Kbuild: add support for ThinLTO

This change adds support for ThinLTO, which greatly improves build times
over full LTO while retaining most of the performance benefits:

  https://clang.llvm.org/docs/ThinLTO.html

Bug: 145210207
Change-Id: I8bfc19028266077be2bc1fb5c2bc001b599d3214
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: mydongistiny <jaysonedson@gmail.com>
Signed-off-by: DennySPB <dennyspb@gmail.com>

Makefile

@@ -694,14 +694,12 @@ ifneq ($(ld-name),lld)
 LDFINAL_vmlinux := $(LD)
 LD		:= $(LDGOLD)
 KBUILD_LDFLAGS		+= -plugin LLVMgold.so
-KBUILD_LDFLAGS		+= -plugin-opt=-function-sections
-KBUILD_LDFLAGS		+= -plugin-opt=-data-sections
 endif
-# use llvm-ar for building symbol tables from IR files, and llvm-dis instead
+# use llvm-ar for building symbol tables from IR files, and llvm-nm instead
 # of objdump for processing symbol versions and exports
-LLVM_AR		:= llvm-ar
+LLVM_AR	:= llvm-ar
-LLVM_DIS	:= llvm-dis
+LLVM_NM	:= llvm-nm
-export LLVM_AR LLVM_DIS
+export LLVM_AR LLVM_NM
 endif
 
 # The arch Makefile can set ARCH_{CPP,A,C}FLAGS to override the default
@@ -752,7 +750,12 @@ KBUILD_CFLAGS	+= $(call cc-option,-fdata-sections,)
 endif
 
 ifdef CONFIG_LTO_CLANG
-lto-clang-flags	:= -flto -fvisibility=hidden
+ifdef CONFIG_THINLTO
+lto-clang-flags := -flto=thin
+else
+lto-clang-flags := -flto
+endif
+lto-clang-flags += -fvisibility=default $(call cc-option, -fsplit-lto-unit)
 
 # Limit inlining across translation units to reduce binary size
 LD_FLAGS_LTO_CLANG := -mllvm -import-instr-limit=5
@@ -763,7 +766,7 @@ KBUILD_LDFLAGS_MODULE += $(LD_FLAGS_LTO_CLANG)
 KBUILD_LDFLAGS_MODULE += -T $(srctree)/scripts/module-lto.lds
 
 # allow disabling only clang LTO where needed
-DISABLE_LTO_CLANG := -fno-lto -fvisibility=default
+DISABLE_LTO_CLANG := -fno-lto
 export DISABLE_LTO_CLANG
 endif
 

arch/Kconfig

@@ -503,6 +503,18 @@ config ARCH_SUPPORTS_LTO_CLANG
 	  - compiling inline assembly with clang's integrated assembler,
 	  - and linking with either lld or GNU gold w/ LLVMgold.
 
+config ARCH_SUPPORTS_THINLTO
+	bool
+	help
+	  An architecture should select this if it supports clang's ThinLTO.
+
+config THINLTO
+	bool "Use clang ThinLTO (EXPERIMENTAL)"
+	depends on LTO_CLANG && ARCH_SUPPORTS_THINLTO
+	default y
+	help
+	  Use ThinLTO to speed up Link Time Optimization.
+
 choice
 	prompt "Link-Time Optimization (LTO) (EXPERIMENTAL)"
 	default LTO_NONE

scripts/Makefile.build

@@ -218,7 +218,7 @@ cmd_modversions_c =								\
 			    > $(@D)/$(@F).symversions;				\
 		fi;								\
 	else									\
-		if $(LLVM_DIS) -o=- $(@D)/.tmp_$(@F) | grep -q __ksymtab; then	\
+		if $(LLVM_NM) $(@D)/.tmp_$(@F) | grep -q __ksymtab; then	\
 			$(call cmd_gensymtypes_c,$(KBUILD_SYMTYPES),$(@:.o=.symtypes)) \
 			    > $(@D)/$(@F).symversions;				\
 		fi;								\
@@ -570,7 +570,7 @@ cmd_export_list = 					\
 				$(filter_export_list) 	\
 				>>$(ksyms-lds);		\
 		else					\
-			$(LLVM_DIS) -o=- $$o |		\
+			$(LLVM_NM) $$o |		\
 				$(filter_export_list) 	\
 				>>$(ksyms-lds);		\
 		fi; 					\

scripts/Makefile.modpost

@@ -138,10 +138,8 @@ ARCH_POSTLINK := $(wildcard $(srctree)/arch/$(SRCARCH)/Makefile.postlink)
 quiet_cmd_ld_ko_o = LD [M]  $@
 
 ifdef CONFIG_LTO_CLANG
-bad_ld_flags = -plugin-opt=-function-sections -plugin-opt=-data-sections
-filtered_ldflags = $(filter-out $(bad_ld_flags),$(LDFLAGS))
 cmd_ld_ko_o =								\
-	$(LD) -r $(filtered_ld_flags)					\
+	$(LD) -r $(KBUILD_LDFLAGS)                                 	\
 		 $(KBUILD_LDFLAGS_MODULE) $(LDFLAGS_MODULE) 		\
 		 $(shell [ -s $(@:.ko=.o.symversions) ] &&		\
 			echo -T $(@:.ko=.o.symversions))  		\