Re: [PATCH] arm64: enable dead code elimination

From: Marc Zyngier <maz@kernel.org>
To: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] arm64: enable dead code elimination
Date: Mon, 17 Jul 2023 13:15:18 +0100	[thread overview]
Message-ID: <864jm2vind.wl-maz@kernel.org> (raw)
In-Reply-To: <23d94414-b819-eaef-6fac-638bf713d341@huawei.com>

On Mon, 17 Jul 2023 12:56:39 +0100,
Kefeng Wang <wangkefeng.wang@huawei.com> wrote:
> 
> 
> 
> 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@huawei.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?

You tell me!

gcc (Debian 10.2.1-6) 10.2.1 20210110
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

so hardly something special. This is built with the current state of
my NV tree, available here[1] As for the configuration, have a look
here[2].

	M.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/log/?h=kvm-arm64/nv-6.6-WIP
[2] https://paste.debian.net/1286106/

-- 
Without deviation from the norm, progress is not possible.

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