Re: [PATCH] x86: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

[liuyuntao (F)]

On 2024/4/23 0:01, Masahiro Yamada wrote:
> On Mon, Apr 22, 2024 at 3:41 PM Yuntao Liu <liuyuntao12@xxxxxxxxxx> 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.