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

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Message-ID: <e49e2296-11f8-a2fa-6b64-db18ba5db641@c-s.fr>
Date:   Tue, 17 Apr 2018 18:49:44 +0200
From:   Christophe LEROY <christophe.leroy@....fr>
To:     Mathieu Malaterre <malat@...ian.org>
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



Le 17/04/2018 à 18:45, Mathieu Malaterre a écrit :
> 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:

Which version of GCC do you use ?

Can you provide the generated System.map with and without that option 
active ?

Thanks
Christophe

> 
> ---
> 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
>>