linux-kernel - Re: Compilers and RCU readers: Once more unto the breach!

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Message-ID: <20150520133037.GK6776@linux.vnet.ibm.com>
Date:	Wed, 20 May 2015 06:30:37 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	David Howells <dhowells@...hat.com>
Cc:	Will Deacon <will.deacon@....com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"c++std-parallel@...u.org" <c++std-parallel@...u.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"gcc@....gnu.org" <gcc@....gnu.org>,
	p796231 <Peter.Sewell@...cam.ac.uk>,
	"mark.batty@...cam.ac.uk" <Mark.Batty@...cam.ac.uk>,
	Peter Zijlstra <peterz@...radead.org>,
	Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	"michaelw@...ibm.com" <michaelw@...ibm.com>
Subject: Re: Compilers and RCU readers: Once more unto the breach!

On Wed, May 20, 2015 at 02:18:37PM +0100, David Howells wrote:
> Paul E. McKenney <paulmck@...ux.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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