Proposed resolution (January, 2014) [SSUPERSEDED]:

1. Change 6.9.2 [intro.multithread] paragraph 14 as follows:

The visible sequence of side effects on an atomic object M, with respect to a value computation B of M, is a maximal contiguous sub-sequence of side effects in the modification order of M, where the first side effect is visible with respect to B, and for every side effect, it is not the case that B happens before it. The value of an atomic object M, as determined by evaluation B, shall be the value stored by some operation in the visible sequence of M with respect to B side effect A that modifies M, where B does not happen before A. [Note: It can be shown that the visible sequence of side effects of a value computation is unique given The set of side effects that a given evaluation might take its value from is also restricted by the rest of the rules described here, and in particular, by the coherence requirements below. —end note]

2. Change 6.9.2 [intro.multithread] paragraph 20 as follows:

[Note: The visible sequence of side effects value observed by a load of an atomic depends on the “happens before” relation, which depends on the values observed by loads of atomics, which we are restricting here. The intended reading is that there must exist an association of atomic loads with modifications they observe that, together with suitably chosen modification orders and the “happens before” relation derived as described above, satisfy the resulting constraints as imposed here. —end note]

3. Change 6.9.2 [intro.multithread] paragraph 22 as follows:

[Note: Compiler transformations that introduce assignments to a potentially shared memory location that would not be modified by the abstract machine are generally precluded by this standard, since such an assignment might overwrite another assignment by a different thread in cases in which an abstract machine execution would not have encountered a data race. This includes implementations of data member assignment that overwrite adjacent members in separate memory locations. Reordering of atomic loads in cases in which the atomics in question may alias is also generally precluded, since this may violate the “visible sequence” coherence rules. —end note]

4. Change 32.5.4 [atomics.order] paragraph 3 as follows:

There shall be a single total order S on all memory_order_seq_cst operations, consistent with the “happens before” order and modification orders for all affected locations, such that each memory_order_seq_cst operation B that loads a value from an atomic object M observes one of the following values:

• the result of the last modification A of M that precedes B in S, if it exists, or

• if A exists, the result of some modification of M in the visible sequence of side effects with respect to B that is not memory_order_seq_cst and that does not happen before A, or

• if A does not exist, the result of some modification of M in the visible sequence of side effects with respect to B that is not memory_order_seq_cst.

[Note:...