Hi,
On Wed, 20 May 2015, Paul E. McKenney wrote:
> > > I'm not sure... you'd require the compiler to perform static analysis of
> > > loops to determine the state of the machine when they exit (if they exit!)
> > > in order to show whether or not a dependency is carried to subsequent
> > > operations. If it can't prove otherwise, it would have to assume that a
> > > dependency *is* carried, and it's not clear to me how it would use this
> > > information to restrict any subsequent dependency removing optimisations.
> >
> > It'd just convert consume to acquire.
>
> It should not need to, actually.
[with GCC hat, and having only lightly read your document]
Then you need to provide language or at least informal reasons why the
compiler is allowed to not do that. Without that a compiler would have to
be conservative, if it can't _prove_ that a dependency chain is stopped,
then it has to assume it hasn't.
For instance I can't really make out easily what your document says about
the following simple situation (well, actually I have difficulties to
differ between what you're proposing as the good-new model of this all,
and what you're merely describing as different current states of affair):
char * fancy_assign (char *in) { return in; }
...
char *x, *y;
x = atomic_load_explicit(p, memory_order_consume);
y = fancy_assign (x);
atomic_store_explicit(q, y, memory_order_relaxed);
So, is there, or is there not a dependency carried from x to y in your
proposed model (and which rule in your document states so)? Clearly,
without any other language the compiler would have to assume that there is
(because the equivalent 'y = x' assignment would carry the dependency).
If it has to assume this, then the whole model is not going to work very
well, as usual with models that assume a certain less-optimal fact
("carries-dep" is less optimal for code generation purposes that
"not-carries-dep") unless very specific circumstances say it can be
ignored.
Ciao,
Michael.
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