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93 lines
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HTML
93 lines
2.5 KiB
HTML
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<title><atomic> design</title>
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<a href="https://llvm.org/">LLVM Home</a>
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<!--*********************************************************************-->
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<h1><atomic> design</h1>
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<!--*********************************************************************-->
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<p>
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There are currently 3 designs under consideration. They differ in where most
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of the implementation work is done. The functionality exposed to the customer
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should be identical (and conforming) for all three designs.
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</p>
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<ol type="A">
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<li>
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<a href="atomic_design_a.html">Minimal work for the library</a>
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</li>
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<li>
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<a href="atomic_design_b.html">Something in between</a>
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</li>
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<li>
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<a href="atomic_design_c.html">Minimal work for the front end</a>
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</li>
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</ol>
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<p>
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With any design, the (back end) compiler writer should note:
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</p>
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<blockquote>
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<p>
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The decision to implement lock-free operations on any given type (or not) is an
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ABI-binding decision. One can not change from treating a type as not lock free,
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to lock free (or vice-versa) without breaking your ABI.
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</p>
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<p>
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Example:
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</p>
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<blockquote><pre>
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TU1.cc
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-----------
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extern atomic<long long> A;
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int foo() { return A.compare_exchange_strong(w, x); }
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TU2.cc
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-----------
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extern atomic<long long> A;
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void bar() { return A.compare_exchange_strong(y, z); }
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</pre></blockquote>
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</blockquote>
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<p>
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If only <em>one</em> of these calls to <tt>compare_exchange_strong</tt> is
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implemented with mutex-locked code, then that mutex-locked code will not be
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executed mutually exclusively of the one implemented in a lock-free manner.
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</p>
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