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From: Geert Uytterhoeven <geert@linux-m68k.org>
Date: Mon, 11 Mar 2024 10:14:50 +0100
Message-ID: <CAMuHMdXKk9S9voKVPvO+xvn1zFW3FnKyVHQGDkC7b9Ynkcnvmw@mail.gmail.com>
Subject: Re: [PATCH-next v2] arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION
To: "liuyuntao (F)" <liuyuntao12@huawei.com>
Cc: Arnd Bergmann <arnd@arndb.de>, linux-arm-kernel@lists.infradead.org, 
	linux-kernel@vger.kernel.org, Ard Biesheuvel <ardb@kernel.org>, 
	Fangrui Song <maskray@google.com>, Russell King <linux@armlinux.org.uk>, Andrew Davis <afd@ti.com>, 
	Andrew Morton <akpm@linux-foundation.org>, 
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, 
	Geert Uytterhoeven <geert+renesas@glider.be>, Jonathan Corbet <corbet@lwn.net>, 
	Mike Rapoport <rppt@kernel.org>, Rob Herring <robh@kernel.org>, Thomas Gleixner <tglx@linutronix.de>, 
	Linus Walleij <linus.walleij@linaro.org>, llvm@lists.linux.dev
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Hi Yuntao,

On Sat, Mar 9, 2024 at 2:24=E2=80=AFPM liuyuntao (F) <liuyuntao12@huawei.co=
m> wrote:
> On 2024/3/9 16:20, Arnd Bergmann wrote:
> > On Sat, Mar 9, 2024, at 07:14, liuyuntao (F) wrote:
> >> On 2024/3/8 21:15, Arnd Bergmann wrote:
> >>> On Thu, Mar 7, 2024, at 16:12, Yuntao Liu wrote:
> >>
> >> Thanks for the tests, CONFIG_LD_DEAD_CODE_DATA_ELIMINATION and
> >> CONFIG_TRIM_UNUSED_KSYMS do indeed result in a significant improvement=
.
> >> I found that arm32 still doesn't support CONFIG_LTO_CLANG. I've done
> >> some work on it, but without success. I'd like to learn more about the
> >> CONFIG_LTO_CLANG patch. Do you have any relevant links?
> >
> > I did not try to get it to boot and gave up when I did not see
> > any size improvement. I think there were previous attempts to
> > do it elsewhere, which I did not try to find.
> >
>
> I tested this patch, the size improvement was only about one
> ten-thousandth, and the compilation time had increased by about a quarter=
,
> and the kernel did not boot.
>
> Strangely, LTO has actually increased the compilation time
> significantly, which seems contrary to its purpose.

The purpose of LTO is to reduce code size. Doing so requires more
processing, hence the total build time increases.

Gr{oetje,eeting}s,

                        Geert

--=20
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k=
org

In personal conversations with technical people, I call myself a hacker. Bu=
t
when I'm talking to journalists I just say "programmer" or something like t=
hat.
                                -- Linus Torvalds