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

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Date:   Mon, 11 Jul 2022 14:50:46 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Miko Larsson <mikoxyzzz@...il.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Chris Down <chris@...isdown.name>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        John Ogness <john.ogness@...utronix.de>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Oleksandr Natalenko <oleksandr@...hat.com>,
        Elliot Berman <quic_eberman@...cinc.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vlastimil Babka <vbabka@...e.cz>, X86 ML <x86@...nel.org>,
        arcml <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 Wed, Jun 29, 2022 at 7:48 PM Miko Larsson <mikoxyzzz@...il.com> wrote:
>
> 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


This patch focuses on the removal of CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3,
so I think it is OK as-is.

The rest of cleanups, if needed,
should be submitted separately.




>
> --
> ~miko
>
>


-- 
Best Regards
Masahiro Yamada