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Date: Wed, 24 Jun 2020 14:29:02 -0700
From: Sami Tolvanen <samitolvanen@google.com>
Subject: Re: [PATCH 02/22] kbuild: add support for Clang LTO
Message-ID: <20200624212902.GA26253@google.com>
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To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Masahiro Yamada <masahiroy@kernel.org>, Will Deacon <will@kernel.org>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, "Paul E. McKenney" <paulmck@kernel.org>, Kees Cook <keescook@chromium.org>, clang-built-linux <clang-built-linux@googlegroups.com>, Kernel Hardening <kernel-hardening@lists.openwall.com>, linux-arch <linux-arch@vger.kernel.org>, Linux ARM <linux-arm-kernel@lists.infradead.org>, Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>, linux-pci@vger.kernel.org, "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>

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@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