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

Sami Tolvanen
2020-09-08T17:14:36Z

On Sun, Sep 06, 2020 at 05:17:32AM +0900, Masahiro Yamada wrote:
> 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 ?

The idea was to allow compiler-specific LTO flags to be filtered out
separately if needed, but you're right, this is not really necessary
right now. I'll drop CC_FLAGS_LTO_CLANG in v3.

Sami