Zelenko]

cmake/modules/HandleLLVMOptions.cmake

15

Is it really necessary? I don't see any logic behind. Same for white space below.

15

Is it really necessary? I don't see any logic behind. Same for white space below.

[Eugene.Zelenko]

Also use LLD for CMAKE_MODULE_LINKER_FLAGS.

[compnerd]

Please undo the whitespace changes, they make the file harder to follow.

[Eugene.Zelenko]

Will undo white space changes at commit. Just wanted to make white space consistent.

CMakeLists.txt

if(${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
   option(LLVM_ENABLE_LOCAL_SUBMODULE_VISIBILITY "Compile with -fmodules-local-submodule-visibility." OFF)
 else()
   option(LLVM_ENABLE_MODULE_DEBUGGING "Compile with -gmodules." OFF)
   option(LLVM_ENABLE_LOCAL_SUBMODULE_VISIBILITY "Compile with -fmodules-local-submodule-visibility." ON)
 endif()
 option(LLVM_ENABLE_CXX1Y "Compile with C++1y enabled." OFF)
 option(LLVM_ENABLE_LIBCXX "Use libc++ if available." OFF)
 option(LLVM_ENABLE_LIBCXXABI "Use libc++abi when using libc++." OFF)
+option(LLVM_ENABLE_LLD "Use lld as C and C++ linker." OFF)
 option(LLVM_ENABLE_PEDANTIC "Compile with pedantic enabled." ON)
 option(LLVM_ENABLE_WERROR "Fail and stop if a warning is triggered." OFF)
 
 if( NOT uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG" )
   option(LLVM_ENABLE_ASSERTIONS "Enable assertions" OFF)
 else()
   option(LLVM_ENABLE_ASSERTIONS "Enable assertions" ON)
 endif()

cmake/modules/HandleLLVMOptions.cmake

 # This CMake module is responsible for interpreting the user defined LLVM_
 # options and executing the appropriate CMake commands to realize the users'
 # selections.
 
 # This is commonly needed so make sure it's defined before we include anything
 # else.
 string(TOUPPER "${CMAKE_BUILD_TYPE}" uppercase_CMAKE_BUILD_TYPE)
 
 include(CheckCompilerVersion)
 include(HandleLLVMStdlib)
 include(AddLLVMDefinitions)
 include(CheckCCompilerFlag)
 include(CheckCXXCompilerFlag)
 
-
 if (CMAKE_LINKER MATCHES "lld-link.exe")
   # Pass /MANIFEST:NO so that CMake doesn't run mt.exe on our binaries.  Adding
   # manifests with mt.exe breaks LLD's symbol tables and takes as much time as
   # the link. See PR24476.
   append("/MANIFEST:NO"
     CMAKE_EXE_LINKER_FLAGS CMAKE_MODULE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
 endif()
 

 # build might work on ELF but fail on MachO/COFF.
 if(NOT (${CMAKE_SYSTEM_NAME} MATCHES "Darwin" OR WIN32 OR CYGWIN OR
         ${CMAKE_SYSTEM_NAME} MATCHES "FreeBSD" OR
         ${CMAKE_SYSTEM_NAME} MATCHES "OpenBSD") AND
    NOT LLVM_USE_SANITIZER)
   set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -Wl,-z,defs")
 endif()
 
-
 function(append value)
   foreach(variable ${ARGN})
     set(${variable} "${${variable}} ${value}" PARENT_SCOPE)
   endforeach(variable)
 endfunction()
 
 function(append_if condition value)
   if (${condition})
if (C_SUPPORTS_${name} AND CXX_SUPPORTS_${name})
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${flag}" PARENT_SCOPE)
     set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${flag}" PARENT_SCOPE)
     set(CMAKE_ASM_FLAGS "${CMAKE_ASM_FLAGS} ${flag}" PARENT_SCOPE)
   else()
     message(WARNING "${flag} is not supported.")
   endif()
 endfunction()
 
+if(LLVM_ENABLE_LLD)
+  check_cxx_compiler_flag("-fuse-ld=lld" CXX_SUPPORTS_LLD)
+  append_if(CXX_SUPPORTS_LLD "-fuse-ld=lld"
+    CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
+endif()
+
 if( LLVM_ENABLE_PIC )
   if( XCODE )
     # Xcode has -mdynamic-no-pic on by default, which overrides -fPIC. I don't
     # know how to disable this, so just force ENABLE_PIC off for now.
     message(WARNING "-fPIC not supported with Xcode.")
   elseif( WIN32 OR CYGWIN)
     # On Windows all code is PIC. MinGW warns if -fPIC is used.
   else()