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    SubjectRe: [PATCH 02/22] kbuild: add support for Clang LTO
    On Wed, Jun 24, 2020 at 07:26:47PM -0700, Nathan Chancellor wrote:
    > 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 :)

    Yes, that's the reason. I'll add a note about it. Thanks!

    Sami

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