Date: Wed, 20 May 2015 06:30:37 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: David Howells <dhowells@redhat.com>
Cc: 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!
Message-ID: <20150520133037.GK6776@linux.vnet.ibm.com>
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On Wed, May 20, 2015 at 02:18:37PM +0100, David Howells wrote:
> 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)?

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

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