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<!DOCTYPE html>
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<title>Issue 4231: datapar::chunk&lt;N&gt; should use simd-size-type instead of size_t</title>
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<meta property="og:title" content="Issue 4231: datapar::chunk&lt;N&gt; should use simd-size-type instead of size_t">
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<meta property="og:description" content="C++ library issue. Status: New">
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#New">New</a> status.</em></p>
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<h3 id="4231"><a href="lwg-active.html#4231">4231</a>. <code>datapar::chunk&lt;N&gt;</code> should use <code><i>simd-size-type</i></code> instead of <code>size_t</code></h3>
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<p><b>Section:</b> 29.10.3 <a href="https://wg21.link/simd.syn">[simd.syn]</a>, 29.10.7.8 <a href="https://wg21.link/simd.creation">[simd.creation]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
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<b>Submitter:</b> Matthias Kretz <b>Opened:</b> 2025-03-22 <b>Last modified:</b> 2025-03-22</p>
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<p><b>Priority: </b>Not Prioritized
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</p>
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<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
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<p><b>Discussion:</b></p>
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<p>
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All index values and simd widths in subclause "Data-parallel types" use the
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type <code><i>simd-size-type</i></code>. Specifically, the NTTP of <code class='backtick'>std::datapar::resize</code>
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uses <code><i>simd-size-type</i></code> and <code class='backtick'>std::datapar::chunk</code> is "implemented"
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using <code class='backtick'>std::datapar::resize</code>.
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<p/>
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For <code>chunk&lt;N&gt;</code>, <code>N</code> is of type <code class='backtick'>size_t</code> and needs to be
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converted to <code><i>simd-size-type</i></code> in the effects clause where it is
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passed to resize. The NTTP of chunk should use <code><i>simd-size-type</i></code>
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instead of <code class='backtick'>size_t</code>.
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</p>
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<p id="res-4231"><b>Proposed resolution:</b></p>
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<p>
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This wording is relative to <a href="https://wg21.link/N5008">N5008</a>.
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</p>
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<ol>
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<li><p>Modify 29.10.3 <a href="https://wg21.link/simd.syn">[simd.syn]</a>, header <code>&lt;simd&gt;</code> synopsis, as indicated:</p>
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<blockquote>
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<pre>
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namespace std::datapar {
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[&hellip;]
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template&lt;<ins><i>simd-size-type</i></ins><del>size_t</del> N, class T, class Abi&gt;
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constexpr auto chunk(const basic_simd&lt;T, Abi&gt;&amp; x) noexcept;
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template&lt;<ins><i>simd-size-type</i></ins><del>size_t</del> N, size_t Bytes, class Abi&gt;
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constexpr auto chunk(const basic_simd_mask&lt;Bytes, Abi&gt;&amp; x) noexcept;
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[&hellip;]
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}
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</pre>
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</blockquote>
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</li>
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<li><p>Modify 29.10.7.8 <a href="https://wg21.link/simd.creation">[simd.creation]</a> as indicated:</p>
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<blockquote>
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<pre>
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template&lt;<ins><i>simd-size-type</i></ins><del>size_t</del> N, class T, class Abi&gt;
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constexpr auto chunk(const basic_simd&lt;T, Abi&gt;&amp; x) noexcept;
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</pre>
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<blockquote>
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<p>
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-4- <i>Effects</i>: Equivalent to: <code>return chunk&lt;resize_t&lt;N, basic_simd&lt;T, Abi&gt;&gt;&gt;(x);</code>
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</p>
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<pre>
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template&lt;<ins><i>simd-size-type</i></ins><del>size_t</del> N, size_t Bytes, class Abi&gt;
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constexpr auto chunk(const basic_simd_mask&lt;Bytes, Abi&gt;&amp; x) noexcept;
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</pre>
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<blockquote>
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<p>
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-5- <i>Effects</i>: Equivalent to: <code>return chunk&lt;resize_t&lt;N, basic_simd_mask&lt;Bytes, Abi&gt;&gt;&gt;(x);</code>
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</p>
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</blockquote>
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</body>
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</html>

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