From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760243AbYD2PvA (ORCPT ); Tue, 29 Apr 2008 11:51:00 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1758207AbYD2Puh (ORCPT ); Tue, 29 Apr 2008 11:50:37 -0400 Received: from smtp1.linux-foundation.org ([140.211.169.13]:49502 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758136AbYD2Pug (ORCPT ); Tue, 29 Apr 2008 11:50:36 -0400 Date: Tue, 29 Apr 2008 08:50:03 -0700 (PDT) From: Linus Torvalds To: Harvey Harrison cc: Andrew Morton , LKML Subject: Re: [PATCH] lib: bitrev.c micro-optimization In-Reply-To: <1209446696.24729.18.camel@brick> Message-ID: References: <1209446696.24729.18.camel@brick> User-Agent: Alpine 1.10 (LFD 962 2008-03-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Not very good. Why do it in memory, when doing it in a register is much clearer? This generates one byte longer code for me, but avoids the partial memory stalls and looks more obvious. It is also portable, unlike your version (you need to use a union to avoid aliasing issues - admittedly we disable the gcc alias code anyway because of other kernel code, but still). Linus --- lib/bitrev.c | 10 ++++------ 1 files changed, 4 insertions(+), 6 deletions(-) diff --git a/lib/bitrev.c b/lib/bitrev.c index 989aff7..e0cb44e 100644 --- a/lib/bitrev.c +++ b/lib/bitrev.c @@ -42,17 +42,15 @@ const u8 byte_rev_table[256] = { }; EXPORT_SYMBOL_GPL(byte_rev_table); -static __always_inline u16 bitrev16(u16 x) -{ - return (bitrev8(x & 0xff) << 8) | bitrev8(x >> 8); -} - /** * bitrev32 - reverse the order of bits in a u32 value * @x: value to be bit-reversed */ u32 bitrev32(u32 x) { - return (bitrev16(x & 0xffff) << 16) | bitrev16(x >> 16); + return (bitrev8(x) << 24) | + (bitrev8(x >> 8) << 16) | + (bitrev8(x >> 16) << 8) | + (bitrev8(x >> 24)); } EXPORT_SYMBOL(bitrev32);