On Wed, Jul 3, 2019 at 6:33 PM Paul E. McKenney <paul...@linux.ibm.com> wrote:
>
> On Wed, Jul 03, 2019 at 05:47:56PM +0200, Richard Biener wrote:
> > On July 3, 2019 5:14:58 PM GMT+02:00, "Paul E. McKenney"
> > <paul...@linux.ibm.com> wrote:
> > >On Wed, Jul 03, 2019 at 12:39:41AM +0530, Akshat Garg wrote:
> > >> On Tue, Jul 2, 2019 at 8:40 PM Paul E. McKenney
> > ><paul...@linux.ibm.com>
> > >> wrote:
> > >>
> > >> > On Tue, Jul 02, 2019 at 02:15:55PM +0100, Ramana Radhakrishnan
> > >wrote:
> > >> > > On Tue, Jul 2, 2019 at 1:38 PM Paul E. McKenney
> > ><paul...@linux.ibm.com>
> > >> > wrote:
> > >> > >
> > >> > > >
> > >> > > > Once a user-created non-dependent pointer is assigned to, it is
> > >OK to
> > >> > > > break the dependency.
> > >> > >
> > >> > > Ok, that's good.
> > >> > > >
> > >> > > > Or am I missing the point here?
> > >> > >
> > >> > > I was just trying to make sure we were on the same page. I wonder
> > >if
> > >> > > marking this volatile would be sufficient for prototyping. I
> > >suspect
> > >> > > we would need another flag somewhere which someone with gimple
> > >> > > knowledge might be able to help us with.
> > >> >
> > >> > I expect that marking it as volatile would do the trick. ;-)
> > >> >
> > >> > Thanx, Paul
> > >> >
> > >> So, marking this pointer as volatile will not allow the compiler to
> > >> modify/optimize the statements, the pointer is appearing in. And we
> > >don't
> > >> need to push any other code inside any of the passes. Due to this, we
> > >want
> > >> to automatically say those dependent pointers are volatile and
> > >introduce a
> > >> new flag for this. Am I getting you guys correctly? Kindly, let me
> > >know?
> > >
> > >While I suspect that this might work, it would suppress way more
> > >optimizations than would be good. For but one example, consider:
> > >
> > > _Dependent_ptr int *p;
> > >
> > > p = atomic_load_explicit(gp, memory_order_consume);
> > > a = p->a;
> > > b = p->b;
> > >
> > >If "p" is volatile, then the compiler will be prevented from keeping
> > >it in a register, which would not make people coding fastpaths at
> > >all happy. ;-)
> > >
> > >Still, use of volatile might be a good technique for prototyping and
> > >analysis of _Dependent_ptr.
> >
> > With this example can you quickly summarize what kind of guarantees
> > _Dependent_ptr gives and how a compiler
> > Could possibly break those?
>
> First I suppose I should fix the bug in the above code. Or one of the
> bugs, at least. :-/
>
> struct foo {
> int a;
> int b;
> };
>
> _Dependent_ptr struct foo *p;
>
> p = atomic_load_explicit(gp, memory_order_consume);
> a = p->a;
> b = p->b;
>
> And then let me tweak the example a bit. For the first tweak:
>
> struct foo {
> int a;
> int b;
> };
>
> struct foo default_foo = { .a = 42, .b = 43 };
> int *gp = &default_foo;
>
> ...
>
> _Dependent_ptr int *p;
>
> p = atomic_load_explicit(gp, memory_order_consume);
> a = p->a;
> b = p->b;
>
> Suppose that the compiler used feedback-driven optimization, and noticed
> that the value of gp was almost always &default_foo. The compiler might
> decide to transform the last three lines as follows:
>
> p = atomic_load_explicit(gp, memory_order_consume);
> if (p == &default_foo) {
> a = default_foo.a;
> b = default_foo.b;
> } else {
> a = p->a;
> b = p->b;
> }
>
> Now, as long as the value of gp had remained &default_foo for the full
> duration of execution, no harm done. But suppose the following code
> was executing concurrently with the above transformed code:
>
> struct foo *q;
>
> q = malloc(sizeof(*q));
> assert(q);
> q->a = 1729;
> q->b = 1730;
> atomic_store_explicit(gp, q, memory_order_release);
> do_something();
> default_foo.a = 1;
> default_foo.b = 2;
> atomic_store_explicit(gp, &default_foo, memory_order_release);
>
> In this case, if the memory_order_consume() came just after the pointer
> was reset to &default_foo, it is possible that the transformed code
> would set "a" to 42 and "b" to 43, which might not be what the guy
> writing the code wanted to happen.
>
> One of the purposes of _Dependent_ptr is to prevent this transformation.
>
> This transformation can also happen if the developer's code contained a
> comparison to &default_foo -- an ARM or PowerPC compiler backend, upon
> seeing two pointers containing the same bits, would likely consider the
> two pointers as being interchangeable, and thus might do the dereferences
> using the copy that was not tagged with the hardware dependencies.
>
> There are quite a few other examples. The C++ standards committee
> working papers shown below go through a number of them, in case the
> above example is not convincing. Or you could tell me what you would
> like to see, and I would attempt to find/create a suitable example.
>
> Does that help, or am I missing your point?
Browsed through that resources, so yes. So the important thing
is that data dependences in the source are not to be removed or
replaced by control dependences because that enables out-of-order
execution on the CPU (or by the compiler). This is only needed
for data typed with the _Dependent_ptr qualifier.
I think fully guaranteeing this is hard (besides when you use
volatile), we have the very same issue when dealing with
pointer provenance rules, known for years and not fixed
(and I don't see a good way to fix these issues without
sacrifying performance everywhere).
Good luck ;)
Maybe performance isn't so much of an issue for _Dependent_ptr
(compared to when all pointers are affected).
Richard.
>
> Thanx, Paul
>
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0098r0.pdf
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0190r4.pdf
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0462r1.pdf
>
> > Richard.
> >
> > >
> > > Thanx, Paul
> > >
> > >> Akshat
> > >>
> > >> >
> > >> > > regards
> > >> > > Ramana
> > >> > >
> > >> > > >
> > >> > > > Thanx,
> > >Paul
> > >> > > >
> > >> > > > > Ramana
> > >> > > > > >>
> > >> > > > > >>
> > >> > > > > >> > Does this sounds like a workable plan for ? Let me know
> > >your
> > >> > thoughts. If this sounds good then, we can do this for all the
> > >> > optimizations that may kill the dependencies at somepoint.
> > >> > > > > >> >
> > >> > > > > >> > -Akshat
> > >> > > > >
> > >> > > >
> > >> > >
> > >> >
> > >> >
> >
>
