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35 <h1>&lt;atomic&gt; design</h1>
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37
38<p>
Howard Hinnant79a77392010-10-06 16:15:10 +0000[diff] [blame]39There are currently 3 designs under consideration. They differ in where most
40of the implmentation work is done. The functionality exposed to the customer
41should be identical (and conforming) for all three designs.
Howard Hinnantbded6c92010-10-05 16:44:40 +0000[diff] [blame]42</p>
43
Howard Hinnant79a77392010-10-06 16:15:10 +0000[diff] [blame]44<ol type="A">
Howard Hinnantbded6c92010-10-05 16:44:40 +0000[diff] [blame]45<li>
Howard Hinnant79a77392010-10-06 16:15:10 +0000[diff] [blame]46<a href="atomic_design_a.html">Minimal work for the library</a>
Howard Hinnantbded6c92010-10-05 16:44:40 +0000[diff] [blame]47</li>
48<li>
Howard Hinnant79a77392010-10-06 16:15:10 +0000[diff] [blame]49<a href="atomic_design_b.html">Something in between</a>
Howard Hinnantbded6c92010-10-05 16:44:40 +0000[diff] [blame]50</li>
51<li>
Howard Hinnant79a77392010-10-06 16:15:10 +0000[diff] [blame]52<a href="atomic_design_c.html">Minimal work for the front end</a>
Howard Hinnantbded6c92010-10-05 16:44:40 +0000[diff] [blame]53</li>
Howard Hinnant79a77392010-10-06 16:15:10 +0000[diff] [blame]54</ol>
Howard Hinnantbded6c92010-10-05 16:44:40 +0000[diff] [blame]55
Howard Hinnant84a7e212010-10-08 17:36:50 +0000[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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