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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Wed, 10 Mar 2021 14:02:24 -0800
Message-ID: <CAKwvOdk4y3Ekc_TiWuk6D2KNbH_6YHZLRK9TtAnOP=cLP6E0Rw@mail.gmail.com>
Subject: Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION
To:     kernel-toolchains@vger.kernel.org
Cc:     Masahiro Yamada <masahiroy@kernel.org>,
        Nicholas Piggin <npiggin@gmail.com>,
        Fangrui Song <maskray@google.com>,
        Ard Biesheuvel <ardb@kernel.org>,
        Andrew Scull <ascull@google.com>,
        Mark Brown <broonie@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        David Brazdil <dbrazdil@google.com>,
        Geert Uytterhoeven <geert+renesas@glider.be>,
        Ionela Voinescu <ionela.voinescu@arm.com>,
        Kees Cook <keescook@chromium.org>,
        Kristina Martsenko <kristina.martsenko@arm.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Marc Zyngier <maz@kernel.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Will Deacon <will@kernel.org>,
        Nicolas Pitre <nico@fluxnic.net>,
        Arnd Bergmann <arnd@kernel.org>
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On Wed, Mar 10, 2021 at 1:08 PM Arnd Bergmann <arnd@kernel.org> wrote:
>
> On Wed, Mar 10, 2021 at 9:50 PM Masahiro Yamada <masahiroy@kernel.org> wrote:
> > On Mon, Mar 1, 2021 at 10:11 AM Nicholas Piggin <npiggin@gmail.com> wrote:
> > > Excerpts from Arnd Bergmann's message of February 27, 2021 7:49 pm:
>
> >
> > masahiro@oscar:~/ref/linux$ echo  'void this_func_is_unused(void) {}'
> > >>  kernel/cpu.c
> > masahiro@oscar:~/ref/linux$ export
> > CROSS_COMPILE=/home/masahiro/tools/powerpc-10.1.0/bin/powerpc-linux-
> > masahiro@oscar:~/ref/linux$ make ARCH=powerpc  defconfig
> > masahiro@oscar:~/ref/linux$ ./scripts/config  -e EXPERT
> > masahiro@oscar:~/ref/linux$ ./scripts/config  -e LD_DEAD_CODE_DATA_ELIMINATION
> > masahiro@oscar:~/ref/linux$
> > ~/tools/powerpc-10.1.0/bin/powerpc-linux-nm -n  vmlinux | grep
> > this_func
> > c000000000170560 T .this_func_is_unused
> > c000000001d8d560 D this_func_is_unused
> > masahiro@oscar:~/ref/linux$ grep DEAD_CODE_ .config
> > CONFIG_HAVE_LD_DEAD_CODE_DATA_ELIMINATION=y
> > CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y
> >
> >
> > If I remember correctly,
> > LD_DEAD_CODE_DATA_ELIMINATION dropped unused functions
> > when I tried it last time.
> >
> >
> > I also tried arm64 with a HAVE_LD_DEAD_CODE_DATA_ELIMINATION hack.
> > The result was the same.
> >
> >
> >
> > Am I missing something?
>
> It's possible that it only works in combination with CLANG_LTO now
> because something broke. I definitely saw a reduction in kernel
> size when both options are enabled, but did not try a simple test
> case like you did.
>
> Maybe some other reference gets created that prevents the function
> from being garbage-collected unless that other option is removed
> as well?

I wish the linker had a debug flag that could let developers discover
the decisions it made during --gc-sections as to why certain symbols
were retained/kept or not.
-- 
Thanks,
~Nick Desaulniers