Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

[Fangrui Song]

On 2021-02-25, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
>
> When looking at kernel size optimizations, I found that arm64
> does not currently support HAVE_LD_DEAD_CODE_DATA_ELIMINATION,
> which enables the --gc-sections flag to the linker.
>
> I see that for a defconfig build with llvm, there are some
> notable improvements from enabling this, in particular when
> combined with the recently added CONFIG_LTO_CLANG_THIN
> and CONFIG_TRIM_UNUSED_KSYMS:
>
>   text    data     bss     dec     hex filename
> 16570322 10998617 506468 28075407 1ac658f defconfig/vmlinux
> 16318793 10569913 506468 27395174 1a20466 trim_defconfig/vmlinux
> 16281234 10984848 504291 27770373 1a7be05 gc_defconfig/vmlinux
> 16029705 10556880 504355 27090940 19d5ffc gc+trim_defconfig/vmlinux
> 17040142 11102945 504196 28647283 1b51f73 thinlto_defconfig/vmlinux
> 16788613 10663201 504196 27956010 1aa932a thinlto+trim_defconfig/vmlinux
> 16347062 11043384 502499 27892945 1a99cd1 gc+thinlto_defconfig/vmlinux
> 15759453 10532792 502395 26794640 198da90 gc+thinlto+trim_defconfig/vmlinux
>
> I needed a small change to the linker script to get clean randconfig
> builds, but I have not done any meaningful boot testing on it to
> see if it works. If there are no regressions, I wonder whether this
> should be autmatically done for LTO builds, given that it improves
> both kernel size and compile speed.
>
> Link: https://lore.kernel.org/lkml/CAK8P3a05VZ9hSKRzVTxTn+1nf9E+gqebJWTj6N23nfm+ELHt9A@xxxxxxxxxxxxxx/
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>

For folks who are interested in --gc-sections on metadata sections,
I want to bring you awareness of the implication of __start_/__stop_ symbols and C identifier name sections.
You can see https://github.com/ClangBuiltLinux/linux/issues/1307 for a summary.
(Its linked blog article has some examples.)

In the kernel linker scripts, most C identifier name sections begin with double-underscore __.
Some are surrounded by `KEEP(...)`, some are not.

* A `KEEP` keyword has GC root semantics and makes ld --gc-sections ineffectful.
* Without `KEEP`, __start_/__stop_ references from a live input section
  can unnecessarily retain all the associated C identifier name input
  sections. The new ld.lld option `-z start-stop-gc` can defeat this rule.

As an example, a __start___jump_table reference from a live section
causes all `__jump_table` input section to be retained, even if you
change `KEEP(__jump_table)` to `(__jump_table)`.
(If you change the symbol name from `__start_${section}` to something
else (e.g. `__start${section}`), the rule will not apply.)


There are a lot of KEEP usage. Perhaps some can be dropped to facilitate
ld --gc-sections.