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2445. "Stronger" memory ordering

Section: 99 [depr.util.smartptr.shared.atomic], 99 [atomics.types.operations.req] Status: Resolved Submitter: JF Bastien Opened: 2014-10-08 Last modified: 2017-11-29

Priority: Not Prioritized

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Discussion:

The definitions of compare and exchange in [util.smartptr.shared.atomic] p32 and 99 [atomics.types.operations.req] p20 state:

Requires: The failure argument shall not be memory_order_release nor memory_order_acq_rel. The failure argument shall be no stronger than the success argument.

The term "stronger" isn't defined by the standard.

It is hinted at by 99 [atomics.types.operations.req] p21:

When only one memory_order argument is supplied, the value of success is order, and the value of failure is order except that a value of memory_order_acq_rel shall be replaced by the value memory_order_acquire and a value of memory_order_release shall be replaced by the value memory_order_relaxed.

Should the standard define a partial ordering for memory orders, where consume and acquire are incomparable with release?

[2014-11 Urbana]

Move to SG1.

[2016-11-12, Issaquah]

Resolved by P0418R2

Proposed resolution: