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[195.198.44.182]) by smtp.gmail.com with ESMTPSA id l2-20020a170906078200b006fe89cafc42sm7676800ejc.172.2022.06.29.03.48.33 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 29 Jun 2022 03:48:33 -0700 (PDT) From: Miko Larsson To: Masahiro Yamada , Linus Torvalds , Nick Desaulniers 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 Subject: Re: [PATCH] kbuild: drop support for CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3 Date: Wed, 29 Jun 2022 12:48:45 +0200 Message-ID: <2842572.mvXUDI8C0e@sakura.myxoz.lan> In-Reply-To: <20220628210407.3343118-1-ndesaulniers@google.com> References: <20220628210407.3343118-1-ndesaulniers@google.com> MIME-Version: 1.0 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20220629_034837_295628_B74C91FD X-CRM114-Status: GOOD ( 16.94 ) X-BeenThere: linux-snps-arc@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: Linux on Synopsys ARC Processors List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-snps-arc" Errors-To: linux-snps-arc-bounces+linux-snps-arc=archiver.kernel.org@lists.infradead.org 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 _______________________________________________ linux-snps-arc mailing list linux-snps-arc@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-snps-arc