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

[liuyuntao (F)]

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@xxxxxxxxxx>
>
> 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?