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34 <!--*********************************************************************-->
35 <h1>&lt;atomic&gt; design</h1>
36 <!--*********************************************************************-->
37
38<p>
Howard Hinnant086b7182010-10-06 16:15:10[diff] [blame]39There are currently 3 designs under consideration. They differ in where most
Howard Hinnant0dee9cd2013-02-15 15:37:50[diff] [blame]40of the implementation work is done. The functionality exposed to the customer
Howard Hinnant086b7182010-10-06 16:15:10[diff] [blame]41should be identical (and conforming) for all three designs.
Howard Hinnant56f0d5b2010-10-05 16:44:40[diff] [blame]42</p>
43
Howard Hinnant086b7182010-10-06 16:15:10[diff] [blame]44<ol type="A">
Howard Hinnant56f0d5b2010-10-05 16:44:40[diff] [blame]45<li>
Howard Hinnant086b7182010-10-06 16:15:10[diff] [blame]46<a href="atomic_design_a.html">Minimal work for the library</a>
Howard Hinnant56f0d5b2010-10-05 16:44:40[diff] [blame]47</li>
48<li>
Howard Hinnant086b7182010-10-06 16:15:10[diff] [blame]49<a href="atomic_design_b.html">Something in between</a>
Howard Hinnant56f0d5b2010-10-05 16:44:40[diff] [blame]50</li>
51<li>
Howard Hinnant086b7182010-10-06 16:15:10[diff] [blame]52<a href="atomic_design_c.html">Minimal work for the front end</a>
Howard Hinnant56f0d5b2010-10-05 16:44:40[diff] [blame]53</li>
Howard Hinnant086b7182010-10-06 16:15:10[diff] [blame]54</ol>
Howard Hinnant56f0d5b2010-10-05 16:44:40[diff] [blame]55
Howard Hinnant08f29692010-10-08 17:36:50[diff] [blame]56<p>
57With any design, the (back end) compiler writer should note:
58</p>
59
60<blockquote>
61<p>
62The decision to implement lock-free operations on any given type (or not) is an
63ABI-binding decision. One can not change from treating a type as not lock free,
64to lock free (or vice-versa) without breaking your ABI.
65</p>
66
67<p>
68Example:
69</p>
70
71<blockquote><pre>
72TU1.cc
73-----------
74extern atomic&lt;long long&gt; A;
75int foo() { return A.compare_exchange_strong(w, x); }
76
77TU2.cc
78-----------
79extern atomic&lt;long long&gt; A;
80void bar() { return A.compare_exchange_strong(y, z); }
81</pre></blockquote>
82</blockquote>
83
84<p>
85If only <em>one</em> of these calls to <tt>compare_exchange_strong</tt> is
86implemented with mutex-locked code, then that mutex-locked code will not be
87executed mutually exclusively of the one implemented in a lock-free manner.
88</p>
89
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