[PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

[Rasmus Villemoes]

On 10/03/2021 21.49, Masahiro Yamada wrote:
> On Mon, Mar 1, 2021 at 10:11 AM Nicholas Piggin <npiggin at gmail.com> wrote:

> 
> I tested LD_DEAD_CODE_DATA_ELIMINATION for the latest kernel.
> 
> I added an unused function, this_func_is_unused(),
> then built the ppc kernel with LD_DEAD_CODE_DATA_ELIMINATION.
> 
> It remained in vmlinux.
> 
> 
> masahiro at oscar:~/ref/linux$ echo  'void this_func_is_unused(void) {}'
>>>  kernel/cpu.c
> masahiro at oscar:~/ref/linux$ export
> CROSS_COMPILE=/home/masahiro/tools/powerpc-10.1.0/bin/powerpc-linux-
> masahiro at oscar:~/ref/linux$ make ARCH=powerpc  defconfig
> masahiro at oscar:~/ref/linux$ ./scripts/config  -e EXPERT
> masahiro at oscar:~/ref/linux$ ./scripts/config  -e LD_DEAD_CODE_DATA_ELIMINATION
> masahiro at 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

Dunno, works just fine for my ppc32 target in v4.19 (i.e., the function
gets eliminated when enabling LD_DEAD_CODE_DATA_ELIMINATION).

But yes, I can reproduce for master ppc64 defconfig. kernel/.cpu.o.cmd
says that it wasn't even compiled with -ffunction-sections, nor does
.vmlinux.cmd mention --gc-sections.

> masahiro at oscar:~/ref/linux$ grep DEAD_CODE_ .config
> CONFIG_HAVE_LD_DEAD_CODE_DATA_ELIMINATION=y
> CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y

Ah, but scripts/config just blindly adds that config option - I don't
think ppc64 actually supports this, and
CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y automagically vanishes from
.config when building.

Rasmus