Re: Does Itanium permit speculative stores?

[Paul E. McKenney]

On Tue, Nov 12, 2013 at 07:26:17PM +0100, Peter Zijlstra wrote:
> On Tue, Nov 12, 2013 at 06:00:26PM +0000, Luck, Tony wrote:
> > > Does Itanium permit speculative stores?  For example, on Itanium what are
> > > the permitted outcomes of the following litmus test, where both x and y
> > > are initially zero?
> > 
> > We have a complier visible speculative read via the "ld.s" and "chk" instructions. But
> > there is no speculative write ("st.s") instruction.  I think you are asking "can out of order
> > writes become visible in this scenario?"
> > 
> > 	CPU 0				CPU 1
> > 
> > 	r1 = ACCESS_ONCE(x);		r2 = ACCESS_ONCE(y);
> > 	if (r1)				if (r2)
> > 		ACCESS_ONCE(y) = 1;		ACCESS_ONCE(x) = 1;
> > 
> > > In particular, is the outcome (r1 == 1 && r2 == 1) possible on Itanium
> > > given this litmus test?
> > 
> > The "ACCESS_ONCE" macro casts to volatile - which will make gcc generate
> > ordered "ld.acq" and "st.rel" instructions for your code snippets. So I think
> > you should be fine.
> 
> Cute that volatile generates barrier instructions.
> 
> But no; I think Paul accidentally formulated his question in C (since we
> all speak C) but meant to ask an architectural question.

I got both answers, so I am good.  ;-)

> So the point we're having a discussion on is if any architecture has
> visible speculative STORES and if there's an architecture that doesn't
> have control dependencies.
> 
> On the visible speculative STORES; can, if in the above example we have
> regular loads/stores:
> 
>   LOAD r1, x			LOAD r2, y
>   IF (r1)			IF (r2)
> 	STORE y, 1			STORE x, 1
> 
> we observe: r1==1 && r2==1
> 
> In order for that to be true; we must be able to observe the stores
> before the loads are complete -- and therefore before the branches are a
> certainty.
> 
> Typically if an architecture speculates on branches the result doesn't
> become visible/committed until the branch is a certainty -- ie. linear
> branch history.
> 
> Alternatively:
> 
> 	x:=0
> 
> 	IF (cond)			LOAD r1,x
> 		STORE x,1
> 	STORE x,2
> 
> Can r1 ever be 1 if we know 'cond' will never be true (runtime
> constraint, not compile time so the branch cannot be omitted).

I would have been OK mandating use of ACCESS_ONCE() to prevent speculative
stores, but it is even nicer that it is not necessary.

							Thanx, Paul

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