Re: [PATCH] kbuild: drop support for CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3

From: Miko Larsson <mikoxyzzz@gmail.com>
To: Masahiro Yamada <masahiroy@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Nick Desaulniers <ndesaulniers@google.com>
Cc: arnd@arndb.de, bigeasy@linutronix.de, chris@chrisdown.name,
	dmitry.torokhov@gmail.com, gustavoars@kernel.org,
	john.ogness@linutronix.de, linux-kbuild@vger.kernel.org,
	linux-kernel@vger.kernel.org, nathan@kernel.org,
	ndesaulniers@google.com, oleksandr@redhat.com,
	quic_eberman@quicinc.com, seanjc@google.com, tglx@linutronix.de,
	vbabka@suse.cz, x86@kernel.org,
	linux-snps-arc@lists.infradead.org,
	Vineet Gupta <vgupta@kernel.org>
Subject: Re: [PATCH] kbuild: drop support for CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3
Date: Wed, 29 Jun 2022 12:48:45 +0200	[thread overview]
Message-ID: <2842572.mvXUDI8C0e@sakura.myxoz.lan> (raw)
In-Reply-To: <20220628210407.3343118-1-ndesaulniers@google.com>

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

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