Re: [PATCH-next v2] arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

From: "liuyuntao (F)" <liuyuntao12@huawei.com>
To: Arnd Bergmann <arnd@arndb.de>,
	<linux-arm-kernel@lists.infradead.org>,
	<linux-kernel@vger.kernel.org>, Ard Biesheuvel <ardb@kernel.org>,
	Fangrui Song <maskray@google.com>
Cc: Russell King <linux@armlinux.org.uk>, Andrew Davis <afd@ti.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Geert Uytterhoeven <geert+renesas@glider.be>,
	Jonathan Corbet <corbet@lwn.net>, Mike Rapoport <rppt@kernel.org>,
	Rob Herring <robh@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Linus Walleij <linus.walleij@linaro.org>, <llvm@lists.linux.dev>
Subject: Re: [PATCH-next v2] arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION
Date: Sat, 9 Mar 2024 14:14:49 +0800	[thread overview]
Message-ID: <42892794-7668-4eb0-8d2f-c78ca0daf370@huawei.com> (raw)
In-Reply-To: <58cc1053-7208-4b22-99cb-210fdf700569@app.fastmail.com>

On 2024/3/8 21:15, Arnd Bergmann wrote:
> On Thu, Mar 7, 2024, at 16:12, Yuntao Liu wrote:
>> The current arm32 architecture does not yet support the
>> HAVE_LD_DEAD_CODE_DATA_ELIMINATION feature. arm32 is widely used in
>> embedded scenarios, and enabling this feature would be beneficial for
>> reducing the size of the kernel image.
>>
>> In order to make this work, 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.
>>
>> It boots normally with defconfig, vexpress_defconfig and tinyconfig.
>>
>> The size comparison of zImage is as follows:
>> defconfig       vexpress_defconfig      tinyconfig
>> 5137712         5138024                 424192          no dce
>> 5032560         4997824                 298384          dce
>> 2.0%            2.7%                    29.7%           shrink
>>
>> When using smaller config file, there is a significant reduction in the
>> size of the zImage.
>>
>> We also tested this patch on a commercially available single-board
>> computer, and the comparison is as follows:
>> a15eb_config
>> 2161384         no dce
>> 2092240         dce
>> 3.2%            shrink
>>
>> The zImage size has been reduced by approximately 3.2%, which is 70KB on
>> 2.1M.
>>
>> Signed-off-by: Yuntao Liu <liuyuntao12@huawei.com>
> 
> I've retested with both gcc-13 and clang-18, and so no
> more build issues. Your previous version already worked
> fine for me.
> 
> I did some tests combining this with CONFIG_TRIM_UNUSED_KSYMS,
> which showed a significant improvement as expected. I also
> tried combining it with an experimental CONFIG_LTO_CLANG
> patch, but that did not show any further improvements.
> 

Thanks for the tests, CONFIG_LD_DEAD_CODE_DATA_ELIMINATION and 
CONFIG_TRIM_UNUSED_KSYMS do indeed result in a significant improvement.
I found that arm32 still doesn't support CONFIG_LTO_CLANG. I've done 
some work on it, but without success. I'd like to learn more about the 
CONFIG_LTO_CLANG patch. Do you have any relevant links?