linux-kernel - Re: [PATCH-next v2] arm32: enable HAVE_LD_DEAD_CODE_DATA

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Date: Mon, 11 Mar 2024 10:14:50 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: "liuyuntao (F)" <liuyuntao12@...wei.com>
Cc: Arnd Bergmann <arnd@...db.de>, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>, 
	Fangrui Song <maskray@...gle.com>, Russell King <linux@...linux.org.uk>, Andrew Davis <afd@...com>, 
	Andrew Morton <akpm@...ux-foundation.org>, 
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, 
	Geert Uytterhoeven <geert+renesas@...der.be>, Jonathan Corbet <corbet@....net>, 
	Mike Rapoport <rppt@...nel.org>, Rob Herring <robh@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, 
	Linus Walleij <linus.walleij@...aro.org>, llvm@...ts.linux.dev
Subject: Re: [PATCH-next v2] arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

Hi Yuntao,

On Sat, Mar 9, 2024 at 2:24 PM liuyuntao (F) <liuyuntao12@...wei.com> 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

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68korg

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds