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35  <h1>&lt;atomic&gt; design</h1>
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37
38<p>
39There are currently 3 designs under consideration.  They differ in where most
40of the implementation work is done.  The functionality exposed to the customer
41should be identical (and conforming) for all three designs.
42</p>
43
44<ol type="A">
45<li>
46<a href="atomic_design_a.html">Minimal work for the library</a>
47</li>
48<li>
49<a href="atomic_design_b.html">Something in between</a>
50</li>
51<li>
52<a href="atomic_design_c.html">Minimal work for the front end</a>
53</li>
54</ol>
55
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.
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