Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932408Ab0BDPOR (ORCPT ); Thu, 4 Feb 2010 10:14:17 -0500 Received: from casper.infradead.org ([85.118.1.10]:45488 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756515Ab0BDPOQ (ORCPT ); Thu, 4 Feb 2010 10:14:16 -0500 Subject: Re: [PATCH 2/5] bitops: compile time optimization for hweight_long(CONSTANT) From: Peter Zijlstra To: Borislav Petkov Cc: "H. Peter Anvin" , Andrew Morton , Wu Fengguang , LKML , Jamie Lokier , Roland Dreier , Al Viro , "linux-fsdevel@vger.kernel.org" , Ingo Molnar In-Reply-To: <20100204151050.GC32711@aftab> 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> Content-Type: text/plain; charset="UTF-8" Date: Thu, 04 Feb 2010 16:13:52 +0100 Message-ID: <1265296432.22001.18.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.28.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2331 Lines: 59 On Thu, 2010-02-04 at 16:10 +0100, Borislav Petkov wrote: > On Wed, Feb 03, 2010 at 11:49:54AM -0800, H. Peter Anvin wrote: > > On 02/03/2010 10:47 AM, Peter Zijlstra wrote: > > > On Wed, 2010-02-03 at 19:14 +0100, Borislav Petkov wrote: > > > > > >> alternative("call hweightXX", "popcnt", X86_FEATURE_POPCNT) > > > > > > Make sure to apply a 0xff bitmask to the popcnt r16 call for hweight8(), > > > and hweight64() needs a bit of magic for 32bit, but yes, something like > > > that ought to work nicely. > > > > > > > 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. > +int arch_hweight_long(unsigned long w) > +{ > + if (sizeof(w) == 4) { > + asm volatile("movl %[w], %%eax" :: [w] "r" (w)); > + alternative("call _hweight32", > + "call _popcnt32", > + X86_FEATURE_POPCNT); > + asm volatile("" : "=a" (w)); > + > + } else { > + asm volatile("movq %[w], %%rax" :: [w] "r" (w)); > + alternative("call _hweight64", > + "call _popcnt64", > + X86_FEATURE_POPCNT); > + asm volatile("" : "=a" (w)); > + } > + return w; > +} hweight_long() isn't an arch primitive, only __arch_hweight{8,16,32,64} are. -- 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/