1458. Overlapping evaluations are allowed

Section: 32.4 [atomics.order] Status: Dup Submitter: BSI Opened: 2010-08-25 Last modified: 2016-02-10

Priority: Not Prioritized

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Duplicate of: 1459

Discussion:

Addresses GB-131

32.5 [atomics.lockfree] p.8 states:

An atomic store shall only store a value that has been computed from constants and program input values by a finite sequence of program evaluations, such that each evaluation observes the values of variables as computed by the last prior assignment in the sequence.

... but 6.8.1 [intro.execution] p.13 states:

If A is not sequenced before B and B is not sequenced before A, then A and B are unsequenced. [ Note: The execution of unsequenced evaluations can overlap. — end note ]

Overlapping executions can make it impossible to construct the sequence described in 32.5 [atomics.lockfree] p.8. We are not sure of the intention here and do not offer a suggestion for change, but note that 32.5 [atomics.lockfree] p.8 is the condition that prevents out-of-thin-air reads.

Proposed resolution:

Request the concurrency working group to determine if changes are needed. Consider changing the use of "sequence" in 32.5 [atomics.lockfree]