New paper: N3710, Specifying the absence of "out of thin air" results (LWG2265) -- Hans Boehm et al.

A new WG21 paper is available. A copy is linked below, and the paper will also appear in the next normal WG21 mailing. If you are not a committee member, please use the comments section below or the std-proposals forum for public discussion.

Document number: N3710

Date: 2013-08-29

Specifying the absence of "out of thin air" results (LWG2265)

by Hans Boehm, Mark Batty, Brian Demsky, Olivier Giroux, Paul McKenney, Peter Sewell, Francesco Zappa Nardelli, et al.


The C++11 memory model specified the semantics of shared ordinary and atomic objects. There is one small part of this specification that we always recognized was dubious. The specification in 1.10 of weakly ordered atomics allows an atomic load to essentially see, or not see, the result of any racing store. In the case of explicit very weak memory ordering specifications, this can lead to "causal cycles" and highly counterintuitive results. ...

Essentially this issue has been an open issue in the Java specification for about 10 years. The major advantage that we have in C++ is that the problem is confined to non-synchronizing atomics, i.e. memory_order_relaxed, and some memory_order_consume uses (or read-modify-write operations that effectively weaken the ordering on either the read or write to one of those). Many of us expect those to be rarely used. ...

This proposal benefitted from a lengthy email discussion involving participants listed at the beginning of this paper, and others. This provided very useful input resulting in improvements to this proposal. It unfortunately resulted in neither a clear consensus in that group, nor an alternate proposal with clearly better consensus behind it. Not all of the listed contributors necessarily support this proposal. Since I don't believe there is support for maintaining the status quo, I nonetheless bring forward this proposal. Mark Batty and Peter Sewell are currently considering submitting additional late papers on this topic, including an alternate proposal. ...

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