From: "H. Peter Anvin" Subject: Re: linux/bitops.h Date: Wed, 04 May 2016 18:27:24 -0700 Message-ID: References: <1462170413-7164-1-git-send-email-tytso@mit.edu> <1462170413-7164-2-git-send-email-tytso@mit.edu> <20160504174901.GC3901@thunk.org> <20160504190723.GD3901@thunk.org> <572A6CDD.10503@av8n.com> <572A724C.6010704@av8n.com> <20160504230638.GO13997@two.firstfloor.org> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Cc: John Denker , "Theodore Ts'o" , linux-kernel@vger.kernel.org, Stephan Mueller , Herbert Xu , Sandy Harris , cryptography@lakedaemon.net, linux-crypto@vger.kernel.org To: noloader@gmail.com, Andi Kleen Return-path: Received: from terminus.zytor.com ([198.137.202.10]:34300 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753875AbcEEB17 (ORCPT ); Wed, 4 May 2016 21:27:59 -0400 In-Reply-To: Sender: linux-crypto-owner@vger.kernel.org List-ID: On May 4, 2016 6:20:32 PM PDT, Jeffrey Walton wrote: >On Wed, May 4, 2016 at 7:06 PM, Andi Kleen wrote: >> On Wed, May 04, 2016 at 03:06:04PM -0700, John Denker wrote: >>> On 05/04/2016 02:56 PM, H. Peter Anvin wrote: >>> >> Beware that shifting by an amount >= the number of bits in the >>> >> word remains Undefined Behavior. >>> >>> > This construct has been supported as a rotate since at least gcc2. >>> >>> How then should we understand the story told in commit d7e35dfa? >>> Is the story wrong? >> >> I don't think Linux runs on a system where it would make a difference >> (like a VAX), and also gcc always converts it before it could. >> Even UBSan should not complain because it runs after the conversion >> to ROTATE. >> >From what I understand, its a limitation in the barrel shifter and the >way the shift bits are handled. > >Linux runs on a great number of devices, so its conceivable (likely?) >a low-cost board would have hardware limitations that not found in >modern desktops and servers or VAX. > >Jeff This is a compiler feature, though. And if icc or clang don't support the idiom they would never have compiled a working kernel. -- Sent from my Android device with K-9 Mail. Please excuse brevity and formatting.