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

[Nicolas Pitre]

On Wed, 10 Mar 2021, Nick Desaulniers wrote:

> On Wed, Mar 10, 2021 at 1:08 PM Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
> >
> > 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?
> 
> 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.

The GNU LD has --print-gc-sections to list those sections that were 
dropped. And normally you should be able to find why a section wasn't 
dropped by looking for dependencies in the linker map file.


Nicolas