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    <p>I guess the "Lack of memory model" in these cases just means that the optimizers were written before the C++11 memory model got published, and might perform now invalid optimizations. It's very difficult and time-consuming to validate optimizations against the memory model, so it's no big surprise that the clang/gcc teams haven't finished that yet.</p> <blockquote> <p>Does a lack of memory model support means that legal C++11 programs that use std::atomic arent seq consistent?</p> </blockquote> <p>Yes, that's a possibility. It's even worse: the compiler might introduce data races into (according to the C++11 standard) race-free programs, e.g. by introducing speculative writes.</p> <p>For example, several C++ compilers used to perform this optimization:</p> <pre><code>for (p = q; p = p -&gt; next; ++p) { if (p -&gt; data &gt; 0) ++count; } </code></pre> <p>Could get optimized into:</p> <pre><code>register int r1 = count; for (p = q; p = p -&gt; next; ++p) { if (p -&gt; data &gt; 0) ++r1; } count = r1; </code></pre> <p>If all <code>p-&gt;data</code> are non-negative, the original source code did not write to <code>count</code>, but the optimized code does. This can introduce a data race in an otherwise race-free program, so the C++11 specification disallows such optimizations. Existing compilers now have to verify (and adjust if necessary) all optimizations.</p> <p>See <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2338.html" rel="noreferrer">Concurrency memory model compiler consequences</a> for details.</p>
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    1. POHow can C++ compilers support C++11 atomic, but not support C++11 memory model
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    1. USDaniel
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    1. COu know what is funny? MS says for VC++ 11: Memory model: N2429 made the Core Language recognize the existence of multithreading, but there appears to be nothing for a compiler implementation to do (at least, one that already supported multithreading). So it's N/A in the table.
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      1. USNoSenseEtAl
    2. CO@NoSenseEtAl: That's only a very specific part of the entire memory model. The same table also has "Data-dependency ordering" (N2664) which did require work. Ordinarily, we'd count that as part of the C++11 memory model, too.
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