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

[Sedat Dilek <sedat.dilek@gmail.com>]

On Wed, Mar 10, 2021 at 10:08 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?
>

The best results on size-reduction of vmlinux I got with Clang-CFI on x86-64.

Clang-LTO and Clang-CFI:
I was able to build with CONFIG_TRIM_UNUSED_KSYMS=y which needs to add
a whitelist file or add a whitelist to scripts/gen_autoksyms.sh.
And boot on bare metal.
Furthermore, I was able to compile
CONFIG_HAVE_LD_DEAD_CODE_DATA_ELIMINATION=y with and without
CONFIG_TRIM_UNUSED_KSYMS=y.
Every kernel I had CONFIG_HAVE_LD_DEAD_CODE_DATA_ELIMINATION=y does not boot.
Yes, there is a size reduction with both enabled but not that good as
with Clang-CFI.
All testings with several iterations of LLVM/Clang v13-git.
With CONFIG_TRIM_UNUSED_KSYMS=y I see a 3x-loops of building .version
and folowing steps - got no answer if this is intended.
Means longer build-time.
I did not follow this anymore as both Kconfigs with Clang-LTO consume
more build-time and the resulting vmlinux is some MiB bigger than with
Clang-CFI.

If someone is interested in x86-64 I can provide the whitelist files
and or (alternatively) changes to scripts/gen_autoksyms.sh.
AFAICS I had open a thread for this - damn digital dementia.

- Sedat -