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<title>Issue 4321: How are evaluations occurring within a store and a load operation ordered where the store synchronized with the load</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#New">New</a> status.</em></p>
<h3 id="4321"><a href="lwg-active.html#4321">4321</a>. How are evaluations occurring within a store and a load operation ordered where the store synchronized with the load</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#New">New</a>
 <b>Submitter:</b> jim x <b>Opened:</b> 2025-08-20 <b>Last modified:</b> 2025-08-25</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#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Consider this example:
</p>
<blockquote><pre>
std::atomic&lt;int&gt; v = 0;
// thread 1:
v.store(1, memory_order::release); // #1
// thread 2:
v.load(memory_order::acquire); // #2
</pre></blockquote>
<p>
Say, <code class='backtick'>#2</code> reads the value written by <code class='backtick'>#1</code>, <code class='backtick'>#1</code> synchronizes with <code class='backtick'>#2</code>. According to 6.10.2.2 <a href="https://wg21.link/intro.races">[intro.races]</a> p7:
</p>
<blockquote>
<p>
 An evaluation <i>A</i> <i>happens before an evaluation</i> <i>B</i> (or, equivalently, <i>B</i> 
 happens after <i>A</i>) if either
</p>
<ol style="list-style-type: none">
<li><p>(7.1) &mdash; [&hellip;]</p></li>
<li><p>(7.2) &mdash; <i>A</i> synchronizes with <i>B</i>, or</p></li>
<li><p>(7.3) &mdash; [&hellip;]</p></li>
</ol>
</blockquote>
<p>
So, <code class='backtick'>#1</code> happens before <i>B</i>. However, 6.10.1 <a href="https://wg21.link/intro.execution">[intro.execution]</a> p12 says:
</p>
<blockquote>
<p>
For each
</p>
<ol style="list-style-type: none">
<li><p>(12.1) &mdash; function invocation,</p></li>
<li><p>(12.2) &mdash; [&hellip;]</p></li>
<li><p>(12.3) &mdash; [&hellip;]</p></li>
</ol>
<p>
<i>F</i>, each evaluation that does not occur within <i>F</i> but is evaluated 
on the same thread and as part of the same signal handler (if any) is either 
sequenced before all evaluations that occur within <i>F</i> or sequenced after 
all evaluations that occur within <i>F</i>;
</p>
</blockquote>
<p>
Because both <code class='backtick'>v.store(...)</code> and <code class='backtick'>v.load(...)</code> are function invocations, and we can think that the member 
functions comprise some evaluations to form the operation, therefore, how are these evaluations that 
occur within the <code class='backtick'>store</code> ordered with those within the <code class='backtick'>load</code>?
<p/>
The rule only says the <code class='backtick'>store</code> synchronizes with the <code class='backtick'>load</code>, hence, the evaluation of the function 
call expression <code class='backtick'>v.store(...)</code> happens before the evaluation of the function call expression <code class='backtick'>v.load(...)</code>, 
but how about these evaluations occurring within these functions?
<p/>
A possible resolution might be: The order between all evaluations occurring within a function invocation 
and another evaluation <i>B</i> is determined by how the evaluation of the function call expression is 
ordered in relation to the expression <i>B</i>.
<p/>
For example, if <code class='backtick'>v.store()</code> happens-before <i>E</i>, then all evaluations occurring within the <code class='backtick'>store</code> 
happen-before <i>E</i>. As well, <code class='backtick'>v.store(...)</code> synchronizes with <code class='backtick'>v.load(...)</code>, then all evaluations 
occurring within <code class='backtick'>v.store(...)</code> synchronize with all evaluations occurring within <code class='backtick'>v.load(...)</code>.
</p>


<p id="res-4321"><b>Proposed resolution:</b></p>





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