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?
Ciao,
Michael.
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