kbuild: Support explicitly selecting LD linker

https://lld.llvm.org/

Enable CONFIG_LD_(BFD|GOLD|LLD) to force compilation with desired LD
linker. This enables experimental support for LLVM's LLD linker
which has been shown to result in faster link time and potentially
reduce kernel image size when used in conjunction with Clang LTO.
GNU gold will remain as the preferred linker due to it's universal
compatibility.

Co-authored-by: Danny Lin <danny@kdrag0n.dev>
[@0ctobot: This is an adaptation of the following commits:
kdrag0n/proton_bluecross@0b08017,
kdrag0n/proton_bluecross@aba5259,
kdrag0n/proton_bluecross@70c8925]
Co-authored-by: Adam W. Willis <return.of.octobot@gmail.com>

Makefile

@@ -345,6 +345,7 @@ AS		= $(CROSS_COMPILE)as
 LD		= $(CROSS_COMPILE)ld
 CC		= $(CROSS_COMPILE)gcc
 LDGOLD		= $(CROSS_COMPILE)ld.gold
+LDLLD		= ld.lld
 CPP		= $(CC) -E
 AR		= $(CROSS_COMPILE)ar
 NM		= $(CROSS_COMPILE)nm
@@ -638,11 +639,19 @@ export CFLAGS_GCOV CFLAGS_KCOV
 
 # Make toolchain changes before including arch/$(SRCARCH)/Makefile to ensure
 # ar/cc/ld-* macros return correct values.
-ifdef CONFIG_LTO_CLANG
-# 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		:= $(LDLLD)
+endif
+
+ifdef CONFIG_LTO_CLANG
+# use GNU gold and LD for vmlinux_link, or LLD for LTO linking
+ifeq ($(ld-name),gold)
 LDFLAGS		+= -plugin LLVMgold.so
+endif
 LDFLAGS		+= -plugin-opt=-function-sections
 LDFLAGS		+= -plugin-opt=-data-sections
 # use llvm-ar for building symbol tables from IR files, and llvm-dis instead
@@ -691,6 +700,10 @@ ifdef CONFIG_LTO
 lto-flags	:= $(lto-clang-flags)
 KBUILD_CFLAGS	+= $(lto-flags)
 
+ifeq ($(ld-name),lld)
+LDFLAGS		+= --lto-O3
+endif
+
 DISABLE_LTO	:= $(DISABLE_LTO_CLANG)
 export DISABLE_LTO
 
@@ -1186,8 +1199,10 @@ ifdef CONFIG_LTO_CLANG
   ifneq ($(call clang-ifversion, -ge, 0500, y), y)
 	@echo Cannot use CONFIG_LTO_CLANG: requires clang 5.0 or later >&2 && exit 1
   endif
-  ifneq ($(call gold-ifversion, -ge, 112000000, y), y)
-	@echo Cannot use CONFIG_LTO_CLANG: requires GNU gold 1.12 or later >&2 && exit 1
+  ifneq ($(ld-name), lld)
+    ifneq ($(call gold-ifversion, -ge, 112000000, y), y)
+	@echo Cannot use CONFIG_LTO_CLANG: requires LLD or GNU gold 1.12 or later >&2 && exit 1
+    endif
   endif
 endif
 # Make sure compiler supports LTO flags

arch/Kconfig

@@ -492,6 +492,34 @@ 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
+	  Select the desired kernel linker from a supported list.
+
+config LD_BFD
+	bool "bfd"
+	depends on !LTO_CLANG
+	help
+	  This option selects the default ld.bfd linker, which is part of
+	  GNU binutils.
+
+config LD_GOLD
+	bool "gold"
+	help
+	  This option selects the ld.gold linker, which is a part of GNU
+	  binutils and required when compiling with Clang LTO.
+
+config LD_LLD
+	bool "lld"
+	help
+	  This option selects ld.lld as a drop-in replacement for ld.gold
+	  and may optionally be used when compiling with Clang LTO.
+
+endchoice
+
 config LTO
 	def_bool n
 
@@ -527,6 +555,7 @@ 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

init/Kconfig

@@ -1584,6 +1584,7 @@ config LD_DEAD_CODE_DATA_ELIMINATION
 	bool "Dead code and data elimination (EXPERIMENTAL)"
 	depends on HAVE_LD_DEAD_CODE_DATA_ELIMINATION
 	depends on EXPERT
+	select LD_GOLD
 	help
 	  Enable this if you want to do dead code and data elimination with
 	  the linker by compiling with -ffunction-sections -fdata-sections,

scripts/Kbuild.include

@@ -205,8 +205,8 @@ ld-option = $(call try-run, $(LD) $(LDFLAGS) $(1) -v,$(1),$(2))
 ar-option = $(call try-run, $(AR) rc$(1) "$$TMP",$(1),$(2))
 
 # ld-name
-# Expands to either bfd or gold
-ld-name = $(shell $(LD) -v 2>&1 | grep -q "GNU gold" && echo gold || echo bfd)
+# Expands to bfd, gold, or lld
+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