Messages in this thread | | | Date | Wed, 10 Mar 2021 17:08:36 -0500 (EST) | From | Nicolas Pitre <> | Subject | Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION |
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On Wed, 10 Mar 2021, Nick Desaulniers wrote:
> On Wed, Mar 10, 2021 at 1:08 PM Arnd Bergmann <arnd@kernel.org> wrote: > > > > On Wed, Mar 10, 2021 at 9:50 PM Masahiro Yamada <masahiroy@kernel.org> wrote: > > > On Mon, Mar 1, 2021 at 10:11 AM Nicholas Piggin <npiggin@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.
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
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