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Subject: Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION
To:     Masahiro Yamada <masahiroy@kernel.org>,
        Nicholas Piggin <npiggin@gmail.com>
Cc:     Arnd Bergmann <arnd@kernel.org>, Fangrui Song <maskray@google.com>,
        Ard Biesheuvel <ardb@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Andrew Scull <ascull@google.com>,
        Mark Brown <broonie@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        David Brazdil <dbrazdil@google.com>,
        Geert Uytterhoeven <geert+renesas@glider.be>,
        Ionela Voinescu <ionela.voinescu@arm.com>,
        Kees Cook <keescook@chromium.org>,
        Kristina Martsenko <kristina.martsenko@arm.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Marc Zyngier <maz@kernel.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Will Deacon <will@kernel.org>, Nicolas Pitre <nico@fluxnic.net>
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From:   Rasmus Villemoes <rasmus.villemoes@prevas.dk>
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Date:   Wed, 10 Mar 2021 22:45:58 +0100
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On 10/03/2021 21.49, Masahiro Yamada wrote:
> On Mon, Mar 1, 2021 at 10:11 AM Nicholas Piggin <npiggin@gmail.com> wrote:

> 
> I tested LD_DEAD_CODE_DATA_ELIMINATION for the latest kernel.
> 
> I added an unused function, this_func_is_unused(),
> then built the ppc kernel with LD_DEAD_CODE_DATA_ELIMINATION.
> 
> It remained in vmlinux.
> 
> 
> 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

Dunno, works just fine for my ppc32 target in v4.19 (i.e., the function
gets eliminated when enabling LD_DEAD_CODE_DATA_ELIMINATION).

But yes, I can reproduce for master ppc64 defconfig. kernel/.cpu.o.cmd
says that it wasn't even compiled with -ffunction-sections, nor does
.vmlinux.cmd mention --gc-sections.

> masahiro@oscar:~/ref/linux$ grep DEAD_CODE_ .config
> CONFIG_HAVE_LD_DEAD_CODE_DATA_ELIMINATION=y
> CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y

Ah, but scripts/config just blindly adds that config option - I don't
think ppc64 actually supports this, and
CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y automagically vanishes from
.config when building.

Rasmus