linux-kernel - Re: [PATCH] arm64: enable dead code elimination

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Date:   Mon, 17 Jul 2023 19:20:40 +0800
From:   Kefeng Wang <wangkefeng.wang@...wei.com>
To:     Will Deacon <will@...nel.org>
CC:     Catalin Marinas <catalin.marinas@....com>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arm64: enable dead code elimination



On 2023/7/17 17:24, Will Deacon wrote:
> On Mon, Jul 17, 2023 at 04:07:39PM +0800, Kefeng Wang wrote:
>> Select CONFIG_HAVE_LD_DEAD_CODE_DATA_ELIMINATION for arm64, allowing the
>> user to enable dead code elimination. In order for this to work, ensure
>> that we keep the necessary tables by annotating them with KEEP, also it
>> requires further changes to linker script to KEEP some tables and wildcard
>> compiler generated sections into the right place.
>>
>> The following comparison is based 6.5-rc2 with defconfig,
>>
>> $ ./scripts/bloat-o-meter vmlinux-base vmlinux-new
>> add/remove: 3/1106 grow/shrink: 4102/6964 up/down: 35704/-99980 (-64276)
>> Function                                     old     new   delta
>> ...
>> Total: Before=17888959, After=17824683, chg -0.36%
>>
>> add/remove: 0/1 grow/shrink: 0/1 up/down: 0/-44 (-44)
>> Data                                         old     new   delta
>> ...
>> Total: Before=4820808, After=4820764, chg -0.00%
>>
>> add/remove: 0/1 grow/shrink: 0/1 up/down: 0/-1096 (-1096)
>> RO Data                                      old     new   delta
>> ...
>> Total: Before=5179123, After=5178027, chg -0.02%
>>
>> $ size vmlinux-base vmlinux
>>     text	   data	     bss      dec       hex	filename
>> 25433734  15385766  630656  41450156  2787aac	vmlinux-base
>> 24756738  15360870  629888  40747496  26dc1e8	vmlinux-new
>>
>> Memory available after booting, saving 704k on qemu,
>> base: 8084532K/8388608K
>> new:  8085236K/8388608K
> 
> Is that a 0.009% improvement? Is it really worth the hassle?
> 
> x86 doesn't select this and risc-v had to turn it off for LLD, so it feels
> like we're just creating a rod for our own back by selecting it.


The LD_DEAD_CODE_DATA_ELIMINATION is particularly used for small configs
on small systems, risc-v is aimed to resource limited board platforms,
maybe x86 has no strong requirement, and we will try to use it on some
embedded board, if no one try it, this feature will never become stable :)

> 
> Will
>