ANDROID: 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>

Makefile

@@ -663,8 +663,8 @@ endif
 # use llvm-ar for building symbol tables from IR files, and llvm-dis instead
 # of objdump for processing symbol versions and exports
 LLVM_AR		:= llvm-ar
-LLVM_DIS	:= llvm-dis
-export LLVM_AR LLVM_DIS
+LLVM_NM		:= llvm-nm
+export LLVM_AR LLVM_NM
 endif
 
 # The arch Makefile can set ARCH_{CPP,A,C}FLAGS to override the default
@@ -831,10 +831,16 @@ 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
+LDFLAGS		+= --thinlto-cache-dir=.thinlto-cache
+else
+lto-clang-flags	:= -flto
+endif
+lto-clang-flags += -fvisibility=default $(call cc-option, -fsplit-lto-unit)
 
 # allow disabling only clang LTO where needed
-DISABLE_LTO_CLANG := -fno-lto -fvisibility=default
+DISABLE_LTO_CLANG := -fno-lto
 export DISABLE_LTO_CLANG
 endif
 
@@ -851,7 +857,7 @@ export LDFINAL_vmlinux LDFLAGS_FINAL_vmlinux
 endif
 
 ifdef CONFIG_CFI_CLANG
-cfi-clang-flags	+= -fsanitize=cfi $(call cc-option, -fsplit-lto-unit)
+cfi-clang-flags	+= -fsanitize=cfi
 DISABLE_CFI_CLANG := -fno-sanitize=cfi
 ifdef CONFIG_MODULES
 cfi-clang-flags	+= -fsanitize-cfi-cross-dso

arch/Kconfig

@@ -622,6 +622,18 @@ 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

@@ -220,7 +220,7 @@ 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;								\
@@ -594,7 +594,7 @@ cmd_export_list = 					\
 				$(filter_export_list) 	\
 				>>$(ksyms-lds);		\
 		else					\
-			$(LLVM_DIS) -o=- $$o |		\
+			$(LLVM_NM) $$o |		\
 				$(filter_export_list) 	\
 				>>$(ksyms-lds);		\
 		fi; 					\