Re: [PATCH 02/22] kbuild: add support for Clang LTO

Nathan Chancellor
2020-06-25T02:26:50Z

Hi Sami,

On Wed, Jun 24, 2020 at 01:31:40PM -0700, 'Sami Tolvanen' via Clang Built Linux wrote:
> 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

I am curious, what is the reason for gating this at clang 11.0.0?

Presumably this? https://github.com/ClangBuiltLinux/linux/issues/510

It might be nice to notate this so that we do not have to wonder :)

Cheers,
Nathan