Re: [PATCH] arm64: enable dead code elimination

[Kefeng Wang]

On 2023/7/17 17:42, Marc Zyngier wrote:
> On 2023-07-17 09:07, Kefeng Wang wrote:
> > Select CONFIG_HAVE_LD_DEAD_CODE_DATA_ELIMINATION for arm64, allowing the
> > user to enable dead code elimination. In order for this to work, ensure
> > that 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.
> >
> > The following comparison is based 6.5-rc2 with defconfig,
> >
> > $ ./scripts/bloat-o-meter vmlinux-base vmlinux-new
> > add/remove: 3/1106 grow/shrink: 4102/6964 up/down: 35704/-99980 (-64276)
> > Function                                     old     new   delta
> > ...
> > Total: Before=17888959, After=17824683, chg -0.36%
> >
> > add/remove: 0/1 grow/shrink: 0/1 up/down: 0/-44 (-44)
> > Data                                         old     new   delta
> > ...
> > Total: Before=4820808, After=4820764, chg -0.00%
> >
> > add/remove: 0/1 grow/shrink: 0/1 up/down: 0/-1096 (-1096)
> > RO Data                                      old     new   delta
> > ...
> > Total: Before=5179123, After=5178027, chg -0.02%
> >
> > $ size vmlinux-base vmlinux
> >    text       data         bss      dec       hex    filename
> > 25433734  15385766  630656  41450156  2787aac    vmlinux-base
> > 24756738  15360870  629888  40747496  26dc1e8    vmlinux-new
> >
> > Memory available after booting, saving 704k on qemu,
> > base: 8084532K/8388608K
> > new:  8085236K/8388608K
> >
> > Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
>
> I took this patch for a spin in my tree, and ended up with:
>
>    CC      .vmlinux.export.o
>    UPD     include/generated/utsversion.h
>    CC      init/version-timestamp.o
>    LD      .tmp_vmlinux.kallsyms1
> ld: init/main.o(__patchable_function_entries): error: need linked-to 
> section for --gc-sections
> make[2]: *** [scripts/Makefile.vmlinux:36: vmlinux] Error 1
> make[1]: *** [/home/maz/hot-poop/arm-platforms/Makefile:1238: vmlinux] 
> Error 2
> make: *** [Makefile:234: __sub-make] Error 2

I don't find this error with CONFIG_FTRACE_MCOUNT_RECORD or 
allyesconfig, does it need special config or gcc version?
> so it's probably not ready for prime time.
>
>          M.