110 lines
8.9 KiB
HTML
110 lines
8.9 KiB
HTML
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<title>std::kill_dependency</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_atomic_kill_dependency skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>kill_dependency</h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-atomic.html">atomic</a></span></div>
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<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
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<table class="t-dcl-begin">
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<td>
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<div>定义于头文件 <code><a href="cpp-header-atomic.html"><atomic></a></code></div>
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<td></td>
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<td></td>
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<tr class="t-dcl t-since-cxx11">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> T <span class="sy1">></span><br />
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T kill_dependency<span class="br0">(</span> T y <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<p>告诉编译器 <span class="t-lc"><a href="cpp-atomic-memory_order.html">std::memory_order_consume</a></span> 原子加载操作所开始的依赖树不会扩张过 <code>std::kill_dependency</code> 的返回值；即参数不会将依赖携带进返回值。</p>
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<p>这可用于在依赖链离开函数作用域（而函数无 <code>[[<a href="cpp-language-attributes-carries_dependency.html">carries_dependency</a>]]</code> 属性）时，避免不必要的 <span class="t-lc"><a href="cpp-atomic-memory_order.html">std::memory_order_acquire</a></span> 栅栏。</p>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<table class="t-par-begin">
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<tr class="t-par">
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<td>y</td>
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<td>-</td>
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<td>要从依赖树移除其返回值的表达式</td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
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<p>返回 <code>y</code> ，它不再是依赖树的一部分。</p>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<dl>
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<dt>file1.cpp:</dt>
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</dl>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="kw1">struct</span> foo <span class="br0">{</span> <span class="kw4">int</span><span class="sy2">*</span> a<span class="sy4">;</span> <span class="kw4">int</span><span class="sy2">*</span> b<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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<a href="cpp-atomic-atomic.html"><span class="kw2066">std::<span class="me2">atomic</span></span></a><span class="sy1"><</span><span class="kw1">struct</span> foo<span class="sy2">*</span><span class="sy1">></span> foo_head<span class="br0">[</span><span class="nu0">10</span><span class="br0">]</span><span class="sy4">;</span>
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<span class="kw4">int</span> foo_array<span class="br0">[</span><span class="nu0">10</span><span class="br0">]</span><span class="br0">[</span><span class="nu0">10</span><span class="br0">]</span><span class="sy4">;</span>
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<span class="co1">// 消费操作开始依赖链，它将脱离此函数</span>
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<span class="br0">[</span><span class="br0">[</span>carries_dependency<span class="br0">]</span><span class="br0">]</span> <span class="kw1">struct</span> foo<span class="sy2">*</span> f<span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">return</span> foo_head<span class="br0">[</span>i<span class="br0">]</span>.<span class="me1">load</span><span class="br0">(</span>memory_order_consume<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="co1">// 依赖链通过有参数进入此函数，而在函数前被杀掉（故不发生额外的获得操作）</span>
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<span class="kw4">int</span> g<span class="br0">(</span><span class="kw4">int</span><span class="sy2">*</span> x, <span class="kw4">int</span><span class="sy2">*</span> y <span class="br0">[</span><span class="br0">[</span>carries_dependency<span class="br0">]</span><span class="br0">]</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">return</span> std<span class="sy4">::</span><span class="me2">kill_dependency</span><span class="br0">(</span>foo_array<span class="br0">[</span><span class="sy2">*</span>x<span class="br0">]</span><span class="br0">[</span><span class="sy2">*</span>y<span class="br0">]</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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<dl>
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<dt>file2.cpp:</dt>
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</dl>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="br0">[</span><span class="br0">[</span>carries_dependency<span class="br0">]</span><span class="br0">]</span> <span class="kw1">struct</span> foo<span class="sy2">*</span> f<span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">int</span> g<span class="br0">(</span><span class="kw4">int</span><span class="sy2">*</span> x, <span class="kw4">int</span><span class="sy2">*</span> y <span class="br0">[</span><span class="br0">[</span>carries_dependency<span class="br0">]</span><span class="br0">]</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">int</span> c <span class="sy1">=</span> <span class="nu0">3</span><span class="sy4">;</span>
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<span class="kw4">void</span> h<span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">struct</span> foo<span class="sy2">*</span> p<span class="sy4">;</span>
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p <span class="sy1">=</span> f<span class="br0">(</span>i<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// f 内开始的依赖链持续进入 p ，而无过度的获得</span>
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do_something_with<span class="br0">(</span>g<span class="br0">(</span><span class="sy3">&</span>c, p<span class="sy2">-</span><span class="sy1">></span>a<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// p->b 不从缓存进口</span>
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do_something_with<span class="br0">(</span>g<span class="br0">(</span>p<span class="sy2">-</span><span class="sy1">></span>a, <span class="sy3">&</span>c<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 左参数无 carries_dependency 属性：</span>
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<span class="co1">// 可能发生内存获得栅栏</span>
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<span class="co1">// p->b 在进入 g() 前变为可见</span>
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<span class="br0">}</span></pre></div>
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</div>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<table class="t-dsc-begin">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-atomic-memory_order.html"><span class="t-lines"><span>memory_order</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>为给定的原子操作定义内存顺序约束<br />
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<span class="t-mark">(枚举)</span></td>
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