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

[Paul E. McKenney]

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? ÂP0190R0 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++. Â;-)
> 
> If an implementation must support it, then it is not an annotation but a keyword. So no [[]]Â

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

> SentÂfromÂmyÂBlackBerryÂportableÂBabbageÂDevice
> Â Original Message Â
> From: Paul E. McKenney
> Sent: Thursday, February 18, 2016 4:58 AM
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> 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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