On Thu, Sep 03, 2020 at 03:08:59PM -0700, Kees Cook wrote: > On Thu, Sep 03, 2020 at 01:30:34PM -0700, Sami Tolvanen 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> > > I remain crazy excited about being able to use this in upstream. :) > > The only suggestion I have here, if it might help with clarity, would be > to remove DISABLE_LTO globally as a separate patch, since it's entirely > unused in the kernel right now. This series removes it as it goes, which > I think is fine, but it might cause some reviewers to ponder "what's > this DISABLE_LTO thing? Don't we need that?" without realizing currently > unused in the kernel. Sure, that makes sense. I'll add a patch to remove DISABLE_LTO treewide in v3. Sami
