On Mon, Feb 24, 2014 at 05:55:50PM +0100, Michael Matz wrote:
> Hi,
>
> On Mon, 24 Feb 2014, Linus Torvalds wrote:
>
> > > To me that reads like
> > >
> > > int i;
> > > int *q = &i;
> > > int **p = &q;
> > >
> > > atomic_XXX (p, CONSUME);
> > >
> > > orders against accesses '*p', '**p', '*q' and 'i'. Thus it seems they
> > > want to say that it orders against aliased storage - but then go further
> > > and include "indirectly through a chain of pointers"?! Thus an
> > > atomic read of a int * orders against any 'int' memory operation but
> > > not against 'float' memory operations?
> >
> > No, it's not about type at all, and the "chain of pointers" can be
> > much more complex than that, since the "int *" can point to within an
> > object that contains other things than just that "int" (the "int" can
> > be part of a structure that then has pointers to other structures
> > etc).
>
> So, let me try to poke holes into your definition or increase my
> understanding :) . You said "chain of pointers"(dereferences I assume),
> e.g. if p is result of consume load, then access to
> p->here->there->next->prev->stuff is supposed to be ordered with that load
> (or only when that last load/store itself is also an atomic load or
> store?).
>
> So, what happens if the pointer deref chain is partly hidden in some
> functions:
>
> A * adjustptr (B *ptr) { return &ptr->here->there->next; }
> B * p = atomic_XXX (&somewhere, consume);
> adjustptr(p)->prev->stuff = bla;
>
> As far as I understood you, this whole ptrderef chain business would be
> only an optimization opportunity, right? So if the compiler can't be sure
> how p is actually used (as in my function-using case, assume adjustptr is
> defined in another unit), then the consume load would simply be
> transformed into an acquire (or whatever, with some barrier I mean)? Only
> _if_ the compiler sees all obvious uses of p (indirectly through pointer
> derefs) can it, yeah, do what with the consume load?
Good point, I left that out of my list. Adding it:
13. By default, pointer chains do not propagate into or out of functions.
In implementations having attributes, a [[carries_dependency]]
may be used to mark a function argument or return as passing
a pointer chain into or out of that function.
If a function does not contain memory_order_consume loads and
also does not contain [[carries_dependency]] attributes, then
that function may be compiled using any desired dependency-breaking
optimizations.
The ordering effects are implementation defined when a given
pointer chain passes into or out of a function through a parameter
or return not marked with a [[carries_dependency]] attributed.
Note that this last paragraph differs from the current standard, which
would require ordering regardless.
Thanx, Paul
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