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

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Message-ID: <23d94414-b819-eaef-6fac-638bf713d341@huawei.com>
Date:   Mon, 17 Jul 2023 19:56:39 +0800
From:   Kefeng Wang <wangkefeng.wang@...wei.com>
To:     Marc Zyngier <maz@...nel.org>
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 2023/7/17 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@...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

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.