[Squash] Treewide: Update for ld.lld usage

kbuild: Allow forcing the alternative LLD linker via Kconfig

LLD is a relatively new linker from the LLVM Project that aims to be a
faster and more modern alternative to the GNU gold and bfd linkers from
binutils: https://lld.llvm.org/

I've also found that it offers more insightful diagnostics when
something goes wrong, e.g. when there are undefined references. It does
also appear to speed up the overall build time by 4-10s as compared to
ld.bfd.

These new config options will only allow fully-working configurations:
  - gold/lld when Clang LTO is enabled
  - bfd/lld otherwise

Signed-off-by: Danny Lin <[email protected]>
Signed-off-by: Adam W. Willis <[email protected]>
Signed-off-by: Yaroslav Furman <[email protected]>

BACKPORT: FROMLIST: Makefile: lld: set -O2 linker flag when linking w…

…ith LLD

For arm64:
0.34% size improvement with lld -O2 over lld for vmlinux.
3.3% size improvement with lld -O2 over lld for Image.lz4-dtb.

Link: ClangBuiltLinux/linux#343
Suggested-by: Rui Ueyama <[email protected]>
Suggested-by: Nathan Chancellor <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
Tested-by: Nathan Chancellor <[email protected]>
Signed-off-by: Nick Desaulniers <[email protected]>
(am from https://patchwork.kernel.org/patch/10806729/)
Signed-off-by: Adam W. Willis <[email protected]>
Signed-off-by: Yaroslav Furman <[email protected]>

BACKPORT: FROMLIST: Makefile: lld: tell clang to use lld

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 BACKPORT: FROMLIST: Makefile: lld: tell clang to use lld

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: ClangBuiltLinux/linux#342
Link: ClangBuiltLinux/linux#366
Link: ClangBuiltLinux/linux#357
Suggested-by: Nathan Chancellor <[email protected]>
Suggested-by: Masahiro Yamada <[email protected]>
Signed-off-by: Nick Desaulniers <[email protected]>
Reviewed-by: Nathan Chancellor <natechancellor@gmail
Signed-off-by: Divyanshu-Modi <[email protected]>
Signed-off-by: Kunmun <[email protected]>

Makefile

@@ -340,6 +340,7 @@ include scripts/Kbuild.include
 AS		= $(CROSS_COMPILE)as
 LD		= $(CROSS_COMPILE)ld
 LDGOLD		= $(CROSS_COMPILE)ld.gold
+LDLLD		= ld.lld
 CC		= $(CROSS_COMPILE)gcc
 CPP		= $(CC) -E
 AR         ?= $(CROSS_COMPILE)ar
@@ -641,8 +642,9 @@ KBUILD_CFLAGS += $(call cc-option, -fcatch-undefined-behavior)
 KBUILD_CFLAGS += $(call cc-option, -no-integrated-as)
 KBUILD_AFLAGS += $(call cc-option, -no-integrated-as)
 ifeq ($(ld-name),lld)
-KBUILD_CFLAGS += -fuse-ld=lld
+KBUILD_CFLAGS += -fuse-ld=$(shell which $(LD))
 endif
+KBUILD_CPPFLAGS += -Qunused-arguments
 endif
 
 # These warnings generated too much noise in a regular build.
@@ -651,11 +653,16 @@ KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable)
 
 # Make toolchain changes before including arch/$(SRCARCH)/Makefile to ensure
 # ar/cc/ld-* macros return correct values.
-ifdef CONFIG_LTO_CLANG
-ifneq ($(ld-name),lld)
-# use GNU gold with LLVMgold for LTO linking, and LD for vmlinux_link
+ifdef CONFIG_LD_GOLD
 LDFINAL_vmlinux := $(LD)
 LD		:= $(LDGOLD)
+endif
+ifdef CONFIG_LD_LLD
+LD		:= $(LDLLD)
+endif
+ifdef CONFIG_LTO_CLANG
+# use GNU gold with LLVMgold or LLD for LTO linking, and LD for vmlinux_link
+ifeq ($(ld-name),gold)
 LDFLAGS		+= -plugin LLVMgold.so
 endif
 # use llvm-ar for building symbol tables from IR files, and llvm-dis instead
@@ -803,7 +810,7 @@ ifdef CONFIG_KCOV
 endif
 
 ifeq ($(ld-name),lld)
-LDFLAGS += -O2
+LDFLAGS += -O3
 endif
 
 KBUILD_CFLAGS += $(call cc-disable-warning, unused-const-variable)
@@ -1174,7 +1181,7 @@ ifdef CONFIG_LTO_CLANG
   endif
   ifneq ($(ld-name),lld)
     ifneq ($(call gold-ifversion, -ge, 112000000, y), y)
-         @echo Cannot use CONFIG_LTO_CLANG: requires GNU gold 1.12 or later >&2 && exit 1
+    	@echo Cannot use CONFIG_LTO_CLANG: requires LLD or GNU gold 1.12 or later >&2 && exit 1
     endif
   endif
 endif

arch/Kconfig

@@ -429,6 +429,37 @@ config THIN_ARCHIVES
 	  Select this if the architecture wants to use thin archives
 	  instead of ld -r to create the built-in.o files.
 
+choice
+	prompt "Kernel linker"
+	default LD_BFD if !LTO_CLANG
+	default LD_GOLD if LTO_CLANG
+	help
+	  This selects the linker that will be used to link the kernel and
+	  its composite objects.
+
+	  Note that using either ld.gold or lld is required for Clang LTO.
+	  ld.bfd will not work.
+
+config LD_BFD
+	bool "bfd"
+	depends on !LTO_CLANG
+	help
+	  Use the standard ld.bfd linker from binutils, which is usually the
+	  default. This linker does not work with Clang LTO.
+
+config LD_GOLD
+	bool "gold"
+	depends on LTO_CLANG
+	help
+	  Use the alternative ld.gold linker from binutils.
+
+config LD_LLD
+	bool "lld"
+	help
+	  Use the alternative lld linker from LLVM.
+
+endchoice
+
 config LD_DEAD_CODE_DATA_ELIMINATION
 	bool
 	help
@@ -483,6 +514,7 @@ config LTO_CLANG
 	select LTO
 	select THIN_ARCHIVES
 	select LD_DEAD_CODE_DATA_ELIMINATION
+	select LD_GOLD	
 	help
           This option enables clang's Link Time Optimization (LTO), which allows
           the compiler to optimize the kernel globally at link time. If you

scripts/Kbuild.include

@@ -208,18 +208,7 @@ ar-option = $(call try-run, $(AR) rc$(1) "$$TMP",$(1),$(2))
 
 # ld-name
 # Expands to either bfd, gold, or lld
-ifneq (,$(LD))
-	__ld-name = $(shell $(LD) -v 2>&1)
-endif
-ifneq (,$(findstring GNU gold,$(__ld-name)))
-	ld-name = gold
-else
-	ifneq (,$(findstring LLD,$(__ld-name)))
-		ld-name = lld
-	else
-		ld-name = bfd
-	endif
-endif
+ld-name = $(shell if $(LD) -v 2>&1 | grep -q "GNU gold"; then echo gold; elif $(LD) -v 2>&1 | grep -q "LLD"; then echo lld; else echo bfd; fi)
 
 # ld-version
 # Note this is mainly for HJ Lu's 3 number binutil versions