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<title>Issue 1043: Clarify that compare_exchange is not a read-modify-write operation</title>
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<h3 id="1043"><a href="lwg-defects.html#1043">1043</a>. Clarify that <code>compare_exchange</code> is not a read-modify-write operation</h3>
<p><b>Section:</b> 32.5.8.2 <a href="https://wg21.link/atomics.types.operations">[atomics.types.operations]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Alisdair Meredith <b>Opened:</b> 2009-03-12 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#atomics.types.operations">active issues</a> in [atomics.types.operations].</p>
<p><b>View all other</b> <a href="lwg-index.html#atomics.types.operations">issues</a> in [atomics.types.operations].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>

<p><b>Addresses US 91 [CD1]</b></p>

<p>
It is unclear whether or not a failed <code>compare_exchange</code> is a RMW operation
(as used in 6.10.2 <a href="https://wg21.link/intro.multithread">[intro.multithread]</a>).
</p>

<p>
Suggested solution:
</p>

<p>
Make failing <code>compare_exchange</code> operations <b>not</b> be RMW.
</p>

<p><i>[
Anthony Williams adds:
]</i></p>


<blockquote><p>
In 32.5.8.2 <a href="https://wg21.link/atomics.types.operations">[atomics.types.operations]</a> p18 it says that "These
operations are atomic read-modify-write operations" (final sentence).
This is overly restrictive on the implementations of
<code>compare_exchange_weak</code> and <code>compare_exchange_strong</code> on platforms without a
native CAS instruction.
</p></blockquote>


<p><i>[
Summit:
]</i></p>


<blockquote><p>
Group agrees with the resolution as proposed by Anthony Williams in the attached note.
</p></blockquote>

<p><i>[
Batavia (2009-05):
]</i></p>

<blockquote><p>
We recommend the proposed resolution be reviewed
by members of the Concurrency Subgroup.
</p></blockquote>

<p><i>[
2009-07 post-Frankfurt:
]</i></p>


<blockquote><p>
This is likely to be addressed by Lawrence's upcoming paper. He will
adopt the proposed resolution.
</p></blockquote>

<p><i>[
2009-08-17 Handled by
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2925.html">N2925</a>.
]</i></p>


<p><i>[
2009-10 Santa Cruz:
]</i></p>


<blockquote><p>
<del>NAD Editorial</del><ins>Resolved</ins>.  Addressed by
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2992.htm">N2992</a>.
</p></blockquote>



<p id="res-1043"><b>Proposed resolution:</b></p>
<p>
Change 32.5.8.2 <a href="https://wg21.link/atomics.types.operations">[atomics.types.operations]</a> p18:
</p>

<blockquote><p>
-18- <i>Effects:</i> Atomically, compares the value pointed to by
<code>object</code> or by <code>this</code> for equality with that in
<code>expected</code>, and if true, replaces the value pointed to by
<code>object</code> or by <code>this</code> with desired, and if false, updates
the value in <code>expected</code> with the value pointed to by
<code>object</code> or by <code>this</code>. Further, if the comparison is true,
memory is affected according to the value of <code>success</code>, and if the
comparison is false, memory is affected according to the value of
<code>failure</code>. When only one <code>memory_order</code> argument is
supplied, the value of <code>success</code> is <code>order</code>, and the value
of <code>failure</code> is <code>order</code> except that a value of
<code>memory_order_acq_rel</code> shall be replaced by the value
<code>memory_order_acquire</code> and a value of
<code>memory_order_release</code> shall be replaced by the value
<code>memory_order_relaxed</code>. <ins>If the comparison is <code>true</code>, </ins>
<del>T</del><ins>t</ins>hese operations are atomic
read-modify-write operations (1.10). 
<ins>If the comparison is <code>false</code>, these
operations are atomic load operations.</ins>
</p></blockquote>






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