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New issue from jim x: "How does [atomics.order] p3 apply when then mo…
…dification is an initialization?"
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| <?xml version='1.0' encoding='utf-8' standalone='no'?> | ||
| <!DOCTYPE issue SYSTEM "lwg-issue.dtd"> | ||
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| <issue num="4174" status="New"> | ||
| <title>How does [atomics.order] p3 apply when then modification is an initialization?</title> | ||
| <section><sref ref="[atomics.order]"/></section> | ||
| <submitter>jim x</submitter> | ||
| <date>13 Nov 2024</date> | ||
| <priority>99</priority> | ||
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| <discussion> | ||
| <p> | ||
| Consider this example | ||
| </p> | ||
| <blockquote><pre> | ||
| std::atomic<int> v = 0; | ||
| // thread 1: | ||
| v.load(std::memory_order::seq_cst); | ||
| //thread 2: | ||
| v.store(1,std::memory_order::seq_cst); | ||
| </pre></blockquote> | ||
| <p> | ||
| If the load operation reads the value `0`, how are load and store operations ordered in the single total order? | ||
| According to <sref ref="[atomics.order]"/> p3 (emphasize mine) | ||
| </p> | ||
| <blockquote style="border-left: 3px solid #ccc;padding-left: 15px;"> | ||
| <p> | ||
| An atomic operation <i>A</i> on some atomic object <i>M</i> is <i>coherence-ordered before</i> | ||
| another atomic operation <i>B</i> on <i>M</i> if | ||
| </p> | ||
| <ol style="list-style-type: none"> | ||
| <li><p>[…]</p></li> | ||
| <li><p>(3.3) — <i>A</i> and <i>B</i> are not the same atomic read-modify-write operation, | ||
| and there exists an <b>atomic modification <i>X</i></b> of <i>M</i> such that <i>A</i> reads the value stored by | ||
| <i>X</i> and <i>X</i> precedes <i>B</i> in the modification order of <i>M</i>, or</p></li> | ||
| </ol> | ||
| </blockquote> | ||
| <p> | ||
| According to <sref ref="[atomics.types.operations]"/> p3 (emphasize mine) | ||
| </p> | ||
| <blockquote style="border-left: 3px solid #ccc;padding-left: 15px;"> | ||
| <i>Effects</i>: Initializes the object with the value <tt>desired</tt>. <b>Initialization is not an atomic operation</b> | ||
| (<sref ref="[intro.multithread]"/>). | ||
| </blockquote> | ||
| <p> | ||
| So, how does <sref ref="[atomics.order]"/> p3 apply to this example such that the load operation precedes | ||
| the store operation in the single total order <i>S</i>? | ||
| </p> | ||
| </discussion> | ||
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| <resolution> | ||
| <p> | ||
| </p> | ||
| </resolution> | ||
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| </issue> |