Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1162540AbaDCIB0 (ORCPT ); Thu, 3 Apr 2014 04:01:26 -0400 Received: from mail-ig0-f173.google.com ([209.85.213.173]:62500 "EHLO mail-ig0-f173.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1162421AbaDCIBY convert rfc822-to-8bit (ORCPT ); Thu, 3 Apr 2014 04:01:24 -0400 MIME-Version: 1.0 In-Reply-To: <20140402181354.GB13479@quack.suse.cz> References: <1396451946-13924-1-git-send-email-nasa4836@gmail.com> <20140402181354.GB13479@quack.suse.cz> From: Zhan Jianyu Date: Thu, 3 Apr 2014 16:00:44 +0800 Message-ID: Subject: Re: [PATCH] blkdev: use an efficient way to check merge flags To: Jan Kara Cc: axboe@kernel.dk, hch@lst.de, viro@zeniv.linux.org.uk, kmo@daterainc.com, namjae.jeon@samsung.com, LKML Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 3, 2014 at 2:13 AM, Jan Kara wrote: > OK, but have you checked the generated code is actually any better? This > is something I'd expect a compiler might be able to optimize anyway. And the > original code looks more readable to me. Hi, Jan, I've disassemble the code on my x86_64 box (it's inline though, I just look at its call site), and found that this patch DOES make it more efficient. Orig asm snippt with patch asm snippt ============ ================ mov %edx,%ecx mov %rdx,%r9 xor %r8d,%ecx xor %r8d,%r8d test $0x80,%cl and $0x380,%r9d jne 14c5 test $0x380,%ecx and $0x3,%ch sete %r8b jne 14c5 cmp %r8,%r9 je 14b5 This saves a branch. Furthermore, I found that gcc is smart enough to try to optimize the code, so if we do like this, it will generate the most optimal and smallest code : static inline bool blk_check_merge_flags(unsigned int flags1, ¦unsigned int flags2) { return ((flags1 ^ flags2) & (REQ_DISCARD | REQ_SECURE | REQ_WRITE_SAME)) == 0; } this gives out : mov %edx,%r8d xor %ecx,%r8d and $0x380,%r8d jne 14a5 But yes, it compromises readibility. Regards, Jianyu Zhan -- 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/