Re: [PATCH] x86: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

[Alexander Lobakin]

From: Yuntao Liu <liuyuntao12@xxxxxxxxxx>
Date: Mon, 22 Apr 2024 06:05:56 +0000

> The current x86 architecture does not yet support the
> HAVE_LD_DEAD_CODE_DATA_ELIMINATION feature. x86 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.
> 
> Enabling CONFIG_UNWINDER_ORC or CONFIG_MITIGATION_RETPOLINE will enable
> the objtool's --orc and --retpoline parameters, which will alter the
> layout of the binary file, thereby preventing gc-sections from functioning
> properly. Therefore, HAVE_LD_DEAD_CODE_DATA_ELIMINATION should only be
> selected when they are not enabled.

Dunno, I have DCE enabled for years on my home kernel, see commit [0]
with both ORC and retpolines enabled, and I didn't have any issues.
vmlinux still shrinks well, even with Clang LTO.

> 
> Enabling CONFIG_LTO_CLANG or CONFIG_X86_KERNEL_IBT will use vmlinux.o
> instead of performing the slow LTO link again. This can also prevent
> gc-sections from functioning properly. Therefore, using this optimization
> when CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is not enabled.
> 
> The size comparison of zImage is as follows:
> x86_def_defconfig  i386_defconfig    tinyconfig
> 10892288           10826240          607232          no dce
> 10748928           10719744          529408          dce
> 1.3%               0.98%             12.8%           shrink
> 
> When using smaller config file, there is a significant reduction in the
> size of the zImage.

[0] https://github.com/solbjorn/linux/commit/25c4953ea73d

Thanks,
Olek