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<title>P0020r00 : Floating Point Atomic View</title>
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<div class="document" id="p0020r00-floating-point-atomic-view">
<h1 class="title">P0020r00 : Floating Point Atomic View</h1>
<table class="docinfo" frame="void" rules="none">
<col class="docinfo-name" />
<col class="docinfo-content" />
<tbody valign="top">
<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:hboehm&#64;google.com">hboehm&#64;google.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:jfb&#64;google.com">jfb&#64;google.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">Number:</th><td class="field-body">P0020</td>
</tr>
<tr><th class="docinfo-name">Version:</th>
<td>00</td></tr>
<tr><th class="docinfo-name">Date:</th>
<td>2015-09-23</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>
<tr class="field"><th class="docinfo-name">WG21:</th><td class="field-body">SG1 Concurrency</td>
</tr>
</tbody>
</table>
<p>This paper proposes an extension to the atomic operations library [atomics]
for atomic addition on an object conforming to the <em>atomic-view-concept</em> (see P0019, Atomic View)
instantiated for a floating point type.
A class conforming to the <em>atomic-view-concept</em> shall also provide
the following operations when T is a floating point (FP) type.
This capability is critical for high performance computing (HPC) applications.
We leave open the question of whether the existing class <strong>atomic&lt;T&gt;</strong> should
be similarly extended when T is a floating point type.</p>
<blockquote>
<div class="line-block">
<div class="line">template&lt;&gt; struct atomic&lt; <em>FP</em> &gt; {</div>
<div class="line-block">
<div class="line"><em>FP</em> fetch_add( <em>FP</em> operand , memory_order order = memory_order_seq_cst ) volatile noexcept;</div>
<div class="line"><em>FP</em> fetch_add( <em>FP</em> operand , memory_order order = memory_order_seq_cst ) noexcept;</div>
<div class="line"><em>FP</em> fetch_sub( <em>FP</em> operand , memory_order order = memory_order_seq_cst ) volatile noexcept;</div>
<div class="line"><em>FP</em> fetch_sub( <em>FP</em> operand , memory_order order = memory_order_seq_cst ) noexcept;</div>
<div class="line">void add( fp operand , memory_order order = memory_order_seq_cst ) volatile noexcept;</div>
<div class="line">void add( fp operand , memory_order order = memory_order_seq_cst ) noexcept;</div>
<div class="line">void sub( fp operand , memory_order order = memory_order_seq_cst ) volatile noexcept;</div>
<div class="line">void sub( fp operand , memory_order order = memory_order_seq_cst ) noexcept;</div>
<div class="line">void operator+=( <em>FP</em> operand ) volatile noexcept;</div>
<div class="line">void operator+=( <em>FP</em> operand ) noexcept;</div>
<div class="line">void operator-=( <em>FP</em> operand ) volatile noexcept;</div>
<div class="line">void operator-=( <em>FP</em> operand ) noexcept;</div>
</div>
<div class="line">};</div>
<div class="line"><br /></div>
<div class="line">template&lt;&gt; struct atomic-view-concept &lt; <em>FP</em> &gt; {</div>
<div class="line-block">
<div class="line"><em>FP</em> fetch_add( <em>FP</em> operand , memory_order order = memory_order_seq_cst ) const noexcept;</div>
<div class="line"><em>FP</em> fetch_sub( <em>FP</em> operand , memory_order order = memory_order_seq_cst ) const noexcept;</div>
<div class="line">void add( fp operand , memory_order order = memory_order_seq_cst ) const noexcept;</div>
<div class="line">void sub( fp operand , memory_order order = memory_order_seq_cst ) const noexcept;</div>
<div class="line">void operator+=( <em>FP</em> operand ) const noexcept;</div>
<div class="line">void operator-=( <em>FP</em> operand ) const noexcept;</div>
</div>
<div class="line">};</div>
</div>
</blockquote>
<div class="line-block">
<div class="line"><em>FP</em> <strong>fetch_add(</strong> <em>FP</em> <strong>operand , memory_order order = memory_order_seq_cst )</strong></div>
<div class="line"><em>FP</em> <strong>fetch_sub(</strong> <em>FP</em> <strong>operand , memory_order order = memory_order_seq_cst )</strong></div>
<div class="line"><strong>void add(</strong> <em>FP</em> <strong>operand , memory_order order = memory_order_seq_cst )</strong></div>
<div class="line"><strong>void sub(</strong> <em>FP</em> <strong>operand , memory_order order = memory_order_seq_cst )</strong></div>
</div>
<blockquote>
<p>Requires: The memory order shall not be memory_order_acquire, memory_order_acq_rel, or memory_order_consume.</p>
<p>Effects: Atomically replaces the value of the floating point object
referenced by the atomic view with the sum of the previous value and
operand for <em>add</em> or -operand for <em>sub</em>.
Memory is affected according to the value of <strong>order</strong>.
This is an atomic read-modify-write operation (1.10).</p>
<p>Returns: fetch_<em>op</em>  Atomically, the value of the referenced object immediately before the effects.</p>
<p>Remark: The <strong>add</strong> and <strong>sub</strong> functions omit returning the value before the effects.
Omission of a return value may allow a better performing implementation.</p>
<p>Remark: In the event of arithmetic overflow or other floating point exception,
the behavior of this operation is implementation defined.
It is a quality-of-implementation as to whether
arithmetic overflow or other floating point exception
is detectable in the resulting value; <em>e.g.</em>, a value of NaN.</p>
</blockquote>
<div class="line-block">
<div class="line"><strong>void operator+=(</strong> <em>FP</em> <strong>operand )</strong></div>
<div class="line"><strong>void operator-=(</strong> <em>FP</em> <strong>operand )</strong></div>
</div>
<blockquote>
<p>Effects: <strong>+=</strong> performs <strong>add(operand)</strong> and <strong>-=</strong> performs <strong>sub(operand)</strong></p>
<p>Remark: These operators omit a return value to avoid conflicting
with either the corresponding arithmetic operators for <em>FP</em> or
the arithmetic operators for <strong>atomic&lt;integral&gt;</strong>.
Note that the arithmetic operators for <strong>atomic&lt;</strong><em>integral</em><strong>&gt;</strong>
are inconsistent with the corresponding arithmetic operators for <em>integral</em>.</p>
</blockquote>
<pre class="code c++ literal-block">
<span class="keyword type">int</span> <span class="name function">i</span><span class="punctuation">(</span><span class="literal number integer">0</span><span class="punctuation">);</span>
<span class="punctuation">(</span> <span class="name">i</span> <span class="operator">+=</span> <span class="literal number integer">1</span> <span class="punctuation">)</span> <span class="operator">+=</span> <span class="literal number integer">2</span> <span class="punctuation">;</span> <span class="comment single">// i+= returns an lvalue
</span>
<span class="name">atomic</span><span class="operator">&lt;</span><span class="keyword type">int</span><span class="operator">&gt;</span> <span class="name">ai</span><span class="punctuation">(</span><span class="literal number integer">0</span><span class="punctuation">);</span>
<span class="punctuation">(</span> <span class="name">ai</span> <span class="operator">+=</span> <span class="literal number integer">1</span> <span class="punctuation">)</span> <span class="operator">+=</span> <span class="literal number integer">2</span> <span class="punctuation">;</span> <span class="comment single">// error: i+= returns an rvalue</span>
</pre>
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