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<!DOCTYPE html>
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<title>Issue 4238: simd_mask&lt;complex&lt;double&gt;&gt;::operator+/-/~ return a disabled simd specialization</title>
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<meta property="og:title" content="Issue 4238: simd_mask&lt;complex&lt;double&gt;&gt;::operator+/-/~ return a disabled simd specialization">
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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="4238"><a href="lwg-active.html#4238">4238</a>. <code>simd_mask&lt;complex&lt;double&gt;&gt;::operator+/-/~</code> return a disabled <code class='backtick'>simd</code> specialization</h3>
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<p><b>Section:</b> 29.10.8.4 <a href="https://wg21.link/simd.mask.unary">[simd.mask.unary]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
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<b>Submitter:</b> Matthias Kretz <b>Opened:</b> 2025-03-27 <b>Last modified:</b> 2025-03-29</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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Consider:
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</p>
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<blockquote><pre>
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simd&lt;complex&lt;double&gt;&gt; c = {};
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simd_mask&lt;complex&lt;double&gt;&gt; k = (c == c); // sizeof(complex&lt;double&gt;) == 16
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auto i = -k; // simd&lt;__int128&gt; !
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</pre></blockquote>
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<p>
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<code class='backtick'>basic_simd_mask</code> unary <code class='backtick'>+</code>, <code class='backtick'>-</code>, and <code class='backtick'>~</code> return <code>basic_simd&lt;<i>integer-from</i>&lt;Bytes, Abi&gt;&gt;</code>.
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29.10.2.1 <a href="https://wg21.link/simd.expos.defn">[simd.expos.defn]</a>/2 says:
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</p>
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<blockquote style="border-left: 3px solid #ccc;padding-left: 15px;">
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<p>
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<code><i>integer-from</i>&lt;Bytes&gt;</code> is an alias for a signed integer type <code class='backtick'>T</code> such that <code class='backtick'>sizeof(T)</code> equals <code class='backtick'>Bytes</code>.
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</p>
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</blockquote>
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<p>
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But <code class='backtick'>__int128</code> isn't a vectorizable type. Consequently, <code>simd&lt;__int128&gt;</code> currently
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is a disabled specialization. So it seems <code>simd&lt;complex&lt;double&gt;&gt;</code> wants <code class='backtick'>__int128</code>
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to be added to the list of vectorizable types.
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<p/>
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<b>Options:</b>
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</p>
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<ol style="list-style-type:lower-alpha">
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<li><p>"Specialize" <code>basic_simd_mask&lt;16, Abi&gt;</code> to return
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<code>rebind_t&lt;<i>integer-from</i>&lt;8&gt;, basic_simd_mask&lt;<i>integer-from</i>&lt;16&gt;, Abi&gt;&gt;</code>
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(reduces a 128-bit value of a vector-mask to a 64-bit value in the simd)</p></li>
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<li><p>"Specialize" <code>basic_simd_mask&lt;16, Abi&gt;</code> to return
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<code>resize_t&lt;size() * 2, rebind_t&lt;<i>integer-from</i>&lt;8&gt;,
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basic_simd_mask&lt;<i>integer-from</i>&lt;16&gt;, Abi&gt;&gt;&gt;</code>
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(duplicates a 128-bit value of a vector-mask to two 64-bit values in the simd)</p></li>
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<li><p>delete unary <code class='backtick'>+</code>, <code class='backtick'>-</code>, and <code class='backtick'>~</code> for <code>basic_simd_mask&lt;16, Abi&gt;</code>
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(closest to the status quo)</p></li>
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</ol>
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<p id="res-4238"><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 29.10.8.4 <a href="https://wg21.link/simd.mask.unary">[simd.mask.unary]</a> as indicated:</p>
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<blockquote>
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<pre>
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constexpr basic_simd_mask operator!() const noexcept;
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constexpr basic_simd&lt;<i>integer-from</i>&lt;Bytes&gt;, Abi&gt; operator+() const noexcept;
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constexpr basic_simd&lt;<i>integer-from</i>&lt;Bytes&gt;, Abi&gt; operator-() const noexcept;
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constexpr basic_simd&lt;<i>integer-from</i>&lt;Bytes&gt;, Abi&gt; operator~() const noexcept;
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</pre>
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<blockquote>
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<p>
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-1- Let <code><i>op</i></code> be the operator.
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<p/>
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-2- <i>Returns</i>: A data-parallel object where the <code><i>i</i></code><sup>th</sup> element
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is initialized to the results of applying <code><i>op</i></code> to <code>operator[](<i>i</i>)</code>
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for all <code><i>i</i></code> in the range of <code>[0, size())</code>.
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<p/>
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<ins>-?- <i>Remarks</i>: If <code>Bytes</code> is greater than <code>8</code>, <code class='backtick'>operator+()</code>, <code class='backtick'>operator-()</code>, and
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<code class='backtick'>operator~()</code> are deleted.
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</ins>
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</p>
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</blockquote>
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</html>

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