From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751087AbbDLFrz (ORCPT ); Sun, 12 Apr 2015 01:47:55 -0400 Received: from mail-wg0-f52.google.com ([74.125.82.52]:36089 "EHLO mail-wg0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750800AbbDLFrw (ORCPT ); Sun, 12 Apr 2015 01:47:52 -0400 Date: Sun, 12 Apr 2015 07:47:47 +0200 From: Ingo Molnar To: Linus Torvalds Cc: Thomas Gleixner , Jakub Jelinek , Denys Vlasenko , Borislav Petkov , Tim Chen , Andy Lutomirski , Jason Low , Brian Gerst , Aswin Chandramouleeswaran , "Paul E. McKenney" , Davidlohr Bueso , Peter Zijlstra , "H. Peter Anvin" , LKML , Peter Zijlstra Subject: Re: [PATCH] x86: Turn off GCC branch probability heuristics Message-ID: <20150412054747.GA9062@gmail.com> References: <20150410090051.GA28549@gmail.com> <20150410091252.GA27630@gmail.com> <20150410092152.GA21332@gmail.com> <20150410111427.GA30477@gmail.com> <20150410112748.GB30477@gmail.com> <20150410120846.GA17101@gmail.com> <20150411092021.GA9478@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Linus Torvalds wrote: > On Sat, Apr 11, 2015 at 11:57 AM, Thomas Gleixner wrote: > > > > I thinks its just the no-guess one: > > > > text data dec patch reduction > > 7563475 1781048 10302987 > > 7192973 1780024 9931461 no-guess -4.8% > > 7354819 1781048 958464 align-1 -2.7% > > 7192973 1780024 9931461 no-guess + align-1 -4.8% > > Yeah, a 5% code expansion is a big deal. Sadly, it looks like > 'no-guess' also disables our explicit likely/unlikely handling. So I spent some time trying to get as much code size reduction as possible via GCC optimization options, and the total savings possible are 10.1%: text data bss dec filename 12566391 1617840 1089536 15273767 vmlinux.vanilla 11416805 1617840 1089536 14124181 vmlinux.combo 10532552 1596080 1089536 13218168 vmlinux.Os (combo patch attached below.) The -Os savings are 19% total - but as you mentioned before, it's sometimes achieved through unacceptable techniques. Unfortunately I found no other GCC options to achieve what -Os does - the missing 9% can purely be achieved via -Os, with no cherry-picking possible. The other, smaller savings are: + # Reduces vmlinux size by 0.25%: + KBUILD_CFLAGS += -fno-caller-saves + + # Reduces vmlinux size by 1.10%: + KBUILD_CFLAGS += -fno-inline-small-functions + + # Reduces vmlinux size by about 0.95%: + KBUILD_CFLAGS += -fno-tree-ch (each of them has to be double checked to make sure it leads to nothing silly and unacceptable - I just blindly tried to find GCC options that impacted kernel code size.) Thanks, Ingo --- arch/x86/Makefile | 24 ++++++++++++++++++++++++ 1 file changed, 24 insertions(+) diff --git a/arch/x86/Makefile b/arch/x86/Makefile index 5ba2d9ce82dc..999e94685d12 100644 --- a/arch/x86/Makefile +++ b/arch/x86/Makefile @@ -77,10 +77,34 @@ else KBUILD_AFLAGS += -m64 KBUILD_CFLAGS += -m64 + # Pack jump targets tightly, don't align them to the default 16 bytes: + KBUILD_CFLAGS += -falign-jumps=1 + + # Pack functions tightly as well: + KBUILD_CFLAGS += -falign-functions=1 + + # Pack loops tightly as well: + KBUILD_CFLAGS += -falign-loops=1 + # Don't autogenerate traditional x87 instructions KBUILD_CFLAGS += $(call cc-option,-mno-80387) KBUILD_CFLAGS += $(call cc-option,-mno-fp-ret-in-387) + # + # Don't guess branch probabilities, follow the code and unlikely()/likely() hints, + # which reduces vmlinux size by about 5.4%: + # + KBUILD_CFLAGS += -fno-guess-branch-probability + + # Reduces vmlinux size by 0.25%: + KBUILD_CFLAGS += -fno-caller-saves + + # Reduces vmlinux size by 1.10%: + KBUILD_CFLAGS += -fno-inline-small-functions + + # Reduces vmlinux size by about 0.95%: + KBUILD_CFLAGS += -fno-tree-ch + # Use -mpreferred-stack-boundary=3 if supported. KBUILD_CFLAGS += $(call cc-option,-mpreferred-stack-boundary=3)