Re: [PATCH] x86: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

["liuyuntao (F)" <liuyuntao12@huawei.com>]

On 2024/4/23 0:01, Masahiro Yamada wrote:
> On Mon, Apr 22, 2024 at 3:41 PM Yuntao Liu <liuyuntao12@huawei.com> wrote:
>>
>> 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.
>>
>> 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.
>> ---
> 
>> diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
>> index 7862a8101747..7287b5a9f17d 100755
>> --- a/scripts/link-vmlinux.sh
>> +++ b/scripts/link-vmlinux.sh
>> @@ -60,7 +60,7 @@ vmlinux_link()
>>          # skip output file argument
>>          shift
>>
>> -       if is_enabled CONFIG_LTO_CLANG || is_enabled CONFIG_X86_KERNEL_IBT; then
>> +       if [ is_enabled CONFIG_LTO_CLANG || is_enabled CONFIG_X86_KERNEL_IBT ] && [ ! is_enabled CONFIG_LD_DEAD_CODE_DATA_ELIMINATION ]; then
>>                  # Use vmlinux.o instead of performing the slow LTO link again.
>>                  objs=vmlinux.o
>>                  libs=
>> --
> 
> 
> This is wrong.
> You should not put is_enabled inside [ ... ]
> 

My mistake.

> (is_enabled CONFIG_LTO_CLANG || is_enabled CONFIG_X86_KERNEL_IBT) && !
> is_enabled CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
> 
> is still weird.
> 
> 
> When CONFIG_LTO_CLANG=y and CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y,
> the result of LTO will be discarded.
>

On arm and risc-v, these two configs can both be enabled without any 
issues, i think, it should be the same for x86 as well.