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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
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