[cmake] Pass LLVM_USE_LINKER flag to CrossCompile toolchain if set

[hintonda]

Summary

Pass LLVM_USE_LINKER flag to CrossCompile toolchain if set.

This change only affects linking the NATIVE toolchain.

Noticed this when passing:

-DLLVM_OPTIMIZED_TABLEGEN=ON
-DLLVM_USE_LINKER=gold

[beanz]

I don't think we can do this unconditionally like this. It seems to me like you could have a situation where you want to use different linkers for the host tools and target binaries.

[hintonda]

In D39734#925253, @beanz wrote:

I don't think we can do this unconditionally like this. It seems to me like you could have a situation where you want to use different linkers for the host tools and target binaries.

I can make it conditional, e.g., iff LLVM_OPTIMIZED_TABLEGEN=ON, but honestly can't think of a case where you would want to use different linkers.

[beanz]

The code path you're modifying isn't just for building optimized tools, it also gets triggered when building the host tools in cross compilations. It is totally reasonable that I might want to use a different linker for host binaries from the linker used for target binaries. One example would be if I'm cross compiling to say PPC and wanting to use gold, but want to use lld as the linker for the host tools.

[hintonda]

• Only pass linker flags when not CMAKE_CROSSCOMPLING.

[beanz]

LGTM.

[hintonda]

Rebase after rL319176.

cmake/modules/CrossCompile.cmake

if (EXISTS ${LLVM_MAIN_SRC_DIR}/cmake/platforms/${toolchain}.cmake)
     set(CROSS_TOOLCHAIN_FLAGS_${target_name} 
         -DCMAKE_TOOLCHAIN_FILE=\"${LLVM_MAIN_SRC_DIR}/cmake/platforms/${toolchain}.cmake\"
         CACHE STRING "Toolchain file for ${target_name}")
   endif()
 
   if (buildtype)
     set(build_type_flags "-DCMAKE_BUILD_TYPE=${buildtype}")
   endif()
+  if (LLVM_USE_LINKER AND NOT CMAKE_CROSSCOMPILING)
+    set(linker_flag "-DLLVM_USE_LINKER=${LLVM_USE_LINKER}")
+  endif()
   if (LLVM_EXTERNAL_CLANG_SOURCE_DIR)
     # Propagate LLVM_EXTERNAL_CLANG_SOURCE_DIR so that clang-tblgen can be built
     set(external_clang_dir "-DLLVM_EXTERNAL_CLANG_SOURCE_DIR=${LLVM_EXTERNAL_CLANG_SOURCE_DIR}")
   endif()
 
   add_custom_command(OUTPUT ${LLVM_${target_name}_BUILD}
     COMMAND ${CMAKE_COMMAND} -E make_directory ${LLVM_${target_name}_BUILD}
     COMMENT "Creating ${LLVM_${target_name}_BUILD}...")
 
   add_custom_target(CREATE_LLVM_${target_name}
                     DEPENDS ${LLVM_${target_name}_BUILD})
 
   add_custom_command(OUTPUT ${LLVM_${target_name}_BUILD}/CMakeCache.txt
     COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}"
         ${CROSS_TOOLCHAIN_FLAGS_${target_name}} ${CMAKE_SOURCE_DIR}
         -DLLVM_TARGET_IS_CROSSCOMPILE_HOST=TRUE
         -DLLVM_TARGETS_TO_BUILD=${LLVM_TARGETS_TO_BUILD}
-        ${build_type_flags} ${external_clang_dir}
+        ${build_type_flags} ${linker_flag} ${external_clang_dir}
     WORKING_DIRECTORY ${LLVM_${target_name}_BUILD}
     DEPENDS CREATE_LLVM_${target_name}
     COMMENT "Configuring ${target_name} LLVM...")
 
   add_custom_target(CONFIGURE_LLVM_${target_name}
                     DEPENDS ${LLVM_${target_name}_BUILD}/CMakeCache.txt)
 
 endfunction()
 
 function(llvm_create_cross_target target_name sysroot)
   llvm_create_cross_target_internal(${target_name} ${sysroot} ${CMAKE_BUILD_TYPE})
 endfunction()
 
 llvm_create_cross_target_internal(NATIVE "" Release)