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

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Message-Id: <d3a7a06c-e82f-4e5e-9ceb-6c659a228bbe@app.fastmail.com>
Date: Mon, 11 Mar 2024 12:41:48 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Geert Uytterhoeven" <geert@...ux-m68k.org>,
 "Yuntao Liu" <liuyuntao12@...wei.com>
Cc: 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

On Mon, Mar 11, 2024, at 10:14, Geert Uytterhoeven wrote:
> On Sat, Mar 9, 2024 at 2:24 PM liuyuntao (F) <liuyuntao12@...wei.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 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.

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@kernelorg/