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  <title>p0518r0: Allowing copies as arguments to function objects given to parallel algorithms in response to CH11</title>
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   <h1 class="p-name no-ref" id="title">p0518r0<br>Allowing copies as arguments to function objects given to parallel algorithms in response to CH11</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2017-01-31">31 January 2017</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://wg21.link/p0518">http://wg21.link/p0518</a>
     <dt>Authors:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:dshollm@sandia.gov">David S. Hollman</a> (<span class="p-org org">Sandia National Labs</span>)
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:jliffla@sandia.gov">Jonathan Lifflander</a> (<span class="p-org org">Sandia National Labs</span>)
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:michael@codeplay.com">Michael Wong</a> (<span class="p-org org">Codeplay</span>)
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:dv@vollmann.ch">Detlef Vollmann</a> (<span class="p-org org">Vollmann Engineering</span>)
     <dt>Audience:
     <dd>SG1
     <dt>Toggle Diffs:
     <dd>
      <label for="hidedel" id="hidedel-label">Hide deleted text</label>
     <dt>Project:
     <dd><span>ISO JTC1/SC22/WG21: Programming Language C++</span>
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
  <div class="p-summary" data-fill-with="abstract">
   <p>This paper addresses NB comment CH 11 in <a data-link-type="biblio" href="#biblio-p0488r0">[P0488R0]</a></p>
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li><a href="#bg"><span class="secno">1</span> <span class="content">Background</span></a>
    <li><a href="#word"><span class="secno">2</span> <span class="content">Proposed Wording</span></a>
    <li>
     <a href="#discuss"><span class="secno">3</span> <span class="content">Discussion and Justification</span></a>
     <ol class="toc">
      <li><a href="#defpred"><span class="secno">3.1</span> <span class="content">The "Default Function Object" Clause</span></a>
     </ol>
    <li><a href="#summary"><span class="secno">4</span> <span class="content">Summary</span></a>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="bg"><span class="secno">1. </span><span class="content">Background</span><a class="self-link" href="#bg"></a></h2>
   <p>National Body comment CH11 from <a data-link-type="biblio" href="#biblio-p0488r0">[P0488R0]</a> states:</p>
   <blockquote>
    <p>Comments: It may be useful to copy objects to a separate space for non-sequenced policies.</p>
    <p>Proposed Change: Add explicit allowance for non-sequenced policies to copy the objects
they work on.</p>
   </blockquote>
   <p>Discussion in SG1 at Issaquah on this comment led to the suggestion that, at minimum, wording should be added to some or all of the parallel algorithms allowing the implementation to make copies of arguments to the function objects passed to a <em>parallel algorithm</em> [algorithms.parallel.defns].  A straw poll revealed unanimous agreement that a paper exploring this idea should be written.</p>
   <p>In particular, it was suggested that we examine the consequences of expanding or elaborating on the clause in [algorithms.parallel.user] to include language allowing the implementation to make copies of arguments to certain function objects under certain circumstances.</p>
   <h2 class="heading settled" data-level="2" id="word"><span class="secno">2. </span><span class="content">Proposed Wording</span><a class="self-link" href="#word"></a></h2>
   <p>With discussion to follow below (<a href="#discuss">§3 Discussion and Justification</a>), we recommend that the committee change paragraph 1 of [algorithms.parallel.user] to:</p>
   <blockquote>
    <ins>Unless otherwise specified,</ins>
     function objects passed into parallel algorithms as objects of type <code class="highlight"><span class="n">Predicate</span></code>, <code class="highlight"><span class="n">BinaryPredicate</span></code>, <code class="highlight"><span class="n">Compare</span></code>, 
    <ins>UnaryOperation</ins>
     and <code class="highlight"><span class="n">BinaryOperation</span></code> 
    <ins>and the operators used by the analogous overloads to these parallel algorithms that could be formed by the invocation with the specified default predicate or operation (where applicable)</ins>
     shall not directly or indirectly modify objects via their arguments
    <ins>, which may be a copy of the original object(s) from the range(s) on which the algorithm operates.  Copy operations for this purpose may only be performed on <em>trivially copyable</em> types ([basic.types]).</ins>
    <p></p>
    <ins>[<em>Note:</em> Programmers for whom the object identity of the arguments to these function objects is important should consider using a wrapping iterator that returns a noncopied implementation object such as <code class="highlight"><span class="n">reference_wrapper</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span></code> [refwrap] or some equivalent solution. -<em>end note</em>]</ins>
   </blockquote>
   <h2 class="heading settled" data-level="3" id="discuss"><span class="secno">3. </span><span class="content">Discussion and Justification</span><a class="self-link" href="#discuss"></a></h2>
   <p>For just about all of the algorithms, the primary concern is that a predicate or comparison operator would rely on some property of the arguments’ addresses being consistent, whether in an absolute sense or a relative sense.  (The more general phrasing for this concept is "object identity," but the most relevant aspect of object identity in the current context is the object’s address, so the two terms will be used interchangeably here.)  It is important to note that in every case, the parallel algorithm <em>could</em> still be used with a predicate reliant on the address if a layer of indirection that preserves the pertinent portion of the object’s address information (e.g., through a wrapping iterator).  Thus, it is not the actual breadth of capabilities provided by the parallel algorithm specifications that is being considered here; rather, it is the trade-offs between the cost of requiring the user to implement such a layer of indirection and the benefits of implementation flexibility with greater accessibility to performance optimizations.</p>
   <p>In most cases, the algorithms relevant to the wording in <a href="#word">§2 Proposed Wording</a> that require the same type of function object pose similar concerns to each other.  Of greatest concern are those functions accepting <code class="highlight"><span class="n">Predicate</span></code> function objects.  A typical case is <code class="highlight"><span class="n">find_if</span></code> ([alg.find]), where the function object could want to check for equality with a known object by doing address-wise comparison.  However, it is not uncommon for these use cases to have some other need for indirection that would also make this work (for instance, in-place sorting by address).</p>
   <p>In the SG1 meeting at Issaquah, the first case raised for discussion that takes a <code class="highlight"><span class="n">Compare</span></code> function object was <code class="highlight"><span class="n">sort</span></code> ([alg.sort]).  However, given that the standard library implementation of this algorithm is in-place, comparison based on the address of the arguments would be nonsensical, given that the objects’ addresses would change as the sort progresses.  Other needs for preservation of object identity (in the context of <code class="highlight"><span class="n">sort</span></code>) could not immediately be devised, and we have not come up with any further use cases after some thought.  For the non-modifying parallel algorithms that use <code class="highlight"><span class="n">Compare</span></code> (such as <code class="highlight"><span class="n">min_element</span></code>, [alg.min.max]), the conceivable use cases for the addresses of the arguments to <code class="highlight"><span class="n">Compare</span></code> are esoteric enough that the imposition of an indirection requirement is not onerous.</p>
   <p>Most of the algorithms that take a <code class="highlight"><span class="n">BinaryPredicate</span></code> argument are even less problematic that the first two.  An example here would be <code class="highlight"><span class="n">unique</span></code> [alg.unique].  We considered an amendment of the wording in <a href="#word">§2 Proposed Wording</a> for the case of <code class="highlight"><span class="n">BinaryPredicate</span></code> that would require the relationship (in terms of total ordering) between the addresses of the arguments to be the same as in the original objects, but we decided against this in the interest of reduced conceptual overhead.</p>
   <p>The least problematic cases are those that take a <code class="highlight"><span class="n">UnaryOperation</span></code> or <code class="highlight"><span class="n">BinaryOperation</span></code> function object (note that <code class="highlight"><span class="n">for_each</span></code> [alg.foreach] is specifically excluded from this, since it takes an argument with the template parameter name <code class="highlight"><span class="n">Function</span></code> instead).  An example of this is <code class="highlight"><span class="n">transform</span></code> [alg.transform].  Since these take an argument (or two arguments) and return an object by value (for which the function object has no control over object identity, once copied), it is hard to imagine many use cases where the object identity of the arguments to these function objects is important, since that of the return value cannot be.  We do not claim that there are no use cases here affected by the proposed change, but it is a pretty reasonable argument that the potential benefits in terms of implementation flexibility outweigh the inconvenience of requiring indirection in these corner cases.</p>
   <p>The note in <a href="#word">§2 Proposed Wording</a> is intended to mimic that of the note in paragraph 10 of [algorithms.general], which references similar interactions with the function objects themselves.  In other words, the standard already prohibits the reliance on the object identity of these function objects themselves, so the proposed extension of this prohibition to the function objects' arguments is not unprecedented.</p>
   <h3 class="heading settled" data-level="3.1" id="defpred"><span class="secno">3.1. </span><span class="content">The "Default Function Object" Clause</span><a class="self-link" href="#defpred"></a></h3>
   <p>One more piece of the proposed wording in <a href="#word">§2 Proposed Wording</a> requires further elaboration.  We have inserted the following wording relating to the "default" function objects used by these algorithms:</p>
   <blockquote>
     [...] 
    <ins>and the operators used by the analogous overloads to these parallel algorithms that could be formed by the invocation with the specified default predicate or operation (where applicable) </ins>
     [...] 
   </blockquote>
   <p>Consider the following code:</p>
<pre class="highlight"><span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">></span>
<span class="kt">void</span> <span class="n">call_a</span><span class="p">(</span><span class="n">vector</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span><span class="o">&amp;</span> <span class="n">seq</span><span class="p">)</span> <span class="p">{</span>
  <span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">sort</span><span class="p">(</span><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">par</span><span class="p">,</span> <span class="n">seq</span><span class="p">.</span><span class="n">begin</span><span class="p">(</span><span class="p">)</span><span class="p">,</span> <span class="n">seq</span><span class="p">.</span><span class="n">end</span><span class="p">(</span><span class="p">)</span><span class="p">)</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">></span>
<span class="kt">void</span> <span class="n">call_b</span><span class="p">(</span><span class="n">vector</span><span class="o">&lt;</span><span class="n">T</span><span class="o">></span><span class="o">&amp;</span> <span class="n">seq</span><span class="p">)</span> <span class="p">{</span>
  <span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">sort</span><span class="p">(</span><span class="n">std</span><span class="o">:</span><span class="o">:</span><span class="n">par</span><span class="p">,</span> <span class="n">seq</span><span class="p">.</span><span class="n">begin</span><span class="p">(</span><span class="p">)</span><span class="p">,</span> <span class="n">seq</span><span class="p">.</span><span class="n">end</span><span class="p">(</span><span class="p">)</span><span class="p">,</span>
    <span class="p">[</span><span class="o">&amp;</span><span class="p">]</span><span class="p">(</span><span class="n">T</span> <span class="k">const</span><span class="o">&amp;</span> <span class="n">a</span><span class="p">,</span> <span class="n">T</span> <span class="k">const</span><span class="o">&amp;</span> <span class="n">b</span><span class="p">)</span><span class="p">{</span> <span class="k">return</span> <span class="n">a</span> <span class="o">&lt;</span> <span class="n">b</span><span class="p">;</span> <span class="p">}</span>
  <span class="p">)</span><span class="p">;</span>
<span class="p">}</span>
</pre>
   <p>From the description of <code class="highlight"><span class="n">sort</span></code> and other algorithms that take a <code class="highlight"><span class="n">Compare</span></code> function object ([alg.sorting]), one would expect that <code class="highlight"><span class="n">call_a</span></code> and <code class="highlight"><span class="n">call_b</span></code> should have equivalent behavior, regardless of the type of <code class="highlight"><span class="n">T</span></code>.  But consider the following class with a user-defined <code class="highlight"><span class="k">operator</span><span class="o">&lt;</span><span class="p">(</span><span class="p">)</span></code>:</p>
<pre class="highlight"><span class="k">struct</span> <span class="n">Counted</span> <span class="p">{</span>
  <span class="kt">int</span> <span class="n">value</span><span class="p">;</span>
  <span class="k">mutable</span> <span class="kt">int</span> <span class="n">compares</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
  <span class="kt">bool</span> <span class="k">operator</span><span class="o">&lt;</span><span class="p">(</span><span class="n">Counted</span> <span class="k">const</span><span class="o">&amp;</span> <span class="n">other</span><span class="p">)</span> <span class="k">const</span> <span class="p">{</span>
    <span class="o">+</span><span class="o">+</span><span class="n">compares</span><span class="p">;</span>
    <span class="o">+</span><span class="o">+</span><span class="n">other</span><span class="p">.</span><span class="n">compares</span><span class="p">;</span>
    <span class="k">return</span> <span class="n">value</span> <span class="o">&lt;</span> <span class="n">other</span><span class="p">.</span><span class="n">value</span><span class="p">;</span>
  <span class="p">}</span>
<span class="p">}</span>
</pre>
   <p>The intent of paragraph 1 of <code class="highlight"><span class="p">[</span><span class="n">algorithms</span><span class="p">.</span><span class="n">parallel</span><span class="p">.</span><span class="n">user</span><span class="p">]</span></code> is clearly to prohibit this sort of behavior, but under the current wording, invoking <code class="highlight"><span class="n">call_a</span></code> with a <code class="highlight"><span class="n">vector</span><span class="o">&lt;</span><span class="n">Counted</span><span class="o">></span></code> would bee allowed, but invoking <code class="highlight"><span class="n">call_b</span></code> would not, since it gives a user-defined <code class="highlight"><span class="n">Compare</span></code> that modifies objects via the arguments.  The extension of this prohibition to argument object identity, consistent with the rest of this discussion, also makes sense.</p>
   <h2 class="heading settled" data-level="4" id="summary"><span class="secno">4. </span><span class="content">Summary</span><a class="self-link" href="#summary"></a></h2>
   <p>As we see it, the positives and negative of adopting the above wording can be summarized as follows:</p>
   <p>Positives:</p>
   <ul>
    <li data-md="">
     <p>Increased implementation flexibility</p>
     <ul>
      <li data-md="">
       <p>Greater potential for implementation performance would likely lead to wider adoption.</p>
     </ul>
    <li data-md="">
     <p>Better default behavior in future implementations (when such behaviors as the one discussed herein could be subsumed into standard options on an Executor <a data-link-type="biblio" href="#biblio-p0443r0">[P0443R0]</a> or <code class="highlight"><span class="n">ExecutionPolicy</span></code>)</p>
     <ul>
      <li data-md="">
       <p>In particular, some members of SG1 were keen to make sure that C++17 code should have reasonable compatibility "by default" with future implementations that may want to copy arguments to memory addresses not accessible from the calling context (e.g., a GPGPU; note that there is currently no concept of inaccessible memory under the memory model of the current standard).</p>
     </ul>
   </ul>
   <p>Negatives:</p>
   <ul>
    <li data-md="">
     <p>Increased conceptual overhead</p>
     <ul>
      <li data-md="">
       <p>This effect is likely negligible since virtually all violating uses proposed after substantial discussion would be corner cases.</p>
     </ul>
    <li data-md="">
     <p>Users who want address-dependent behavior need to implement a wrapper</p>
     <ul>
      <li data-md="">
       <p>This drawback is made less severe since these users are already providing a custom function object anyway.</p>
     </ul>
   </ul>
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  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-p0443r0">[P0443R0]
   <dd>Jared Hoberock, Michael Garland, Chris Kohlhoff, Chris Mysen, Carter Edwards. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0443r0.html">A Unified Executors Proposal for C++</a>. 17 October 2016. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0443r0.html">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0443r0.html</a>
   <dt id="biblio-p0488r0">[P0488R0]
   <dd>Barry Hedquist. <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0488r0.pdf">WG21 Working paper: NB Comments, ISO/IEC CD 14882</a>. 18 October 2016. URL: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0488r0.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0488r0.pdf</a>
  </dl>