[PATCH 02/22] kbuild: add support for Clang LTO

[Sami Tolvanen]

On Wed, Jun 24, 2020 at 01:53:52PM -0700, Nick Desaulniers wrote:
> On Wed, Jun 24, 2020 at 1:32 PM Sami Tolvanen <samitolvanen at google.com> wrote:
> >
> > 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)
> 
> The kconfig change gates this on clang-11; do we still need the
> cc-option check here, or can we hardcode the use of -fsplit-lto-unit?
> Playing with the flag in godbolt, it looks like clang-8 had support
> for this flag.

True, we don't need cc-option here anymore. I'll remove it, thanks.

> > +KBUILD_LDFLAGS += --thinlto-cache-dir=.thinlto-cache
> 
> It might be nice to have `make distclean` or even `make clean` scrub
> the .thinlto-cache?  Also, I verified that the `.gitignore` rule for
> `.*` properly ignores this dir.

Sure, distclean sounds appropriate to me.

Sami