[PATCH-next v4] arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

[Geert Uytterhoeven]

Hi Yuntao,

On Sat, Mar 16, 2024 at 3:44 AM Yuntao Liu <liuyuntao12 at huawei.com> 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.
> When using ld.lld for linking, KEEP is not recognized within the OVERLAY
> command, and Ard proposed a concise method to solve this problem.
>
> 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 at huawei.com>
> Tested-by: Arnd Bergmann <arnd at arndb.de>
> Reviewed-by: Arnd Bergmann <arnd at arndb.de>

Thanks for your patch, which is now commit ed0f941022515ff4 ("ARM:
9404/1: arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION") in
arm/for-next (next-20240611).

I gave this a try on my custom configs for RSK+RZA1 (RZ/A1H)
and RZA2MEVB (RZ/A2M).  According to bloat-o-meter, enabling
HAVE_LD_DEAD_CODE_DATA_ELIMINATION reduced kernel size by almost
500 KiB (-8.3%).  The figures reported in "Memory: ... available"
were even more impressive: 1032 KiB more free memory than before.

As these boards have only 32 resp. 64 MiB of RAM, and some products
even use RZ/A1H with just the 10 MiB of on-chip SRAM, this is a good
improvement to have!
Thanks!

Tested-by: Geert Uytterhoeven <geert+renesas at glider.be>

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds