Re: [PATCH v2 09/28] kbuild: add support for Clang LTO

Masahiro Yamada
2020-09-05T20:33:03Z

On Fri, Sep 4, 2020 at 5:31 AM Sami Tolvanen <samitolvanen@google.com> 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 | 18 +++++++-
> arch/Kconfig | 68 +++++++++++++++++++++++++++++++
> 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, 154 insertions(+), 17 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index a9dae26c93b5..dd49eaea7c25 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -909,6 +909,22 @@ KBUILD_CFLAGS += $(CC_FLAGS_SCS)
> export CC_FLAGS_SCS
> endif
>
> +ifdef CONFIG_LTO_CLANG
> +ifdef CONFIG_THINLTO
> +CC_FLAGS_LTO_CLANG := -flto=thin -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)

$(CC_FLAGS_LTO_CLANG) is not used elsewhere.

Why didn't you add the flags to CC_FLAGS_LTO
directly?

Will it be useful if LTO_GCC is supported ?

> +KBUILD_CFLAGS += $(CC_FLAGS_LTO)
> +export CC_FLAGS_LTO
> +endif

--
Best Regards
Masahiro Yamada