From mboxrd@z Thu Jan 1 00:00:00 1970 From: "H. Peter Anvin" Subject: Re: [PATCH 2/5] bitops: compile time optimization for hweight_long(CONSTANT) Date: Thu, 04 Feb 2010 07:16:14 -0800 Message-ID: <4B6AE4BE.70600@zytor.com> References: <20100130094515.475881280@intel.com> <20100130094957.692671259@intel.com> <20100201124825.cc024f2a.akpm@linux-foundation.org> <20100203133951.GA24357@localhost> <20100203070825.e36b3932.akpm@linux-foundation.org> <1265210157.24455.646.camel@laptop> <20100203074251.e2caa3f3.akpm@linux-foundation.org> <20100203181425.GB1367@aftab> <1265222875.24455.1020.camel@laptop> <4B69D362.10608@zytor.com> <20100204151050.GC32711@aftab> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Cc: Peter Zijlstra , Andrew Morton , Wu Fengguang , LKML , Jamie Lokier , Roland Dreier , Al Viro , "linux-fsdevel@vger.kernel.org" , Ingo Molnar To: Borislav Petkov Return-path: Received: from terminus.zytor.com ([198.137.202.10]:43448 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932346Ab0BDPVo (ORCPT ); Thu, 4 Feb 2010 10:21:44 -0500 In-Reply-To: <20100204151050.GC32711@aftab> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On 02/04/2010 07:10 AM, Borislav Petkov wrote: >> >> Arguably the "best" option is to have the alternative being a jump to an >> out-of-line stub which does the necessary parameter marshalling before >> calling a stub. This technique is already used in a few other places. > > Ok, here's a first alpha prototype and completely untested. The asm > output looks ok though. I've added separate 32-bit and 64-bit helpers in > order to dispense with the if-else tests. The hw-popcnt versions are the > opcodes for "popcnt %eax, %eax" and "popcnt %rax, %rax", respectively, > so %rAX has to be preloaded with the bitmask and the computed value > has to be retrieved from there afterwards. And yes, it looks not that > elegant so I'm open for suggestions. > > The good thing is, this should work on any toolchain since we don't rely > on the compiler to know about popcnt and we're protected by CPUID flag > so that the hw-popcnt version is used only on processors which support > it. > > Please take a good look and let me know what do you guys think. > I think you misunderstood me. The idea was to use the alternatives mechanism to put the popcnt instruction inline instead of the call. -hpa -- H. Peter Anvin, Intel Open Source Technology Center I work for Intel. I don't speak on their behalf.