Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755150AbbLDDEU (ORCPT ); Thu, 3 Dec 2015 22:04:20 -0500 Received: from aserp1040.oracle.com ([141.146.126.69]:16989 "EHLO aserp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754894AbbLDDET (ORCPT ); Thu, 3 Dec 2015 22:04:19 -0500 From: Sasha Levin To: torvalds@linux-foundation.org Cc: mingo@kernel.org, linux-kernel@vger.kernel.org, Sasha Levin Subject: [PATCH v2] bitops.h: correctly handle rol32 with 0 byte shift Date: Thu, 3 Dec 2015 22:04:01 -0500 Message-Id: <1449198241-20654-1-git-send-email-sasha.levin@oracle.com> X-Mailer: git-send-email 2.5.0 X-Source-IP: userv0022.oracle.com [156.151.31.74] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1818 Lines: 55 ROL on a 32 bit integer with a shift of 32 or more is undefined and the result is arch-dependent. Avoid this by handling the trivial case of roling by 0 correctly. The trivial solution of checking if shift is 0 breaks gcc's detection of this code as a ROL instruction, which is unacceptable. This bug was reported and fixed in GCC (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57157): The standard rotate idiom, (x << n) | (x >> (32 - n)) is recognized by gcc (for concreteness, I discuss only the case that x is an uint32_t here). However, this is portable C only for n in the range 0 < n < 32. For n == 0, we get x >> 32 which gives undefined behaviour according to the C standard (6.5.7, Bitwise shift operators). To portably support n == 0, one has to write the rotate as something like (x << n) | (x >> ((-n) & 31)) And this is apparently not recognized by gcc. Note that this is broken on older GCCs and will result in slower ROL. Acked-by: Linus Torvalds Signed-off-by: Sasha Levin --- include/linux/bitops.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/include/linux/bitops.h b/include/linux/bitops.h index 2b8ed12..defeaac 100644 --- a/include/linux/bitops.h +++ b/include/linux/bitops.h @@ -107,7 +107,7 @@ static inline __u64 ror64(__u64 word, unsigned int shift) */ static inline __u32 rol32(__u32 word, unsigned int shift) { - return (word << shift) | (word >> (32 - shift)); + return (word << shift) | (word >> ((-shift) & 31)); } /** -- 2.5.0 -- 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/