linux-kernel - Re: [PATCH] powerpc: Allow selection of CONFIG_LD_DEAD_CODE_DATA

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Date:   Tue, 17 Apr 2018 18:45:27 +0200
From:   Mathieu Malaterre <malat@...ian.org>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] powerpc: Allow selection of CONFIG_LD_DEAD_CODE_DATA_ELIMINATION

On Tue, Apr 17, 2018 at 12:49 PM, Christophe Leroy
<christophe.leroy@....fr> wrote:
> This option does dead code and data elimination with the linker by
> compiling with -ffunction-sections -fdata-sections and linking with
> --gc-sections.
>
> By selecting this option on mpc885_ads_defconfig,
> vmlinux LOAD segment size gets reduced by 10%
>
> Program Header before the patch:
>     LOAD off    0x00010000 vaddr 0xc0000000 paddr 0x00000000 align 2**16
>          filesz 0x0036eda4 memsz 0x0038de04 flags rwx
>
> Program Header after the patch:
>     LOAD off    0x00010000 vaddr 0xc0000000 paddr 0x00000000 align 2**16
>          filesz 0x00316da4 memsz 0x00334268 flags rwx
>
> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
> ---
>  arch/powerpc/Kconfig | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 8fe4353be5e3..e1fac49cf465 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -888,6 +888,14 @@ config PPC_MEM_KEYS
>
>           If unsure, say y.
>
> +config PPC_UNUSED_ELIMINATION
> +       bool "Eliminate unused functions and data from vmlinux"
> +       default n
> +       select LD_DEAD_CODE_DATA_ELIMINATION
> +       help
> +         Select this to do dead code and data elimination with the linker
> +         by compiling with -ffunction-sections -fdata-sections and linking
> +         with --gc-sections.
>  endmenu
>

Just for reference, I cannot boot my Mac Mini G4 anymore (yaboot). The
messages I can see (prom_init) are:

---
done
copying OF device tree...
Building dt strings...
Building dt structure...
Device tree strings 0x01100000 -> 0x01100e02
Device tree struct   0x01101000 -> 0x01109000
Quiescing Open Firmware ...
Booting Linux via __start() @ 0x00140000 ...
_
---



>  config ISA_DMA_API
> --
> 2.13.3
>