Re: [PATCH] arm64: enable dead code elimination

[Kefeng Wang]

On 2023/7/17 20:15, Marc Zyngier wrote:
> On Mon, 17 Jul 2023 12:56:39 +0100,
> Kefeng Wang <wangkefeng.wang@xxxxxxxxxx> 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,
...
> > > 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].

1) With gcc 10.3.1/ld (GNU Binutils) 2.37， it could be reproduced,
but there is no issue for cross-compiler gcc 9.3/ld (GNU Binutils for
Ubuntu) 2.34.

2) There is same issue like commit f7584322e4fe ("riscv: disable 
HAVE_LD_DEAD_CODE_DATA_ELIMINATION for LLD") said with allyesconfig on
arm64, it takes too long in bfd_flavour_name()

Samples: 257K of event 'cycles', Event count (approx.): 203974259359 

  Overhead  Shared Object             Symbol 
              IPC   [IPC Coverage]
-   61.11%  libbfd-2.34-arm64.so      [.] bfd_flavour_name 
             -      -
     bfd_flavour_name 

-    6.55%  libbfd-2.34-arm64.so      [.] bfd_hash_traverse 
             -      -


Just like you said, it is not ready for prime time, so please ignore
this patch :(


> 	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/