Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754750Ab0BHJ2x (ORCPT ); Mon, 8 Feb 2010 04:28:53 -0500 Received: from mail-fx0-f211.google.com ([209.85.220.211]:46658 "EHLO mail-fx0-f211.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753965Ab0BHJ2v (ORCPT ); Mon, 8 Feb 2010 04:28:51 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=googlemail.com; s=gamma; h=date:from:to:cc:subject:message-id:mail-followup-to:references :mime-version:content-type:content-disposition:in-reply-to :user-agent; b=FBYKkqqXRaKg2rWc5sV778+Q0yhheDt10cdabg+W7e38bO1DpIRWxZIQS5tfhbkVp2 o6E62EyJLhkEY73PNdJ60aOB3o8FmHlx1ELRjWHMAvg+uoqxNEn6n4la90PsgeLmV1f8 Knli+UzYywUaSl89/JhmOfoXaW4Dq/Am8xOG4= Date: Mon, 8 Feb 2010 10:28:45 +0100 From: Borislav Petkov To: "H. Peter Anvin" Cc: Borislav Petkov , Peter Zijlstra , Andrew Morton , Wu Fengguang , LKML , Jamie Lokier , Roland Dreier , Al Viro , "linux-fsdevel@vger.kernel.org" , Ingo Molnar , Brian Gerst Subject: Re: [PATCH 2/5] bitops: compile time optimization for hweight_long(CONSTANT) Message-ID: <20100208092845.GB12618@a1.tnic> Mail-Followup-To: Borislav Petkov , "H. Peter Anvin" , Borislav Petkov , Peter Zijlstra , Andrew Morton , Wu Fengguang , LKML , Jamie Lokier , Roland Dreier , Al Viro , "linux-fsdevel@vger.kernel.org" , Ingo Molnar , Brian Gerst References: <1265222875.24455.1020.camel@laptop> <4B69D362.10608@zytor.com> <20100204151050.GC32711@aftab> <1265296432.22001.18.camel@laptop> <20100204155419.GD32711@aftab> <1265299457.22001.72.camel@laptop> <20100205121139.GA9044@aftab> <4B6C93A2.1090302@zytor.com> <20100206093659.GA28326@aftab> <4B6E1DA3.50204@zytor.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <4B6E1DA3.50204@zytor.com> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1997 Lines: 53 On Sat, Feb 06, 2010 at 05:55:47PM -0800, H. Peter Anvin wrote: > > Well, the example Brian pointed me to - __mutex_fastpath_lock - lists > > the full set of clobbered registers. Please elaborate on the assembly > > wrapper for the function, wouldn't I need to list all the clobbered > > registers there too or am I missing something? > > > > The notion there would be that you do push/pop in the assembly wrapper. Oh yes, something similar to SAVE/RESTORE_ALL in could work. Good idea! > >> d) On the other hand, you do *not* need a "memory" clobber. > > > > Right, in this case we have all non-barrier like inlines so no memory > > clobber, according to the comment above alternative() macro. > > OK, I'm missing something here. > > A few more notions: > > a. This is exactly the kind of code where you don't want to put > "volatile" on your asm statement, because it's a pure compute. > > b. It is really rather pointless to go through the whole alternatives > work if you are then going to put it inside a function which isn't an > inline ... Well, in the second version I did replace a 'call _hweightXX' with the actual popcnt opcode so the alternatives is only needed to do the replacement during boot. We might just as well do if (X86_FEATURE_POPCNT) __hw_popcnt() else __software_hweight() The only advantage of the alternatives is that it would save us the if-else test above each time we do cpumask_weight. However, the if-else approach is much more readable and obviates the need for all that macro magic and taking special care of calling c function from within asm. And since we do not call cpumask_weight all that often I'll honestly opt for alternative-less solution... Hmm... Thanks, Boris. -- 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/