Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1030914AbbKDVFE (ORCPT ); Wed, 4 Nov 2015 16:05:04 -0500 Received: from mail-qk0-f177.google.com ([209.85.220.177]:36227 "EHLO mail-qk0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1030856AbbKDVFA (ORCPT ); Wed, 4 Nov 2015 16:05:00 -0500 Date: Wed, 4 Nov 2015 16:04:57 -0500 (EST) From: Nicolas Pitre To: Arnd Bergmann cc: kbuild test robot , kbuild-all@01.org, Alexey Brodkin , =?ISO-8859-15?Q?M=E5ns_Rullg=E5rd?= , rmk+kernel@arm.linux.org.uk, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/5] do_div(): generic optimization for constant divisor on 32-bit machines In-Reply-To: <21544300.LSjO3j3Xy7@wuerfel> Message-ID: References: <201511031333.Yk9ngQZJ%fengguang.wu@intel.com> <21544300.LSjO3j3Xy7@wuerfel> User-Agent: Alpine 2.20 (LFD 67 2015-01-07) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1608 Lines: 44 On Tue, 3 Nov 2015, Arnd Bergmann wrote: > On Tuesday 03 November 2015 13:32:17 kbuild test robot wrote: > > > > net/can/bcm.c: In function 'bcm_proc_show': > > >> net/can/bcm.c:223:1: warning: the frame size of 1156 bytes is larger than 1024 bytes [-Wframe-larger-than=] > > } > > Interesting, that is a lot of stack for a function that only has a couple > of local variables: > > #define IFNAMSIZ 16 > char ifname[IFNAMSIZ]; > struct sock *sk = (struct sock *)m->private; > struct bcm_sock *bo = bcm_sk(sk); > struct bcm_op *op; > > > This is a parisc-allyesconfig kernel, so I assume that CONFIG_PROFILE_ALL_BRANCHES > is on, which instruments every 'if' in the kernel. If that causes problems, > we could decide to disable the do_div optimization whenever CONFIG_PROFILE_ALL_BRANCHES > is enabled. I have an ARM allyesconfig build here where that function needs a frame of 88 bytes only. And that is with my do_div optimization applied. With the do_div optimization turned off, the stack frame is still 88 bytes. Turning on CONFIG_PROFILE_ALL_BRANCHES makes the frame size to grow to 96 bytes. Keeping CONFIG_PROFILE_ALL_BRANCHES=y and activating the do_div optimization again, and the function frame size goes back to 88 bytes. So I wonder what parisc gcc could be doing with this code. Nicolas -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/