Re: Control dependency between prior load in while condition and later store?

[Daniel Jordan]

On 04/04/2018 04:35 PM, Alan Stern wrote:
> On Wed, 4 Apr 2018, Daniel Jordan wrote:
>
> > A question for memory-barriers.txt aficionados.
> >
> > Is there a control dependency between the prior load of 'a' and the
> > later store of 'c'?:
> >
> >     while (READ_ONCE(a));
> >     WRITE_ONCE(c, 1);
>
> I would say that yes, there is.
>
> > I have my doubts because memory-barriers.txt doesn't talk much about
> > loops and because of what that document says here:
> >
> >     In addition, control dependencies apply only to the then-clause and
> >     else-clause of the if-statement in question.  In particular, it does
> >     not necessarily apply to code following the if-statement:
> >
> >     	q = READ_ONCE(a);
> >     	if (q) {
> >     		WRITE_ONCE(b, 1);
> >     	} else {
> >     		WRITE_ONCE(b, 2);
> >     	}
> >     	WRITE_ONCE(c, 1);  /* BUG: No ordering against the read from 'a'. */
>
> This refers to situations where the two code paths meet up at the end
> of the "if" statement.  If they don't meet up (because one of the paths
> branches away -- especially if it branches backward) then the
> disclaimer doesn't apply, and everything following the "if" is
> dependent.

Ok, that's the part I wasn't getting: this is how the while loop changes 
the situation.

> The reason is because the compiler knows that code following the "if"
> statement will be executed unconditionally if the paths meet up, so it
> can move that code back before the "if" (provided nothing else prevents
> such motion).  But if the paths don't meet up, the compiler can't
> perform the code motion -- if it did then the program might end up
> executing something that should not have been executed!
>
> > It's not obvious to me how the then-clause/else-clause idea maps onto
> > loops, but if we think of the example at the top like this...
> >
> >     while (1) {
> >         if (!READ_ONCE(a)) {
> >             WRITE_ONCE(c, 1);
> >             break;
> >         }
> >     }
> >
> > ...then the dependent store is within the then-clause.  Viewed this way,
> > it seems there would be a control dependency between a and c.
> >
> > Is that right?
>
> Yes, except that a more accurate view of the object code would be
> something like this:
>
> Loop:	r1 = READ_ONCE(a);
> 	if (r1)
> 		goto Loop;
> 	else
> 		;	// Do nothing
> 	WRITE_ONCE(c, 1);
>
> Here you can see that one path branches backward, so everything
> following the "if" is dependent on the READ_ONCE.

That clears it up, thanks very much!