Add a LLVM_USE_LINKER that defines the linker to use when building LLVM

Summary:
This string parameter is passed to -fuse-ld when linking. It can be
an absolute path to your custom linker, otherwise clang will look for
`ld.{name}`.

Reviewers: davide, tejohnson, pcc

Subscribers: llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D28738

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@292047 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/modules/HandleLLVMOptions.cmake

@@ -147,9 +147,19 @@ function(add_flag_or_print_warning flag name)
   endif()
 endfunction()
 
-if(LLVM_ENABLE_LLD)
-  check_cxx_compiler_flag("-fuse-ld=lld" CXX_SUPPORTS_LLD)
-  append_if(CXX_SUPPORTS_LLD "-fuse-ld=lld"
+if( LLVM_ENABLE_LLD )
+	if ( LLVM_USE_LINKER )
+		message(FATAL_ERROR "LLVM_ENABLE_LLD and LLVM_USE_LINKER can't be set at the same time")
+	endif()
+	set(LLVM_USE_LINKER "lld")
+endif()
+
+if( LLVM_USE_LINKER )
+  check_cxx_compiler_flag("-fuse-ld=${LLVM_USE_LINKER}" CXX_SUPPORTS_CUSTOM_LINKER)
+  if ( NOT CXX_SUPPORTS_CUSTOM_LINKER )
+	  message(FATAL_ERROR "Host compiler does not support '-fuse-ld=${LLVM_USE_LINKER}'")
+  endif()
+  append("-fuse-ld=${LLVM_USE_LINKER}"
     CMAKE_EXE_LINKER_FLAGS CMAKE_MODULE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
 endif()
 

docs/CMake.rst

@@ -382,6 +382,18 @@ LLVM-specific variables
   lines, enabling link-time optimization. Possible values are ``Off``,
   ``On``, ``Thin`` and ``Full``. Defaults to OFF.
 
+**LLVM_USE_LINKER**:STRING
+  Add ``-fuse-ld={name}`` to the link invocation. The possible value depend on
+  your compiler, for clang the value can be an absolute path to your custom
+  linker, otherwise clang will prefix the name with ``ld.`` and apply its usual
+  search. For example to link LLVM with the Gold linker, cmake can be invoked
+  with ``-DLLVM_USE_LINKER=gold``.
+
+**LLVM_ENABLE_LLD**:BOOL
+  This option is equivalent to `-DLLVM_USE_LINKER=lld`, except during a 2-stage
+  build where a dependency is added from the first stage to the second ensuring
+  that lld is built before stage2 begins.
+
 **LLVM_PARALLEL_COMPILE_JOBS**:STRING
   Define the maximum number of concurrent compilation jobs.