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### 1474. weak compare-and-exchange confusion

Section: 33.5.8.2 [atomics.types.operations] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2017-06-15

Priority: Not Prioritized

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Duplicate of: 1470, 1475, 1476, 1477

Discussion:

Addresses US-175, US-165, CH-23, GB-135

99 [atomics.types.operations.req] p. 25: The first sentence is grammatically incorrect.

[ 2010-10-28 Daniel adds: ]

Duplicate issue 1475 also has a proposed resolution, but both issues are resolved with below proposed resolution.

[ 2011-02-15 Howard fixes numbering, Hans improves the wording ]

[2011-02-24 Reflector discussion]

Moved to Tentatively Ready after 6 votes.

Proposed resolution:

1. Change 99 [atomics.types.operations.req] p. 23 as indicated:

   > [ Note: For example, t The effect of the compare-and-exchange operations `atomic_compare_exchange_strong` is

   > >     if (memcmp(object, expected, sizeof(*object)) == 0)
   > >       memcpy(object, &desired, sizeof(*object));
   > >     else
   > >       memcpy(expected, object, sizeof(*object));

   > — end note ] [..]

2. Change 99 [atomics.types.operations.req] p. 25 as indicated:

   > 25 Remark: The weak compare-and-exchange operations may fail spuriously, that is, return false while leaving the contents of memory pointed to by `expected` before the operation is the same that same as that of the `object` and the same as that of `expected` after the operation A weak compare-and-exchange operation may fail spuriously. That is, even when the contents of memory referred to by `expected` and `object` are equal, it may return false and store back to `expected` the same memory contents that were originally there.. [ Note: This spurious failure enables implementation of compare-and-exchange on a broader class of machines, e.g., loadlocked store-conditional machines. A consequence of spurious failure is that nearly all uses of weak compare-and-exchange will be in a loop.

   > When a compare-and-exchange is in a loop, the weak version will yield better performance on some platforms. When a weak compare-and-exchange would require a loop and a strong one would not, the strong one is preferable. — end note ]