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<title>P0020r5 : Floating Point Atomic</title>
<meta name="date" content="2017-03-06" />
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<body>
<div class="document" id="p0020r5-floating-point-atomic">
<h1 class="title">P0020r5 : Floating Point Atomic</h1>
<table class="docinfo" frame="void" rules="none">
<col class="docinfo-name" />
<col class="docinfo-content" />
<tbody valign="top">
<tr class="field"><th class="docinfo-name">Project:</th><td class="field-body">ISO JTC1/SC22/WG21: Programming Language C++</td>
</tr>
<tr class="field"><th class="docinfo-name">Number:</th><td class="field-body">P0020r5</td>
</tr>
<tr><th class="docinfo-name">Date:</th>
<td>2017-03-06</td></tr>
<tr class="field"><th class="docinfo-name">Reply-to:</th><td class="field-body"><a class="reference external" href="mailto:hcedwar&#64;sandia.gov">hcedwar&#64;sandia.gov</a></td>
</tr>
<tr><th class="docinfo-name">Author:</th>
<td>H. Carter Edwards</td></tr>
<tr><th class="docinfo-name">Contact:</th>
<td><a class="first last reference external" href="mailto:hcedwar&#64;sandia.gov">hcedwar&#64;sandia.gov</a></td></tr>
<tr><th class="docinfo-name">Author:</th>
<td>Hans Boehm</td></tr>
<tr><th class="docinfo-name">Contact:</th>
<td><a class="first last reference external" href="mailto:hboehm&#64;google.com">hboehm&#64;google.com</a></td></tr>
<tr><th class="docinfo-name">Author:</th>
<td>Olivier Giroux</td></tr>
<tr><th class="docinfo-name">Contact:</th>
<td><a class="first last reference external" href="mailto:ogiroux&#64;nvidia.com">ogiroux&#64;nvidia.com</a></td></tr>
<tr><th class="docinfo-name">Author:</th>
<td>JF Bastien</td></tr>
<tr><th class="docinfo-name">Contact:</th>
<td><a class="first last reference external" href="mailto:jfbastien&#64;apple.com">jfbastien&#64;apple.com</a></td></tr>
<tr><th class="docinfo-name">Author:</th>
<td>James Reus</td></tr>
<tr><th class="docinfo-name">Contact:</th>
<td><a class="first last reference external" href="mailto:reus1&#64;llnl.gov">reus1&#64;llnl.gov</a></td></tr>
<tr class="field"><th class="docinfo-name">Audience:</th><td class="field-body">Library Evolution</td>
</tr>
<tr class="field"><th class="docinfo-name">URL:</th><td class="field-body"><a class="reference external" href="https://github.com/kokkos/ISO-CPP-Papers/blob/master/P0020.rst">https://github.com/kokkos/ISO-CPP-Papers/blob/master/P0020.rst</a></td>
</tr>
</tbody>
</table>
<div class="section" id="revision-history">
<h1>Revision History</h1>
<div class="section" id="p0020r3">
<h2>P0020r3</h2>
<blockquote>
<ul class="simple">
<li>Align proposal with content of corresponding sections in N5131, 2016-07-15.</li>
</ul>
</blockquote>
</div>
<div class="section" id="p0020r4">
<h2>P0020r4</h2>
<blockquote>
<ul class="simple">
<li>Editorial, add hyphenation to &quot;floating point&quot;</li>
<li>2016-11-09 Issaquah SG1 decision: move to LEWG targeting C++20</li>
</ul>
</blockquote>
</div>
<div class="section" id="p0020r5">
<h2>P0020r5</h2>
<blockquote>
<ul class="simple">
<li>2017-03-01 Kona LEWG review and consensus: move to C++20 IS</li>
<li>Change remarks regarding floating point environment from
note to normative.</li>
<li>Query vendors regarding changing &quot;should&quot; to &quot;shall&quot; in
regard to floating point environment.  Response from two
vendors supporting atomic floating point add operations
in hardware was &quot;do not change.&quot;</li>
<li>Align with C++17 working draft atomic wording as per P0558r1</li>
<li>Note: Section and paragraph numbers have not yet been updated
to C++17 working draft new section numbering.</li>
</ul>
</blockquote>
</div>
</div>
<div class="section" id="overview-motivation">
<h1>Overview / Motivation</h1>
<p>This paper proposes an extension to the atomic operations library [atomics]
for atomic addition on an object conforming to the atomic&lt;T&gt; where T is
a <em>floating-point</em> type (N5131 3.9.1p8).</p>
<p>The capability for atomic addition on floating-point types
critical for parallel high performance computing (HPC) applications.</p>
</div>
<div class="section" id="proposal">
<h1>Proposal:</h1>
<div class="section" id="add-section-to-32-7-specializations-for-floating-point-types">
<h2><em>add section to 32.7</em> Specializations for floating-point types</h2>
<p>The shall be specializations of the <strong>atomic</strong> template for
floating-point types <tt class="docutils literal">float</tt>, <tt class="docutils literal">double</tt>, <tt class="docutils literal">long double</tt>.
For each such floating-point type <em>floating-point</em> ,
the specialization <tt class="docutils literal">atomic&lt;</tt> <em>floating-point</em> <tt class="docutils literal">&gt;</tt> provides
additional atomic operations appropriate to floating-point types.</p>
<blockquote>
<div class="line-block">
<div class="line">template&lt;&gt; struct atomic&lt; <em>floating-point</em> &gt; {</div>
<div class="line-block">
<div class="line">static constexpr bool is_always_lock_free = <em>implementation-defined</em> ;</div>
<div class="line">bool is_lock_free() const volatile noexcept;</div>
<div class="line">bool is_lock_free() const noexcept;</div>
<div class="line">void store( <em>floating-point</em> , memory_order = memory_order_seq_cst ) volatile noexcept;</div>
<div class="line">void store( <em>floating-point</em> , memory_order = memory_order_seq_cst ) noexcept;</div>
<div class="line"><em>floating-point</em> load( memory_order = memory_order_seq_cst ) volatile noexcept;</div>
<div class="line"><em>floating-point</em> load( memory_order = memory_order_seq_cst ) noexcept;</div>
<div class="line">operator <em>floating-point</em> () volatile noexcept ;</div>
<div class="line">operator <em>floating-point</em> () noexcept ;</div>
<div class="line"><em>floating-point</em> exchange( <em>floating-point</em> , memory_order = memory_order_seq_cst ) volatile noexcept;</div>
<div class="line"><em>floating-point</em> exchange( <em>floating-point</em> , memory_order = memory_order_seq_cst ) noexcept;</div>
<div class="line">bool compare_exchange_weak( <em>floating-point</em> &amp; , <em>floating-point</em> , memory_order , memory_order ) volatile noexcept;</div>
<div class="line">bool compare_exchange_weak( <em>floating-point</em> &amp; , <em>floating-point</em> , memory_order , memory_order ) noexcept;</div>
<div class="line">bool compare_exchange_strong( <em>floating-point</em> &amp; , <em>floating-point</em>  , memory_order , memory_order ) volatile noexcept;</div>
<div class="line">bool compare_exchange_strong( <em>floating-point</em> &amp; , <em>floating-point</em>  , memory_order , memory_order ) noexcept;</div>
<div class="line">bool compare_exchange_weak( <em>floating-point</em> &amp; , <em>floating-point</em>  , memory_order = memory_order_seq_cst ) volatile noexcept;</div>
<div class="line">bool compare_exchange_weak( <em>floating-point</em> &amp; , <em>floating-point</em>  , memory_order = memory_order_seq_cst ) noexcept;</div>
<div class="line">bool compare_exchange_strong( <em>floating-point</em> &amp;, <em>floating-point</em> , memory_order = memory_order_seq_cst ) volatile noexcept;</div>
<div class="line">bool compare_exchange_strong( <em>floating-point</em> &amp;, <em>floating-point</em> , memory_order = memory_order_seq_cst ) noexcept;</div>
<div class="line"><br /></div>
<div class="line"><em>floating-point</em> fetch_add( <em>floating-point</em> , memory_order = memory_order_seq_cst) volatile noexcept;</div>
<div class="line"><em>floating-point</em> fetch_add( <em>floating-point</em> , memory_order = memory_order_seq_cst) noexcept;</div>
<div class="line"><em>floating-point</em> fetch_sub( <em>floating-point</em> , memory_order = memory_order_seq_cst) volatile noexcept;</div>
<div class="line"><em>floating-point</em> fetch_sub( <em>floating-point</em> , memory_order = memory_order_seq_cst) noexcept;</div>
<div class="line"><br /></div>
<div class="line">atomic() noexcept = default ;</div>
<div class="line">constexpr atomic( <em>floating-point</em> ) noexcept ;</div>
<div class="line">atomic( const atomic &amp; ) = delete ;</div>
<div class="line">atomic &amp; operator = ( const atomic &amp; ) = delete ;</div>
<div class="line">atomic &amp; operator = ( const atomic &amp; ) volatile = delete ;</div>
<div class="line"><em>floating-point</em> operator=( <em>floating-point</em> ) volatile noexcept ;</div>
<div class="line"><em>floating-point</em> operator=( <em>floating-point</em> ) noexcept ;</div>
<div class="line"><br /></div>
<div class="line"><em>floating-point</em> operator+=( <em>floating-point</em> ) volatile noexcept;</div>
<div class="line"><em>floating-point</em> operator+=( <em>floating-point</em> ) noexcept;</div>
<div class="line"><em>floating-point</em> operator-=( <em>floating-point</em> ) volatile noexcept;</div>
<div class="line"><em>floating-point</em> operator-=( <em>floating-point</em> ) noexcept;</div>
</div>
<div class="line">};</div>
</div>
</blockquote>
<p>The atomic floating-point specializations shall be standard-layout structs.
They shall each have a trivial default constructor and a trivial destructor.</p>
<p>Descriptions are provided below only for members that differ from the primary template.</p>
<div class="line-block">
<div class="line"><strong>T A::fetch_</strong><em>key</em><strong>(T operand, memory_order order = memory_order_seq_cst) volatile noexcept;</strong></div>
<div class="line"><strong>T A::fetch_</strong><em>key</em><strong>(T operand, memory_order order = memory_order_seq_cst) noexcept;</strong></div>
</div>
<blockquote>
<p><em>Effects:</em> Atomically replaces the value pointed to by <tt class="docutils literal">this</tt> with the
result of the computation applied to the value pointed to by <tt class="docutils literal">this</tt> and the
given <tt class="docutils literal">operand</tt>. Memory is affected according to the value of <tt class="docutils literal">order</tt>.
These operations are atomic read-modify-write operations (4.7).</p>
<p><em>Returns:</em> Atomically, the value pointed to by <tt class="docutils literal">this</tt> immediately
before the effects.</p>
<p><em>Remark:</em>
If the result is not a representable value for its type (5p4)
the result is unspecified, but the operations
otherwise have no undefined behavior.
Atomic arithmetic operations on <em>floating-point</em>
should conform to <tt class="docutils literal"><span class="pre">std::numeric_limits&lt;</span></tt> <em>floating-point</em> <tt class="docutils literal">&gt;</tt>
traits associated with the floating-point type (18.3.2).
The floating-point environment (26.4) for atomic arithmetic operations
on <em>floating-point</em> may be different than the calling thread's
floating-point environment.</p>
</blockquote>
</div>
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