[cpp-threads] Alternatives to SC

[Alexander Terekhov]

On 1/16/07, Raul Silvera <rauls at ca.ibm.com> wrote:
[...]
> > > Well, lwarx-stwcx-on-P3 version aside for a moment, at least one
> > > "cumulative" barrier is needed on P2/P3 (Book II shows two) I think.
>
> On this particular example, cumulativity is only needed on P2 (lwsync

P2: if (x == 1) lwsync(), y = 2;

Load x is in "A" and store y = 2 is in "B" (A performs before B...
note that {lw}sync() is NOT needed for only that ordering because it
is provided by control dependency).

> provides a cumulative ordering). Basically you need P1's x=1 store to be
> ordered wrt P3's y=2.

You mean that we need P1's x=1 store with respect to P3 to be ordered
before P3's load x after P3 sees P2's y=2 store. Right?

So that P1's x=1 store will be in "A" and P3's load x will be in "B"...

(quoting and editing the definition of "cumulative" barrier from Book II)

"The ordering done by a memory barrier is said to be "cumulative" if
it also orders storage accesses that are performed by processors and
mechanisms other than P2, as follows.

— A includes all applicable storage accesses by any such processor or
mechanism that have been performed with respect to P2 before the
memory barrier is created. (A includes P1's store x=1 with respect to
any other processor... i.e. P3 including)

— B includes all applicable storage accesses by any such processor or
mechanism that are performed after a Load instruction executed by that
processor or mechanism has returned the value stored by a store that
is in B. (B includes P3's load x performed after P3's load y has
returned value 2 stored by P2.)

Correct?

> Cumulativity is not needed on P3: the order of the

Okay, agreed.

But given that for "[s]tores to storage that is Memory Coherence
Required and is neither Write Through Required nor Caching Inhibited"
eieio is also cumulative...

P1: x = 1;
P2: if (x == 1) eieio(), y = 2;
P3: if (y == 2) isync(), assert(x == 1);

Would also do it, I suppose. ("Storage that is Write Through Required
or Caching Inhibited is not intended to be used for general-purpose
programming.")

Now, when are we going to have totally naked cumulative fence (a beast
without extra features of eieio, {lw}sync, or whatever) for
general-purpose programming on Power? ;-)

regards,
alexander.