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

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Date:	Wed, 20 May 2015 07:03:43 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Ramana Radhakrishnan <ramana.radhakrishnan@....com>
Cc:	David Howells <dhowells@...hat.com>,
	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>,
	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 02:44:30PM +0100, Ramana Radhakrishnan wrote:
> 
> 
> On 20/05/15 14:37, David Howells wrote:
> >Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> >
> >>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;
> >
> >Yeah.  I presume it has to maintain the ordering, though.
> 
> The scheduler for e.g. is free to reorder if it can prove there is
> no dependence (or indeed side-effects for y) between insns produced
> for y and `x = z'.

So for example, if y is independent of z, the compiler can do the
following:

	char *x;

	x = z;
	y;

But the dependency ordering is still maintained from z to x, so this
is not a problem.

Or am I missing something subtle here?

							Thanx, Paul

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