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<title>P0020r3 : Floating Point Atomic</title>
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<div class="document" id="p0020r3-floating-point-atomic">
<h1 class="title">P0020r3 : 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">P0020r3</td>
</tr>
<tr><th class="docinfo-name">Date:</th>
<td>2016-10-14</td></tr>
<tr class="field"><th class="docinfo-name">Reply-to:</th><td class="field-body"><a class="reference external" href="mailto:hcedwar@sandia.gov">hcedwar@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@sandia.gov">hcedwar@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@google.com">hboehm@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@nvidia.com">ogiroux@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@apple.com">jfbastien@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@llnl.gov">reus1@llnl.gov</a></td></tr>
<tr class="field"><th class="docinfo-name">Audience:</th><td class="field-body">SG1 Concurrency, 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>
<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<T> where T is
a <em>floating-point</em> type (N5131 3.9.1p8).</p>
<p>This paper <strong>does not</strong> include proposed extension
of the named atomic type (N5131 Tables 134 and 135),
or by implication extension of the C _Atomic qualification
for floating point types.</p>
<p>The capability for atomic addition on floating point types
critical for high performance computing (HPC) applications.
The need is for extension of <em>atomic</em> and
<em>atomic_view</em> (P0019) for floating point types.</p>
</div>
<div class="section" id="proposal">
<h1>Proposal</h1>
<div class="section" id="add-to-29-2-header-atomic-synopsis">
<h2><em>add to</em> 29.2 Header <atomic> synopsis</h2>
<blockquote>
<div class="line-block">
<div class="line">namespace std {</div>
<div class="line-block">
<div class="line">template<> stuct atomic< <em>floating-point</em> >;</div>
<div class="line"><br /></div>
<div class="line">// In the following declarations, <em>atomic-floating</em> is atomic<T>.</div>
<div class="line"><br /></div>
<div class="line"><em>floating-point</em> atomic_fetch_add( volatile <em>atomic-floating-point</em>*, <em>floating-point</em> ) noexcept ;</div>
<div class="line"><em>floating-point</em> atomic_fetch_add( <em>atomic-floating-point</em>*, <em>floating-point</em> ) noexcept ;</div>
<div class="line"><em>floating-point</em> atomic_fetch_add_explicit( volatile <em>atomic-floating-point</em>*, <em>floating-point</em> , memory_order ) noexcept ;</div>
<div class="line"><em>floating-point</em> atomic_fetch_add_explicit( <em>atomic-floating-point</em>*, <em>floating-point</em> , memory_order ) noexcept ;</div>
<div class="line"><em>floating-point</em> atomic_fetch_sub( volatile <em>atomic-floating-point</em>*, <em>floating-point</em> ) noexcept ;</div>
<div class="line"><em>floating-point</em> atomic_fetch_sub( <em>atomic-floating-point</em>*, <em>floating-point</em> ) noexcept ;</div>
<div class="line"><em>floating-point</em> atomic_fetch_sub_explicit( volatile <em>atomic-floating-point</em>*, <em>floating-point</em> , memory_order ) noexcept ;</div>
<div class="line"><em>floating-point</em> atomic_fetch_sub_explicit( <em>atomic-floating-point</em>*, <em>floating-point</em> , memory_order ) noexcept ;</div>
<div class="line"><br /></div>
</div>
<div class="line">}</div>
</div>
</blockquote>
</div>
<div class="section" id="add-to-29-5-atomic-types">
<h2><em>add to</em> 29.5 Atomic Types</h2>
<blockquote>
<div class="line-block">
<div class="line">template<> struct atomic< <em>floating-point</em> > {</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> & , <em>floating-point</em> , memory_order , memory_order ) volatile noexcept;</div>
<div class="line">bool compare_exchange_weak( <em>floating-point</em> & , <em>floating-point</em> , memory_order , memory_order ) noexcept;</div>
<div class="line">bool compare_exchange_strong( <em>floating-point</em> & , <em>floating-point</em> , memory_order , memory_order ) volatile noexcept;</div>
<div class="line">bool compare_exchange_strong( <em>floating-point</em> & , <em>floating-point</em> , memory_order , memory_order ) noexcept;</div>
<div class="line">bool compare_exchange_weak( <em>floating-point</em> & , <em>floating-point</em> , memory_order = memory_order_seq_cst ) volatile noexcept;</div>
<div class="line">bool compare_exchange_weak( <em>floating-point</em> & , <em>floating-point</em> , memory_order = memory_order_seq_cst ) noexcept;</div>
<div class="line">bool compare_exchange_strong( <em>floating-point</em> &, <em>floating-point</em> , memory_order = memory_order_seq_cst ) volatile noexcept;</div>
<div class="line">bool compare_exchange_strong( <em>floating-point</em> &, <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 & ) = delete ;</div>
<div class="line">atomic & operator = ( const atomic & ) = delete ;</div>
<div class="line">atomic & operator = ( const atomic & ) 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>
</div>
<div class="section" id="add-to-29-6-3-arithmetic-operations-on-atomic-types">
<h2><em>add to</em> 29.6.3 Arithmetic operations on atomic types</h2>
<p>In the declarations of these functions and function template specializations,
the name <em>floating-point</em> refers to a floating point type and the name
<em>atomic-floating-point</em> refers to <strong>atomic<</strong><em>floating-point</em><strong>></strong>.</p>
</div>
<div class="section" id="in-29-6-5-requirements-for-operations-on-atomic-types">
<h2><em>in</em> 29.6.5 Requirements for operations on atomic types</h2>
<p><em>regarding arithmetic operations</em></p>
<div class="line-block">
<div class="line"><strong>C A::fetch_</strong><em>key</em><strong>(M operand, memory_order order = memory_order_seq_cst) volatile noexcept;</strong></div>
<div class="line"><strong>C A::fetch_</strong><em>key</em><strong>(M operand, memory_order order = memory_order_seq_cst) noexcept;</strong></div>
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<p><em>update 29.6.5p31 Remark</em></p>
<blockquote>
<em>Remark:</em> For signed integer types, arithmetic is defined to use
two’s complement representation <s>. There</s> <u>and there</u> are no undefined results.
<u>For floating point types, if the result is not mathematically defined or
not in the range of representable values for its type (5p4)
the result is unspecified.
[Note: Atomic arithmetic operations on <em>floating-point</em>
should conform to <strong>std::numeric_limits<</strong> <em>floating-point</em> <strong>></strong>
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. - end note]</u>
For address types, the result may be an undefined address, but the operations
otherwise have no undefined behavior.</blockquote>
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