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  <title>&lt;atomic&gt; design</title>
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  <h1>&lt;atomic&gt; design</h1>
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<p>
There are currently 3 designs under consideration.  They differ in where most
of the implementation work is done.  The functionality exposed to the customer
should be identical (and conforming) for all three designs.
</p>

<ol type="A">
<li>
<a href="atomic_design_a.html">Minimal work for the library</a>
</li>
<li>
<a href="atomic_design_b.html">Something in between</a>
</li>
<li>
<a href="atomic_design_c.html">Minimal work for the front end</a>
</li>
</ol>

<p>
With any design, the (back end) compiler writer should note:
</p>

<blockquote>
<p>
The decision to implement lock-free operations on any given type (or not) is an
ABI-binding decision.  One can not change from treating a type as not lock free,
to lock free (or vice-versa) without breaking your ABI.
</p>

<p>
Example:
</p>

<blockquote><pre>
TU1.cc
-----------
extern atomic&lt;long long&gt; A;
int foo() { return A.compare_exchange_strong(w, x); }

TU2.cc
-----------
extern atomic&lt;long long&gt; A;
void bar() { return A.compare_exchange_strong(y, z); }
</pre></blockquote>
</blockquote>

<p>
If only <em>one</em> of these calls to <tt>compare_exchange_strong</tt> is
implemented with mutex-locked code, then that mutex-locked code will not be
executed mutually exclusively of the one implemented in a lock-free manner.
</p>

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