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From: David Howells <dhowells@redhat.com>
In-Reply-To: <20150520121522.GH6776@linux.vnet.ibm.com>
References: <20150520121522.GH6776@linux.vnet.ibm.com> <20150520005510.GA23559@linux.vnet.ibm.com> <CA+55aFy_8V-rbE9FQMHx6tXjj8HHKZuKSJvnRPVYvpk46EQA1g@mail.gmail.com> <CA+55aFxOtcB8AYCpLQBGSXK=8_Vh4uDs5HEpzGpPy+hgz542ag@mail.gmail.com> <20150520024148.GD6776@linux.vnet.ibm.com> <20150520114745.GC11498@arm.com>
To: paulmck@linux.vnet.ibm.com
Cc: dhowells@redhat.com, Will Deacon <will.deacon@arm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "c++std-parallel@accu.org" <c++std-parallel@accu.org>,
        "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
        "gcc@gcc.gnu.org" <gcc@gcc.gnu.org>,
        p796231 <Peter.Sewell@cl.cam.ac.uk>,
        "mark.batty@cl.cam.ac.uk" <Mark.Batty@cl.cam.ac.uk>,
        Peter Zijlstra <peterz@infradead.org>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@arm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        "michaelw@ca.ibm.com" <michaelw@ca.ibm.com>
Subject: Re: Compilers and RCU readers: Once more unto the breach!
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Date: Wed, 20 May 2015 14:18:37 +0100
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Paul E. McKenney <paulmck@linux.vnet.ibm.com> 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)?

David
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