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34 <h1>&lt;atomic&gt; design</h1>
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36
37<p>
Howard Hinnant086b7182010-10-06 16:15:10[diff] [blame]38There are currently 3 designs under consideration. They differ in where most
Howard Hinnant0dee9cd2013-02-15 15:37:50[diff] [blame]39of the implementation work is done. The functionality exposed to the customer
Howard Hinnant086b7182010-10-06 16:15:10[diff] [blame]40should be identical (and conforming) for all three designs.
Howard Hinnant56f0d5b2010-10-05 16:44:40[diff] [blame]41</p>
42
Howard Hinnant086b7182010-10-06 16:15:10[diff] [blame]43<ol type="A">
Howard Hinnant56f0d5b2010-10-05 16:44:40[diff] [blame]44<li>
Howard Hinnant086b7182010-10-06 16:15:10[diff] [blame]45<a href="atomic_design_a.html">Minimal work for the library</a>
Howard Hinnant56f0d5b2010-10-05 16:44:40[diff] [blame]46</li>
47<li>
Howard Hinnant086b7182010-10-06 16:15:10[diff] [blame]48<a href="atomic_design_b.html">Something in between</a>
Howard Hinnant56f0d5b2010-10-05 16:44:40[diff] [blame]49</li>
50<li>
Howard Hinnant086b7182010-10-06 16:15:10[diff] [blame]51<a href="atomic_design_c.html">Minimal work for the front end</a>
Howard Hinnant56f0d5b2010-10-05 16:44:40[diff] [blame]52</li>
Howard Hinnant086b7182010-10-06 16:15:10[diff] [blame]53</ol>
Howard Hinnant56f0d5b2010-10-05 16:44:40[diff] [blame]54
Howard Hinnant08f29692010-10-08 17:36:50[diff] [blame]55<p>
56With any design, the (back end) compiler writer should note:
57</p>
58
59<blockquote>
60<p>
61The decision to implement lock-free operations on any given type (or not) is an
62ABI-binding decision. One can not change from treating a type as not lock free,
63to lock free (or vice-versa) without breaking your ABI.
64</p>
65
66<p>
67Example:
68</p>
69
70<blockquote><pre>
71TU1.cc
72-----------
73extern atomic&lt;long long&gt; A;
74int foo() { return A.compare_exchange_strong(w, x); }
75
76TU2.cc
77-----------
78extern atomic&lt;long long&gt; A;
79void bar() { return A.compare_exchange_strong(y, z); }
80</pre></blockquote>
81</blockquote>
82
83<p>
84If only <em>one</em> of these calls to <tt>compare_exchange_strong</tt> is
85implemented with mutex-locked code, then that mutex-locked code will not be
86executed mutually exclusively of the one implemented in a lock-free manner.
87</p>
88
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