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

From: Sami Tolvanen <[email-masked]>
Date: Tue, 8 Sep 2020 10:14:11 -0700

On Sun, Sep 06, 2020 at 05:17:32AM +0900, Masahiro Yamada wrote:
> On Fri, Sep 4, 2020 at 5:31 AM Sami Tolvanen <[email-masked]> 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 <[email-masked]>
> > ---
> >  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

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