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Date:   Wed, 10 Mar 2021 17:08:36 -0500 (EST)
From:   Nicolas Pitre <nico@fluxnic.net>
To:     Nick Desaulniers <ndesaulniers@google.com>
cc:     kernel-toolchains@vger.kernel.org,
        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>, Arnd Bergmann <arnd@kernel.org>
Subject: Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION
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On Wed, 10 Mar 2021, Nick Desaulniers wrote:

> 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.

The GNU LD has --print-gc-sections to list those sections that were 
dropped. And normally you should be able to find why a section wasn't 
dropped by looking for dependencies in the linker map file.


Nicolas