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				33	<!--*********************************************************************-->
				34	<h1><atomic> design</h1>
				35	<!--*********************************************************************-->
				36
				37	<p>
Howard Hinnant	79a7739	2010-10-06 16:15:10 +0000	[diff] [blame]	38	There are currently 3 designs under consideration. They differ in where most
Howard Hinnant	7b01897	2013-02-15 15:37:50 +0000	[diff] [blame]	39	of the implementation work is done. The functionality exposed to the customer
Howard Hinnant	79a7739	2010-10-06 16:15:10 +0000	[diff] [blame]	40	should be identical (and conforming) for all three designs.
Howard Hinnant	bded6c9	2010-10-05 16:44:40 +0000	[diff] [blame]	41	</p>
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Howard Hinnant	79a7739	2010-10-06 16:15:10 +0000	[diff] [blame]	45	<a href="atomic_design_a.html">Minimal work for the library</a>
Howard Hinnant	bded6c9	2010-10-05 16:44:40 +0000	[diff] [blame]	46	</li>
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Howard Hinnant	79a7739	2010-10-06 16:15:10 +0000	[diff] [blame]	48	<a href="atomic_design_b.html">Something in between</a>
Howard Hinnant	bded6c9	2010-10-05 16:44:40 +0000	[diff] [blame]	49	</li>
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Howard Hinnant	79a7739	2010-10-06 16:15:10 +0000	[diff] [blame]	51	<a href="atomic_design_c.html">Minimal work for the front end</a>
Howard Hinnant	bded6c9	2010-10-05 16:44:40 +0000	[diff] [blame]	52	</li>
Howard Hinnant	79a7739	2010-10-06 16:15:10 +0000	[diff] [blame]	53	</ol>
Howard Hinnant	bded6c9	2010-10-05 16:44:40 +0000	[diff] [blame]	54
Howard Hinnant	84a7e21	2010-10-08 17:36:50 +0000	[diff] [blame]	55	<p>
				56	With any design, the (back end) compiler writer should note:
				57	</p>
				58
				59	<blockquote>
				60	<p>
				61	The decision to implement lock-free operations on any given type (or not) is an
				62	ABI-binding decision. One can not change from treating a type as not lock free,
				63	to lock free (or vice-versa) without breaking your ABI.
				64	</p>
				65
				66	<p>
				67	Example:
				68	</p>
				69
				70	<blockquote><pre>
				71	TU1.cc
				72	-----------
				73	extern atomic<long long> A;
				74	int foo() { return A.compare_exchange_strong(w, x); }
				75
				76	TU2.cc
				77	-----------
				78	extern atomic<long long> A;
				79	void bar() { return A.compare_exchange_strong(y, z); }
				80	</pre></blockquote>
				81	</blockquote>
				82
				83	<p>
				84	If only <em>one</em> of these calls to <tt>compare_exchange_strong</tt> is
				85	implemented with mutex-locked code, then that mutex-locked code will not be
				86	executed mutually exclusively of the one implemented in a lock-free manner.
				87	</p>
				88
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