linux-kernel - Re: [PATCH] kbuild: drop support for CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE

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Message-ID: <2842572.mvXUDI8C0e@sakura.myxoz.lan>
Date:   Wed, 29 Jun 2022 12:48:45 +0200
From:   Miko Larsson <mikoxyzzz@...il.com>
To:     Masahiro Yamada <masahiroy@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     arnd@...db.de, bigeasy@...utronix.de, chris@...isdown.name,
        dmitry.torokhov@...il.com, gustavoars@...nel.org,
        john.ogness@...utronix.de, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, nathan@...nel.org,
        ndesaulniers@...gle.com, oleksandr@...hat.com,
        quic_eberman@...cinc.com, seanjc@...gle.com, tglx@...utronix.de,
        vbabka@...e.cz, x86@...nel.org, linux-snps-arc@...ts.infradead.org,
        Vineet Gupta <vgupta@...nel.org>
Subject: Re: [PATCH] kbuild: drop support for CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3

On Tuesday, 28 June 2022 23:04:07 CEST Nick Desaulniers wrote:
> The difference in most compilers between `-O3` and `-O2` is mostly down
> to whether loops with statically determinable trip counts are fully
> unrolled vs unrolled to a multiple of SIMD width.
> 
> This patch is effectively a revert of
> commit 15f5db60a137 ("kbuild,arc: add
> CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3 for ARC") without re-adding
> ARCH_CFLAGS
> 
> Ever since
> commit cfdbc2e16e65 ("ARC: Build system: Makefiles, Kconfig, Linker
> script")
> ARC has been built with -O3, though the reason for doing so was not
> specified in inline comments or the commit message. This commit does not
> re-add -O3 to arch/arc/Makefile.
> 
> Folks looking to experiment with `-O3` (or any compiler flag for that
> matter) may pass them along to the command line invocation of make:
> 
> $ make KCFLAGS=-O3
> 
> Code that looks to re-add an explicit Kconfig option for `-O3` should
> provide:
> 1. A rigorous and reproducible performance profile of a reasonable
>    userspace workload that demonstrates a hot loop in the kernel that
>    would benefit from `-O3` over `-O2`.
> 2. Disassembly of said loop body before and after.
> 3. Provides stats on terms of increase in file size.
> 

Might be worth cleaning up the rest of the kernel of instances of -O3,
too. -O3 used to build lz4 and mips vdso, for instance. Might be a bit
of a digression, though

--
~miko