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| 1 | +<?xml version='1.0' encoding='utf-8' standalone='no'?> |
| 2 | +<!DOCTYPE issue SYSTEM "lwg-issue.dtd"> |
| 3 | + |
| 4 | +<issue num="4321" status="New"> |
| 5 | +<title>How are evaluations occurring within a store and a load operation ordered where the store synchronized with the load</title> |
| 6 | +<section> |
| 7 | +<sref ref="[atomics.types.operations]"/> |
| 8 | +</section> |
| 9 | +<submitter>jim x</submitter> |
| 10 | +<date>20 Aug 2025</date> |
| 11 | +<priority>99</priority> |
| 12 | + |
| 13 | +<discussion> |
| 14 | +<p> |
| 15 | +Consider this example: |
| 16 | +</p> |
| 17 | +<blockquote><pre> |
| 18 | +std::atomic<int> v = 0; |
| 19 | +// thread 1: |
| 20 | +v.store(1, memory_order::release); // #1 |
| 21 | +// thread 2: |
| 22 | +v.load(memory_order::acquire); // #2 |
| 23 | +</pre></blockquote> |
| 24 | +<p> |
| 25 | +Say, `#2` reads the value written by `#1`, `#1` synchronizes with `#2`. According to <sref ref="[intro.races]"/> p7: |
| 26 | +</p> |
| 27 | +<blockquote> |
| 28 | +<p> |
| 29 | + An evaluation <i>A</i> <i>happens before an evaluation</i> <i>B</i> (or, equivalently, <i>B</i> |
| 30 | + happens after <i>A</i>) if either |
| 31 | +</p> |
| 32 | +<ol style="list-style-type: none"> |
| 33 | +<li><p>(7.1) — […]</p></li> |
| 34 | +<li><p>(7.2) — <i>A</i> synchronizes with <i>B</i>, or</p></li> |
| 35 | +<li><p>(7.3) — […]</p></li> |
| 36 | +</ol> |
| 37 | +</blockquote> |
| 38 | +<p> |
| 39 | +So, `#1` happens before <i>B</i>. However, <sref ref="[intro.execution]"/> p12 says: |
| 40 | +</p> |
| 41 | +<blockquote> |
| 42 | +<p> |
| 43 | +For each |
| 44 | +</p> |
| 45 | +<ol style="list-style-type: none"> |
| 46 | +<li><p>(12.1) — function invocation,</p></li> |
| 47 | +<li><p>(12.2) — […]</p></li> |
| 48 | +<li><p>(12.3) — […]</p></li> |
| 49 | +</ol> |
| 50 | +<p> |
| 51 | +<i>F</i>, each evaluation that does not occur within <i>F</i> but is evaluated |
| 52 | +on the same thread and as part of the same signal handler (if any) is either |
| 53 | +sequenced before all evaluations that occur within <i>F</i> or sequenced after |
| 54 | +all evaluations that occur within <i>F</i>; |
| 55 | +</p> |
| 56 | +</blockquote> |
| 57 | +<p> |
| 58 | +Because both `v.store(...)` and `v.load(...)` are function invocations, and we can think that the member |
| 59 | +functions comprise some evaluations to form the operation, therefore, how are these evaluations that |
| 60 | +occur within the `store` ordered with those within the `load`? |
| 61 | +<p/> |
| 62 | +The rule only says the `store` synchronizes with the `load`, hence, the evaluation of the function |
| 63 | +call expression `v.store(...)` happens before the evaluation of the function call expression `v.load(...)`, |
| 64 | +but how about these evaluations occurring within these functions? |
| 65 | +</p> |
| 66 | +</discussion> |
| 67 | + |
| 68 | +<resolution> |
| 69 | +</resolution> |
| 70 | + |
| 71 | +</issue> |
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