Re: [isocpp-parallel] Proposal for new memory_order_consume definition

[Paul E. McKenney]

On Thu, Feb 25, 2016 at 04:46:50PM -0800, Hans Boehm wrote:
> If carries_dependency affects semantics, then it should not be an attribute.

I am not picky about the form of the marking.

> The original design, or at least my understanding of it, was that it not
> have semantics; it was only a suggestion to the compiler that it should
> preserve dependencies instead of inserting a fence at the call site.
> Dependency-based ordering would be preserved in either case.  But I think
> we're moving away from that view towards something that doesn't quietly add
> fences.

Yes, we do need to allow typical implementations to avoid quiet fence
addition.

> I do not think we can quite get away with defining a dependency in a way
> that is unconditionally preserved by existing compilers, and thus I think
> that we do probably need annotations along the dependency path.  I just
> don't see a way to otherwise deal with the case in which a compiler infers
> an equivalent pointer and dereferences that instead of the original.  This
> can happen under so many (unlikely but) hard-to-define conditions that it
> seems undefinable in an implementation-independent manner. "If the
> implementation is able then <the semantics change>" is, in my opinion, not
> acceptable standards text.

Hmmm...

But we do already have something very similar with signed integer
overflow.  If the compiler can see a way to generate faster code that
does not handle the overflow case, then the semantics suddenly change
from twos-complement arithmetic to something very strange.  The standard
does not specify all the ways that the implementation might deduce that
faster code can be generated by ignoring the overflow case, it instead
simply says that signed integer overflow invoked undefined behavior.

And if that is a problem, you use unsigned integers instead of signed
integers.

So it seems that we should be able to do something very similar here.
If you don't use marking, and the compiler deduces that a given pointer
that carries a given dependency is equal to some other pointer not
carrying that same dependency, there is no dependency ordering.  And,
just as with the signed-integer-overflow case, if that is a problem for
you, you can mark the pointers that you intend to carry dependencies.

In both the signed-integer-overflow and pointer-value-deduction cases,
most use cases don't need to care.  In the integer case, this is because
most use cases have small integer values that don't overflow.  In the
pointer case, this is because when the data structure is composed of
lots of heap-allocated data items, the compiler really cannot deduce
anything.

Other safe pointer use cases involve statically allocated data items
whose contents are compile-time constants (thus avoiding the need for
any sort of ordering) and sentinel data items (as in the Linux kernel's
cicular linked lists) where there is no dereferencing.

> Thus I see no way to both avoid adding syntax to functions that preserve
> dependencies and continue to allow existing transformations that remove
> dependencies we care about, e.g. due to equality comparisons.  We can
> hopefully ensure that without annotations compilers break things with very
> low probability, so that there is a reasonable path forward for existing
> code relying on dependency ordering (which currently also breaks with very
> low probability unless you understand what the compiler is doing).  But I
> don't see a way for the standard to guarantee correctness without the added
> syntax (or added optimization constraints that effectively assume all
> functions were annotated).

Your second sentence ("We can hopefully ensure...") does give me hope
that we might be able to reach agreement.  The intent of P0190R0 is
to define a subset of operations where dependencies will be carried.
Note that P0190R0 does call out comparisons as potentially unsafe.

							Thanx, Paul

> On Sat, Feb 20, 2016 at 11:53 AM, Paul E. McKenney <
> paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> 
> > On Fri, Feb 19, 2016 at 09:15:16PM -0500, Tony V E wrote:
> > > There's at least one easy answer in there:
> > >
> > > > âIf implementations must support annotation, what form should that
> > >       annotation take?  P0190R0 recommends the [[carries_dependency]]
> > >       attribute, but I am not picky as long as it can be (1) applied
> > >       to all relevant pointer-like objects and (2) used in C as well
> > >       as C++.  ;-)
> > >
> > > If an implementation must support it, then it is not an annotation but a
> > keyword. So no [[]]
> >
> > I would be good with that approach, especially if the WG14 continues
> > to stay away from annotations.
> >
> > For whatever it is worth, the introduction of intrinsics for comparisons
> > that avoid breaking dependencies enables the annotation to remain
> > optional.
> >
> >                                                         Thanx, Paul
> >
> > > Sent from my BlackBerry portable Babbage Device
> > >   Original Message
> > > From: Paul E. McKenney
> > > Sent: Thursday, February 18, 2016 4:58 AM
> > > To: parallel@xxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> > linux-arch@xxxxxxxxxxxxxxx; gcc@xxxxxxxxxxx; llvm-dev@xxxxxxxxxxxxxx
> > > Reply To: parallel@xxxxxxxxxxxxxxxx
> > > Cc: peterz@xxxxxxxxxxxxx; j.alglave@xxxxxxxxx; will.deacon@xxxxxxx;
> > dhowells@xxxxxxxxxx; Ramana.Radhakrishnan@xxxxxxx; luc.maranget@xxxxxxxx;
> > akpm@xxxxxxxxxxxxxxxxxxxx; Peter.Sewell@xxxxxxxxxxxx;
> > torvalds@xxxxxxxxxxxxxxxxxxxx; mingo@xxxxxxxxxx
> > > Subject: [isocpp-parallel] Proposal for new memory_order_consume
> > definition
> > >
> > > Hello!
> > >
> > > A proposal (quaintly identified as P0190R0) for a new
> > memory_order_consume
> > > definition may be found here:
> > >
> > > http://www2.rdrop.com/users/paulmck/submission/consume.2016.02.10b.pdf
> > >
> > > As requested at the October C++ Standards Committee meeting, this
> > > is a follow-on to P0098R1 that picks one alternative and describes
> > > it in detail. This approach focuses on existing practice, with the
> > > goal of supporting existing code with existing compilers. In the last
> > > clang/LLVM patch I saw for basic support of this change, you could count
> > > the changed lines and still have lots of fingers and toes left over.
> > > Those who have been following this story will recognize that this is
> > > a very happy contrast to work that would be required to implement the
> > > definition in the current standard.
> > >
> > > I expect that P0190R0 will be discussed at the upcoming C++ Standards
> > > Committee meeting taking place the week of February 29th. Points of
> > > discussion are likely to include:
> > >
> > > o     May memory_order_consume dependency ordering be used in
> > > unannotated code? I believe that this must be the case,
> > > especially given that this is our experience base. P0190R0
> > > therefore recommends this approach.
> > >
> > > o     If memory_order_consume dependency ordering can be used in
> > > unannotated code, must implementations support annotation?
> > > I believe that annotation support should be required, at the very
> > > least for formal verification, which can be quite difficult to
> > > carry out on unannotated code. In addition, it seems likely
> > > that annotations can enable much better diagnostics. P0190R0
> > > therefore recommends this approach.
> > >
> > > o     If implementations must support annotation, what form should that
> > > annotation take? P0190R0 recommends the [[carries_dependency]]
> > > attribute, but I am not picky as long as it can be (1) applied
> > > to all relevant pointer-like objects and (2) used in C as well
> > > as C++. ;-)
> > >
> > > o     If memory_order_consume dependency ordering can be used in
> > > unannotated code, how best to define the situations where
> > > the compiler can determine the exact value of the pointer in
> > > question? (In current defacto implementations, this can
> > > defeat dependency ordering. Interestingly enough, this case
> > > is not present in the Linux kernel, but still needs to be
> > > defined.)
> > >
> > > Options include:
> > >
> > > o     Provide new intrinsics that carry out the
> > > comparisons, but guarantee to preserve dependencies,
> > > as recommended by P0190R0 (std::pointer_cmp_eq_dep(),
> > > std::pointer_cmp_ne_dep(), std::pointer_cmp_gt_dep(),
> > > std::pointer_cmp_ge_dep(), std::pointer_cmp_lt_dep(),
> > > and std::pointer_cmp_le_dep()).
> > >
> > > o     State that -any- comparison involving an unannotated
> > > pointer loses the dependency.
> > >
> > > o     How is the common idiom of marking pointers by setting low-order
> > > bits to be supported when those pointers carry dependencies?
> > > At the moment, I believe that setting bits in pointers results in
> > > undefined behavior even without dependency ordering, so P0190R0
> > > kicks this particular can down the road. One option that
> > > has been suggested is to provide intrinsics for this purpose.
> > > (Sorry, but I forget who suggested this.)
> > >
> > > Thoughts?
> > >
> > > Thanx, Paul
> > >
> > > _______________________________________________
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