On Wed, Mar 10, 2021 at 9:50 PM Masahiro Yamada <masahi...@kernel.org> wrote: > On Mon, Mar 1, 2021 at 10:11 AM Nicholas Piggin <npig...@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? Arnd