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@xxxxxxxxxx> > > --- > > 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
