Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

From: Sedat Dilek <sedat.dilek@gmail.com>
To: Arnd Bergmann <arnd@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Arnd Bergmann <arnd@arndb.de>, Kees Cook <keescook@chromium.org>,
	Mark Brown <broonie@kernel.org>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Geert Uytterhoeven <geert+renesas@glider.be>,
	Kristina Martsenko <kristina.martsenko@arm.com>,
	Ionela Voinescu <ionela.voinescu@arm.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Andrew Scull <ascull@google.com>,
	David Brazdil <dbrazdil@google.com>,
	Marc Zyngier <maz@kernel.org>, Ard Biesheuvel <ardb@kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Clang-Built-Linux ML <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION
Date: Fri, 26 Feb 2021 11:02:01 +0100	[thread overview]
Message-ID: <CA+icZUWYR0MWRmzV+6rLKzUQUS8WRjF521iNVXk4w2fJk=NVJQ@mail.gmail.com> (raw)
In-Reply-To: <CAK8P3a0cyAYqdPg_OoaVVY2Gjn=vtwAyfsVPjZ5qYddiOOc-eA@mail.gmail.com>

On Fri, Feb 26, 2021 at 10:51 AM Arnd Bergmann <arnd@kernel.org> wrote:
>
> On Fri, Feb 26, 2021 at 10:05 AM Sedat Dilek <sedat.dilek@gmail.com> wrote:
> > On Fri, Feb 26, 2021 at 9:14 AM Arnd Bergmann <arnd@kernel.org> wrote:
> >
> > > > BTW, is CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y setable for x86 (64bit)?
> > > > ( Did not look or check for it. )
> > >
> > > No, in mainline, HAVE_LD_DEAD_CODE_DATA_ELIMINATION is currently
> > > only selected on MIPS and PowerPC. I only sent experimental patches to
> > > enable it on arm64 and m68k, but have not tried booting them. If you
> > > select the symbol on x86, you should see similar results.
> > >
> >
> > OK, i see:
> >
> > $ git grep HAVE_LD_DEAD_CODE_DATA_ELIMINATION arch/mips/
> > arch/mips/Kconfig:      select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
> >
> > $ git grep HAVE_LD_DEAD_CODE_DATA_ELIMINATION arch/powerpc/
> > arch/powerpc/Kconfig:   select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
> >
> > So, I need to add this to arch/x86/Kconfig.
> >
> > You happen to know if changes to arch/x86/kernel/vmlinux.lds.S
> > (sections) are needed?
>
> No idea. I'm still debugging a possible regression on arm64, but both
> issues I found for arm64 are specific to that architecture and won't
> happen on x86. It's likely that something else breaks though.
>

My first experiences with gc were with the freetz project (mips
embedded router) and don't ask me what GCC version.

I will try with gc + trim + cfi later after my current build has finished.

> > Last question:
> > The last days I see a lot of fixes touching inlining with LLVM/Clang v13-git.
> > What git tag are you using?
> > What are your experiences?
> > Pending patches (kernel-side)?
> >
> > I use:
> > $ /opt/llvm-toolchain/bin/clang --version
> > dileks clang version 13.0.0 (https://github.com/llvm/llvm-project.git
> > c465429f286f50e52a8d2b3b39f38344f3381cce)
>
> This is what I have on the build box:
> Ubuntu clang version
> 13.0.0-++20210223104451+ebca13c66504-1~exp1~20210223095200.234
>

Distro-clang takes much longer here.
Selfmade stage1-only LLVM toolchain compiles here 10% faster.
cfi takes approx. 20% longer.
With trim + gc I suppose it will take much longer.

Let me test.
Will report later.

- Sedat -