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Date: Mon, 11 Mar 2024 12:41:48 +0100
From: "Arnd Bergmann" <arnd@arndb.de>
To: "Geert Uytterhoeven" <geert@linux-m68k.org>,
 "Yuntao Liu" <liuyuntao12@huawei.com>
Cc: 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
Subject: Re: [PATCH-next v2] arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION
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On Mon, Mar 11, 2024, at 10:14, Geert Uytterhoeven wrote:
> On Sat, Mar 9, 2024 at 2:24=E2=80=AFPM liuyuntao (F) <liuyuntao12@huaw=
ei.com> wrote:
>> On 2024/3/9 16:20, Arnd Bergmann wrote:
>>
>> I tested this patch, the size improvement was only about one
>> ten-thousandth, and the compilation time had increased by about a qua=
rter,
>> 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.

I think llvm treats it purely as a performance optimization of
the resulting binary, allowing cross-unit inlining and constant
folding, but I don't think it actually tries or succeeds to make
the output smaller. I do remember seeing size improvements with
LTO using gcc in the past, but this never made it into the
mainline kernel. The last time someone tried to add it was 2022[1],
not sure why there was no follow-up.

     Arnd

[1] https://lore.kernel.org/lkml/20221114114344.18650-1-jirislaby@kernel=
org/