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

[Arnd Bergmann]

On Wed, Mar 10, 2021 at 9:50 PM Masahiro Yamada <masahiroy@xxxxxxxxxx> wrote:
> On Mon, Mar 1, 2021 at 10:11 AM Nicholas Piggin <npiggin@xxxxxxxxx> 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?

         Arnd