From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: Compilers and RCU readers: Once more unto the breach! Date: Wed, 20 May 2015 06:30:37 -0700 Message-ID: <20150520133037.GK6776@linux.vnet.ibm.com> References: <20150520121522.GH6776@linux.vnet.ibm.com> <20150520005510.GA23559@linux.vnet.ibm.com> <20150520024148.GD6776@linux.vnet.ibm.com> <20150520114745.GC11498@arm.com> <31547.1432127917@warthog.procyon.org.uk> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from e38.co.us.ibm.com ([32.97.110.159]:45375 "EHLO e38.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752222AbbETNan (ORCPT ); Wed, 20 May 2015 09:30:43 -0400 Received: from /spool/local by e38.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 20 May 2015 07:30:42 -0600 Content-Disposition: inline In-Reply-To: <31547.1432127917@warthog.procyon.org.uk> Sender: linux-arch-owner@vger.kernel.org List-ID: To: David Howells Cc: Will Deacon , Linus Torvalds , Linux Kernel Mailing List , "c++std-parallel@accu.org" , "linux-arch@vger.kernel.org" , "gcc@gcc.gnu.org" , p796231 , "mark.batty@cl.cam.ac.uk" , Peter Zijlstra , Ramana Radhakrishnan , Andrew Morton , Ingo Molnar , "michaelw@ca.ibm.com" On Wed, May 20, 2015 at 02:18:37PM +0100, David Howells wrote: > Paul E. McKenney wrote: > > > > Additionally, what about the following code? > > > > > > char *x = y ? z : z; > > > > > > Does that extend a dependency chain from z to x? If so, I can imagine a > > > CPU breaking that in practice. > > > > I am not seeing this. I would expect the compiler to optimize to > > something like this: > > > > char *x = z; > > Why? What if y has a potential side-effect (say it makes a function call)? I was thinking of "y" as a simple variable, but if it is something more complex, then the compiler could do this, right? char *x; y; x = z; Thanx, Paul