This change adds build system support for Clang's Link Time
Optimization (LTO). With -flto, instead of ELF object files, Clang
produces LLVM bitcode, which is compiled into native code at link
time, allowing the final binary to be optimized globally. For more
details, see:

  https://llvm.org/docs/LinkTimeOptimization.html

The Kconfig option CONFIG_LTO_CLANG is implemented as a choice,
which defaults to LTO being disabled. To use LTO, the architecture
must select ARCH_SUPPORTS_LTO_CLANG and support:

  - compiling with Clang,
  - compiling inline assembly with Clang's integrated assembler,
  - and linking with LLD.

While using full LTO results in the best runtime performance, the
compilation is not scalable in time or memory. CONFIG_THINLTO
enables ThinLTO, which allows parallel optimization and faster
incremental builds. ThinLTO is used by default if the architecture
also selects ARCH_SUPPORTS_THINLTO:

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

To enable LTO, LLVM tools must be used to handle bitcode files. The
easiest way is to pass the LLVM=1 option to make:

  $ make LLVM=1 defconfig
  $ scripts/config -e LTO_CLANG
  $ make LLVM=1

Alternatively, at least the following LLVM tools must be used:

  CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm

To prepare for LTO support with other compilers, common parts are
gated behind the CONFIG_LTO option, and LTO can be disabled for
specific files by filtering out CC_FLAGS_LTO.

Note that support for DYNAMIC_FTRACE and MODVERSIONS are added in
follow-up patches.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
---
 Makefile                          | 16 ++++++++
 arch/Kconfig                      | 66 +++++++++++++++++++++++++++++++
 include/asm-generic/vmlinux.lds.h | 11 ++++--
 scripts/Makefile.build            |  9 ++++-
 scripts/Makefile.modfinal         |  9 ++++-
 scripts/Makefile.modpost          | 24 ++++++++++-
 scripts/link-vmlinux.sh           | 32 +++++++++++----
 7 files changed, 151 insertions(+), 16 deletions(-)

diff --git a/Makefile b/Makefile
index ac2c61c37a73..0c7fe6fb2143 100644
--- a/Makefile
+++ b/Makefile

@@ -886,6 +886,22 @@ KBUILD_CFLAGS	+= $(CC_FLAGS_SCS)
 export CC_FLAGS_SCS
 endif
 
+ifdef CONFIG_LTO_CLANG
+ifdef CONFIG_THINLTO
+CC_FLAGS_LTO_CLANG := -flto=thin $(call cc-option, -fsplit-lto-unit)
+KBUILD_LDFLAGS	+= --thinlto-cache-dir=.thinlto-cache
+else
+CC_FLAGS_LTO_CLANG := -flto
+endif
+CC_FLAGS_LTO_CLANG += -fvisibility=default
+endif
+
+ifdef CONFIG_LTO
+CC_FLAGS_LTO	:= $(CC_FLAGS_LTO_CLANG)
+KBUILD_CFLAGS	+= $(CC_FLAGS_LTO)
+export CC_FLAGS_LTO
+endif
+
 # arch Makefile may override CC so keep this after arch Makefile is included
 NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)
 

diff --git a/arch/Kconfig b/arch/Kconfig
index 8cc35dc556c7..e00b122293f8 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig

@@ -552,6 +552,72 @@ config SHADOW_CALL_STACK
 	  reading and writing arbitrary memory may be able to locate them
 	  and hijack control flow by modifying the stacks.
 
+config LTO
+	bool
+
+config ARCH_SUPPORTS_LTO_CLANG
+	bool
+	help
+	  An architecture should select this option if it supports:
+	  - compiling with Clang,
+	  - compiling inline assembly with Clang's integrated assembler,
+	  - and linking with LLD.
+
+config ARCH_SUPPORTS_THINLTO
+	bool
+	help
+	  An architecture should select this option if it supports Clang's
+	  ThinLTO.
+
+config THINLTO
+	bool "Clang ThinLTO"
+	depends on LTO_CLANG && ARCH_SUPPORTS_THINLTO
+	default y
+	help
+	  This option enables Clang's ThinLTO, which allows for parallel
+	  optimization and faster incremental compiles. More information
+	  can be found from Clang's documentation:
+
+	    https://clang.llvm.org/docs/ThinLTO.html
+
+choice
+	prompt "Link Time Optimization (LTO)"
+	default LTO_NONE
+	help
+	  This option enables Link Time Optimization (LTO), which allows the
+	  compiler to optimize binaries globally.
+
+	  If unsure, select LTO_NONE.
+
+config LTO_NONE
+	bool "None"
+
+config LTO_CLANG
+	bool "Clang's Link Time Optimization (EXPERIMENTAL)"
+	depends on CC_IS_CLANG && CLANG_VERSION >= 110000 && LD_IS_LLD