From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932573AbYB0Xof (ORCPT ); Wed, 27 Feb 2008 18:44:35 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755154AbYB0XoU (ORCPT ); Wed, 27 Feb 2008 18:44:20 -0500 Received: from smtp1.linux-foundation.org ([207.189.120.13]:36045 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1761466AbYB0XoT (ORCPT ); Wed, 27 Feb 2008 18:44:19 -0500 Date: Wed, 27 Feb 2008 15:43:41 -0800 From: Andrew Morton To: Segher Boessenkool Cc: linux-kernel@vger.kernel.org, segher@kernel.crashing.org, Ingo Molnar , Thomas Gleixner , john stultz Subject: Re: [PATCH] Prevent the loop in timespec_add_ns() to be optimised away Message-Id: <20080227154341.824efe3c.akpm@linux-foundation.org> In-Reply-To: <6a95d77ad55b29ff2d856c9e1f7dd79682370300.1203715782.git.segher@kernel.crashing.org> References: <6a95d77ad55b29ff2d856c9e1f7dd79682370300.1203715782.git.segher@kernel.crashing.org> X-Mailer: Sylpheed version 2.2.4 (GTK+ 2.8.20; i486-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 22 Feb 2008 22:40:45 +0100 Segher Boessenkool wrote: > ...since some architectures don't support __udivdi3() (and > we don't want to use that, anyway). > > Signed-off-by: Segher Boessenkool > --- > include/linux/time.h | 4 ++++ > 1 files changed, 4 insertions(+), 0 deletions(-) > > diff --git a/include/linux/time.h b/include/linux/time.h > index 2091a19..d32ef0a 100644 > --- a/include/linux/time.h > +++ b/include/linux/time.h > @@ -174,6 +174,10 @@ static inline void timespec_add_ns(struct timespec *a, u64 ns) > { > ns += a->tv_nsec; > while(unlikely(ns >= NSEC_PER_SEC)) { > + /* The following asm() prevents the compiler from > + * optimising this loop into a modulo operation. */ > + asm("" : "+r"(ns)); > + > ns -= NSEC_PER_SEC; > a->tv_sec++; > } It's pretty sad that we need to turn this into a loop just because of the __udivdi3() thing. otoh, it's rarely occurring, and it could be that the number of times it loops is usually 1 (if it wasn't zero), so perhaps a loop is faster than a divide anyway. This code is probably too large to be inlined. I queued this patch as needed-in-2.6.25, to-be-merged-via-Thomas.