[PATCH v4] Makefile: lld: tell clang to use lld

[Nick Desaulniers <ndesaulniers@google.com>]

This is needed because clang doesn't select which linker to use based on
$LD but rather -fuse-ld={bfd,gold,lld,<absolute path to linker>}.  This
is problematic especially for cc-ldoption, which checks for linker flag
support via invoking the compiler, rather than the linker.

Select the linker via absolute path from $PATH via `which`. This allows
you to build with:

$ make LD=ld.lld
$ make LD=ld.lld-8
$ make LD=/path/to/ld.lld

Add -Qunused-arguments to KBUILD_CPPFLAGS sooner, as otherwise
Clang likes to complain about -fuse-lld= being unused when compiling but
not linking (-c) such as when cc-option is used. There's no need to
guard with cc-option.

Link: https://github.com/ClangBuiltLinux/linux/issues/342
Link: https://github.com/ClangBuiltLinux/linux/issues/366
Link: https://github.com/ClangBuiltLinux/linux/issues/357
Suggested-by: Nathan Chancellor <natechancellor@gmail.com>
Suggested-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
Changes V3->V4:
* Unconditionally add -Qunused-arguments sooners, as per Nathan.
* Slight modification to commit message for that point.
Changes V2->V3:
* Use absolute path based on `which $LD` as per Masahiro.
* Add -Qunused-arguments.
* Drop tested-by/reviewed-by tags, since this patched has changed enough
  to warrant re-testing/re-review, IMO.
* Add more info to the commit message.
Changes V1->V2:
* add reviewed and tested by tags.
* move this addition up 2 statments so that it's properly added to
  KBUILD_*FLAGS as per Nathan.

 Makefile | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index 026fbc450906..b290e76e1ca5 100644
--- a/Makefile
+++ b/Makefile
@@ -514,6 +514,10 @@ ifneq ($(GCC_TOOLCHAIN),)
 CLANG_FLAGS	+= --gcc-toolchain=$(GCC_TOOLCHAIN)
 endif
 CLANG_FLAGS	+= -no-integrated-as
+ifneq ($(shell $(LD) --version 2>&1 | head -n 1 | grep LLD),)
+CLANG_FLAGS	+= -fuse-ld=$(shell which $(LD))
+endif
+KBUILD_CPPFLAGS	+= -Qunused-arguments
 KBUILD_CFLAGS	+= $(CLANG_FLAGS)
 KBUILD_AFLAGS	+= $(CLANG_FLAGS)
 export CLANG_FLAGS
@@ -716,7 +720,6 @@ stackp-flags-$(CONFIG_STACKPROTECTOR_STRONG)      := -fstack-protector-strong
 KBUILD_CFLAGS += $(stackp-flags-y)
 
 ifdef CONFIG_CC_IS_CLANG
-KBUILD_CPPFLAGS += $(call cc-option,-Qunused-arguments,)
 KBUILD_CFLAGS += $(call cc-disable-warning, format-invalid-specifier)
 KBUILD_CFLAGS += $(call cc-disable-warning, gnu)
 KBUILD_CFLAGS += $(call cc-disable-warning, address-of-packed-member)
-- 
2.21.0.392.gf8f6787159e-goog