On Wed, Mar 10, 2021 at 1:08 PM Arnd Bergmann [off-list ref] wrote:

On Wed, Mar 10, 2021 at 9:50 PM Masahiro Yamada [off-list ref] wrote:

quoted

On Mon, Mar 1, 2021 at 10:11 AM Nicholas Piggin [off-list ref] wrote:

quoted

Excerpts from Arnd Bergmann's message of February 27, 2021 7:49 pm:

quoted

masahiro@oscar:~/ref/linux$ echo  'void this_func_is_unused(void) {}'

quoted

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 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

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