On Sun, Sep 06, 2020 at 04:36:32AM +0900, Masahiro Yamada wrote: > On Fri, Sep 4, 2020 at 5:31 AM Sami Tolvanen <samitolvanen@xxxxxxxxxx> 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. > > What is the reason for doing this in a choice? > To turn off LTO_CLANG for compile-testing? > > I would rather want to give LTO_CLANG more chances > to be enabled/tested. It's a choice to prevent LTO from being enabled by default with allyesconfig and allmodconfig. It would take hours to build these even on a fast computer, and probably days on older hardware. > > +ifdef CONFIG_LTO_CLANG > > +ifdef CONFIG_THINLTO > > +CC_FLAGS_LTO_CLANG := -flto=thin -fsplit-lto-unit > > +KBUILD_LDFLAGS += --thinlto-cache-dir=.thinlto-cache > > > I think this would break external module builds > because it would create cache files in the > kernel source tree. > > External module builds should never ever touch > the kernel tree, which is usually located under > the read-only /usr/src/ in distros. > > > .thinlto-cache should be created in the module tree > when it is built with M=. Thanks for pointing this out, I'll fix the path in v3. > > # Directories & files removed with 'make distclean' > > -DISTCLEAN_FILES += tags TAGS cscope* GPATH GTAGS GRTAGS GSYMS > > +DISTCLEAN_FILES += tags TAGS cscope* GPATH GTAGS GRTAGS GSYMS .thinlto-cache > > > > This was suggested in v1, but I could not understand > why doing this in distclean was appropriate. > > Is keeping cache files of kernel objects > useful for external module builds? No, the cache only speeds up incremental kernel builds. > Also, please clean up .thinlto-cache for external module builds. Ack. Sami
