linux-kernel - Re: [PATCH] arm64: enable dead code elimination

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Message-ID: <801e30a093e41c3eebd675f0d224f8d7@kernel.org>
Date:   Mon, 17 Jul 2023 10:42:15 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Kefeng Wang <wangkefeng.wang@...wei.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: enable dead code elimination

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@...wei.com>

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

so it's probably not ready for prime time.

         M.
-- 
Jazz is not dead. It just smells funny...