From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1423550AbcFHUmP (ORCPT ); Wed, 8 Jun 2016 16:42:15 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:35975 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932233AbcFHUmO (ORCPT ); Wed, 8 Jun 2016 16:42:14 -0400 Date: Wed, 8 Jun 2016 13:42:12 -0700 From: Andrew Morton To: Martin Schwidefsky Cc: linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, Benjamin Herrenschmidt , "David S. Miller" Subject: Re: [PATCH] bitmap: bitmap_equal memcmp optimization Message-Id: <20160608134212.9f868a8dd494b88e402d5d31@linux-foundation.org> In-Reply-To: <201606070837.u578YJKt047781@mx0a-001b2d01.pphosted.com> References: <1465288661-3273-1-git-send-email-schwidefsky@de.ibm.com> <201606070837.u578YJKt047781@mx0a-001b2d01.pphosted.com> X-Mailer: Sylpheed 3.4.1 (GTK+ 2.24.23; x86_64-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 Tue, 7 Jun 2016 10:37:41 +0200 Martin Schwidefsky wrote: > The bitmap_equal function has optimized code for small bitmaps with less > than BITS_PER_LONG bits. For larger bitmaps the out-of-line function > __bitmap_equal is called. > > For a constant number of bits divisible by BITS_PER_LONG the memcmp > function can be used. For s390 gcc knows how to optimize this function, > memcmp calls with up to 256 bytes / 2048 bits are translated into a > single instruction. Patch looks simple enough, although it would benefit from a comment explaining what's special about s390. > --- a/include/linux/bitmap.h > +++ b/include/linux/bitmap.h > @@ -267,6 +267,10 @@ static inline int bitmap_equal(const unsigned long *src1, > { > if (small_const_nbits(nbits)) > return ! ((*src1 ^ *src2) & BITMAP_LAST_WORD_MASK(nbits)); > +#ifdef CONFIG_S390 > + else if (__builtin_constant_p(nbits) && (nbits % BITS_PER_LONG) == 0) > + return !memcmp(src1, src2, nbits / 8); > +#endif > else > return __bitmap_equal(src1, src2, nbits); > } Those elses are a bit daffy. This: --- a/include/linux/bitmap.h~bitmap-bitmap_equal-memcmp-optimization-fix +++ a/include/linux/bitmap.h @@ -266,13 +266,12 @@ static inline int bitmap_equal(const uns const unsigned long *src2, unsigned int nbits) { if (small_const_nbits(nbits)) - return ! ((*src1 ^ *src2) & BITMAP_LAST_WORD_MASK(nbits)); + return !((*src1 ^ *src2) & BITMAP_LAST_WORD_MASK(nbits)); #ifdef CONFIG_S390 - else if (__builtin_constant_p(nbits) && (nbits % BITS_PER_LONG) == 0) + if (__builtin_constant_p(nbits) && (nbits % BITS_PER_LONG) == 0) return !memcmp(src1, src2, nbits / 8); #endif - else - return __bitmap_equal(src1, src2, nbits); + return __bitmap_equal(src1, src2, nbits); } static inline int bitmap_intersects(const unsigned long *src1, _