On Wed, Mar 10, 2021 at 1:08 PM Arnd Bergmann <a...@kernel.org> wrote:
>
> 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?
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
