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

[liuyuntao (F)]

On 2024/3/8 22:27, Ard Biesheuvel wrote:
> On Fri, 8 Mar 2024 at 14:16, Arnd Bergmann <arnd@xxxxxxxx> 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.
> >
> > Tested-by: Arnd Bergmann <arnd@xxxxxxxx>
> > Reviewed-by: Arnd Bergmann <arnd@xxxxxxxx>
> >
> > Adding Ard Biesheuvel and Fangrui Song to Cc, so they can comment
> > on the ARM_VECTORS_TEXT workaround. I don't understand enough of
> > the details of what is going on here.
>
> Thanks for the cc
>
> > Full quote of the patch below so they can see the whole thing.
> >
> > If they are also happy with the patch, I think you can send it
> > into Russell's patch tracker at
> > https://www.armlinux.org.uk/developer/patches/info.php
>
> No, not happy at all :-)
>
> The resulting kernel does not boot (built with GCC or Clang). And the
> patch is buggy (see below)
After applying .reloc .text, R_ARM_NONE, ., the resulting kernel boots 
well in QEMU. I tested it with the latest linux-next master branch and 
the mainline master branch.
By the way, I used vexpress_defconfig for testing, it worked.