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diff --git a/2023/05/01/Rust-虚表布局规则介绍/index.html b/2023/05/01/Rust-虚表布局规则介绍/index.html new file mode 100644 index 00000000..4dfa968b --- /dev/null +++ b/2023/05/01/Rust-虚表布局规则介绍/index.html @@ -0,0 +1,309 @@ +<!DOCTYPE html> +<html lang="en"> + <head> + <meta charset="UTF-8"> +<meta name="viewport" + content="width=device-width, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0"> +<meta http-equiv="X-UA-Compatible" content="ie=edge"> + + <meta name="author" content="韩暮秋"> + + + <meta name="subtitle" content="暮秋小屋"> + + + <meta name="description" content="这里是暮秋小屋，思念和灵感的寄存处"> + + + <meta name="keywords" content="韩暮秋,MuqiuHan,'Muqiu Han', 'muqiu han', muqiuhan"> + + + + +<title>Rust 虚表布局规则介绍 | 暮秋小屋</title> + + + + <link rel="icon" href="/favicon.ico"> + + + +<style> + @import url('https://fonts.googleapis.com/css2?family=Inter:wght@300;400;500;600;700&family=Noto+Sans+SC:wght@300;400;500;700&family=Roboto+Mono&display=swap'); +</style> + + + + <!-- stylesheets list from _config.yml 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<li class="menu-item"> + <a href="/tags/Technique/">计算机科学</a> + </li> + + + + <li class="menu-item"> + <a href="/tags/Life/">生活</a> + </li> + + + + <li class="menu-item"> + <a href="/about">关于</a> + </li> + + + + <li class="menu-item search-btn"> + <a href="#">Search</a> + </li> + + </ul> + </nav> + </div> + </div> +</div> + + + <div class="main"> + <div class="flex-container"> + <article id="post"> + + + <div class="post-head"> + <div class="post-info"> + <div class="tag-list"> + + + <span class="post-tag"> + <a href="/tags/Technique/"> + Technique + </a> + </span> + + <span class="post-tag"> + <a href="/tags/Rust/"> + Rust + </a> + </span> + + + </div> + <div class="post-title"> + + + Rust 虚表布局规则介绍 + + + </div> + <span class="post-date"> + May 1, 2023 + </span> + </div> + <div class="post-img"> + + <div class="h-line-primary"></div> + + </div> +</div> + <div class="post-content"> + <p>在 Rust 中，一个指向未知大小对象（!Sized）的引用或指针被实现为一个由两个 usize 大小的域构成的胖指针。这两个域中，其中一个域保存了被引用或被指向的对象的地址，另一个域保存了一个名为 metadata 的数据。对于 slice 的引用或指针来说，其 metadata 为 slice 的长度。对于 trait object 的引用或指针来说，其 metadata 为虚表（vtable）地址。与 C++ 虚表类似，Rust 虚表的存在使得诸多动态语言特性得以实现，例如动态派发（dynamic dispatch）、向上转换（upcasting）、向下转换（downcast）等。本文将对 Rust 中虚表的布局规则进行简要介绍，并在此过程中对 Rust 中若干动态特性的实现方法进行简要介绍。</p> +<blockquote> +<p>注意：Rust 虚表及其结构属于 Rust 语言的内部实现细节，不保证稳定性。本文所介绍的虚表布局仅反映本文创作时最新的 Rust 虚表结构[1]，在将来 Rust 虚表结构可能会发生变化。一个 Rust 程序的正确性不应该以任何方式依赖于 Rust 虚表的结构。</p> +</blockquote> +<h2 id="基本结构"><a href="#基本结构" class="headerlink" title="基本结构"></a>基本结构</h2><p>Rust 程序中的所有虚表均以一个固定结构的 header 开头。Header 中按顺序包含三个usize 大小的字段：drop_in_place ，size 和 align 。在 header 之后是一系列的 usize 大小字段，其数量以及含义在每个虚表中可能都不同。</p> +<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line">+---------------+</span><br><span class="line">| drop_in_place |</span><br><span class="line">+---------------+</span><br><span class="line">| size |</span><br><span class="line">+---------------+</span><br><span class="line">| align |</span><br><span class="line">+---------------+</span><br><span class="line">| entry1 |</span><br><span class="line">+---------------+</span><br><span class="line">| entry2 |</span><br><span class="line">+---------------+</span><br><span class="line">| entry3 |</span><br><span class="line">+---------------+</span><br></pre></td></tr></table></figure> +<p>虚表 header 中的drop_in_place 是一个函数指针，其指向的函数能够原地 drop 当前胖指针所引用的对象。size 和 align 两个域分别给出对象的大小和内存对齐，这两个域共同构成一个 std::alloc::Layout 结构，可用于释放当前胖指针所引用的对象所占据的内存。虚表 header 的存在使得 trait object 总是能被销毁和释放。例如当销毁一个 <code>Box<dyn Trait></code> 时，<code>Box::<dyn Trait>::drop</code> 会首先调用虚表中的 drop_in_place 函数原地销毁 Box 所引用的对象，然后再调用 dealloc 函数并传递虚表中的 size 和 align 释放堆空间。</p> +<p>在虚表 header 之后是一系列的字段。在最普遍的情况下，每个字段代表一个指向 trait 所定义的函数的指针。例如，对于下列 object safe 的 trait:</p> +<figure class="highlight rust"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">pub</span> <span class="keyword">trait</span> <span class="title class_">Trait</span> {</span><br><span class="line"> <span class="keyword">fn</span> <span class="title function_">fun1</span>(&<span class="keyword">self</span>);</span><br><span class="line"> <span class="keyword">fn</span> <span class="title function_">fun2</span>(&<span class="keyword">self</span>);</span><br><span class="line"> <span class="keyword">fn</span> <span class="title function_">fun3</span>(&<span class="keyword">self</span>);</span><br><span class="line">}</span><br></pre></td></tr></table></figure> + +<p>如果类型T 实现了 Trait，那么为 T 生成的 Trait 虚表的结构为：</p> +<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line">+--------------------------+</span><br><span class="line">| fn drop_in_place(*mut T) |</span><br><span class="line">+--------------------------+</span><br><span class="line">| size of T |</span><br><span class="line">+--------------------------+</span><br><span class="line">| align of T |</span><br><span class="line">+--------------------------+</span><br><span class="line">| fn <T as Trait>::fun1 |</span><br><span class="line">+--------------------------+</span><br><span class="line">| fn <T as Trait>::fun2 |</span><br><span class="line">+--------------------------+</span><br><span class="line">| fn <T as Trait>::fun3 |</span><br><span class="line">+--------------------------+</span><br></pre></td></tr></table></figure> + +<p>Trait 中的函数按照声明顺序依次排列在虚表 header 之后。当通过一个指向 T 对象的 &dyn Trait 调用 fun2 函数时，程序会先从虚表的第 5 个域中得到为 T 实现的 Trait::fun2 函数的地址，然后再调用之。</p> +<h2 id="Super-Trait"><a href="#Super-Trait" class="headerlink" title="Super Trait"></a>Super Trait</h2><p>Object safe 的 trait 可以有 super trait。例如：</p> +<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line">pub trait Grand {</span><br><span class="line"> fn grand_fun1(&self);</span><br><span class="line"> fn grand_fun2(&self);</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">pub trait Parent : Grand {</span><br><span class="line"> fn parent_fun1(&self);</span><br><span class="line"> fn parent_fun2(&self);</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">pub trait Trait : Parent {</span><br><span class="line"> fn fun(&self);</span><br><span class="line">}</span><br></pre></td></tr></table></figure> + +<p>如果类型T 实现了 Trait，那么此时为 T 生成的 Trait 虚表的结构为：</p> +<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line">+-------------------------------+</span><br><span class="line">| fn drop_in_place(*mut T) |</span><br><span class="line">+-------------------------------+</span><br><span class="line">| size of T |</span><br><span class="line">+-------------------------------+</span><br><span class="line">| align of T |</span><br><span class="line">+-------------------------------+</span><br><span class="line">| fn <T as Grand>::grand_fun1 |</span><br><span class="line">+-------------------------------+</span><br><span class="line">| fn <T as Grand>::grand_fun2 |</span><br><span class="line">+-------------------------------+</span><br><span class="line">| fn <T as Parent>::parent_fun1 |</span><br><span class="line">+-------------------------------+</span><br><span class="line">| fn <T as Parent>::parent_fun2 |</span><br><span class="line">+-------------------------------+</span><br><span class="line">| fn <T as Trait>::fun |</span><br><span class="line">+-------------------------------+</span><br></pre></td></tr></table></figure> + +<p>可以看到，此时Trait 以及 Trait 的所有直接或间接父 trait 所定义的所有函数均包含在虚表 header 之后，且顺序为后序（即先排布 Trait 的父 trait 所定义的所有函数，最后再排布 Trait 所定义的所有函数）。这样的排布方式使得在得到 T 类型的 Trait 虚表的同时也同时得到了 T 类型的 Parent 虚表和 Grand 虚表。T 类型的 Grand 虚表恰好由 Trait 虚表的前五个域构成，T 类型的 Parent 虚表恰好由 Trait 虚表的前七项构成。这使得向上转换变得非常简单。</p> +<p>所谓向上转换，即 Rust 允许将&dyn Trait 转换为 &dyn Parent 或 &dyn Grand 。在向上转换的过程中，胖指针的对象地址域保持不变，但 metadata 域可能需要进行调整，因为不同的 trait 可能具有不同的虚表地址。但在当前示例中，向上转换不需要调整 metadata 域，因为一个指向 Trait 虚表的指针同时也指向 Parent 虚表和 Grand 虚表。在后文中我们会进一步介绍需要调整 metadata 域的向上转换的情况。</p> +<blockquote> +<p>注意：目前 stable Rust 暂不支持向上转换。要使用向上转换特性，必须使用 nightly 工具链，并向源文件中添加 #![feature(trait_upcasting)] 特性开关。</p> +</blockquote> +<h2 id="多重继承"><a href="#多重继承" class="headerlink" title="多重继承"></a>多重继承</h2><p>Trait 可以有多个 super trait。例如：</p> +<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line">pub trait Base {</span><br><span class="line"> fn base_fun1(&self);</span><br><span class="line"> fn base_fun2(&self);</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">pub trait Left : Base {</span><br><span class="line"> fn left_fun1(&self);</span><br><span class="line"> fn left_fun2(&self);</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">pub trait Right : Base {</span><br><span class="line"> fn right_fun1(&self);</span><br><span class="line"> fn right_fun2(&self);</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">pub trait Trait : Left + Right {</span><br><span class="line"> fn fun(&self);</span><br><span class="line">}</span><br></pre></td></tr></table></figure> + +<p>如果类型T 实现了 Trait，那么此时为 T 生成的 Trait 虚表的结构为：</p> +<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br></pre></td><td class="code"><pre><span class="line">+-----------------------------+</span><br><span class="line">| fn drop_in_place(*mut T) |</span><br><span class="line">+-----------------------------+</span><br><span class="line">| size of T |</span><br><span class="line">+-----------------------------+</span><br><span class="line">| align of T |</span><br><span class="line">+-----------------------------+</span><br><span class="line">| fn <T as Base>::base_fun1 |</span><br><span class="line">+-----------------------------+</span><br><span class="line">| fn <T as Base>::base_fun2 |</span><br><span class="line">+-----------------------------+</span><br><span class="line">| fn <T as Left>::left_fun1 |</span><br><span class="line">+-----------------------------+</span><br><span class="line">| fn <T as Left>::left_fun2 |</span><br><span class="line">+-----------------------------+</span><br><span class="line">| fn <T as Right>::right_fun1 |</span><br><span class="line">+-----------------------------+</span><br><span class="line">| fn <T as Right>::right_fun2 |</span><br><span class="line">+-----------------------------+</span><br><span class="line">| ptr to <T as Right>::vtable |</span><br><span class="line">+-----------------------------+</span><br><span class="line">| fn <T as Trait>::fun |</span><br><span class="line">+-----------------------------+</span><br></pre></td></tr></table></figure> + +<p>可以看到，此时Trait 及其所有直接或间接父 trait 所定义的所有函数仍然包含在虚表内，因此通过 &dyn Trait 调用的函数仍然可以直接从虚表内得到其实际目标函数的地址。另外，Trait 虚表内仍然包含有效的 Base 虚表和 Left 虚表。因此，将 &dyn Trait 向上转换为 &dyn Left 或 &dyn Base 仍然是极其简单的，不需要调整胖指针的 metadata 域。但是，将 &dyn Trait 向上转换为 &dyn Right 就需要调整 metadata 域了，因为 Trait 虚表内并不包含一个有效的 Right 虚表。这也是 Trait 虚表中 ptr to <code><T as Right>::vtable</code> 域的作用：在执行向上转换时，程序会读取 Trait 虚表的这个域作为得到的 &dyn Right 胖指针的 metadata 。这也是 <em>Rust 向上转换与 C++ 向上转换的一个很大不同：在 C++ 中的向上转换通常并不需要访问虚表（除非需要执行跨虚继承边界的转换），但在 Rust 中向上转换可能需要访问虚表。</em></p> +<p>更加一般地，对于一个 object safe 的 traitTr，将其第一个父 trait、第一个父 trait 的第一个父 trait、…… 这一系列直接或间接父 trait 记为这个 trait 的 PrefixTrait 集合。在将 &dyn Tr 向上转换时，如果转换到的目标 trait 包含在 PrefixTrait 集合内，那么这个向上转换是平凡的：不需要调整胖指针的 metadata 域。否则，这个向上转换需要在 Tr 的虚表内读取目标 trait 的虚表指针作为转换结果的 metadata 。在 Tr 的虚表结构中，位于 PrefixTrait 集合中的父 trait 只需要排布他们所定义的函数即可；对于其他父 trait 还需要额外在虚表内排布一个指向其虚表的指针用于向上转换。</p> +<h2 id="向下转换"><a href="#向下转换" class="headerlink" title="向下转换"></a>向下转换</h2><p>Rust 提供了一个特殊的 trait：std::any::Any 。该 trait 支持向下转换，即可以将 &dyn Any 转换为 T 。转换过程中会对胖指针所指向的对象的实际类型进行检查，确认其确实是一个 T 类型的对象。Any trait 的虚表结构有一些特殊；在虚表 header 之后，Any 虚表仅包含一个域，这个域直接给出胖指针指向的对象的类型标识（由一个 std::any::TypeId 类型的值表示）。例如，对于任意的 T: ‘static，编译器为其生成的 Any 虚表为：</p> +<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">+--------------------------+</span><br><span class="line">| fn drop_in_place(*mut T) |</span><br><span class="line">+--------------------------+</span><br><span class="line">| size of T |</span><br><span class="line">+--------------------------+</span><br><span class="line">| align of T |</span><br><span class="line">+--------------------------+</span><br><span class="line">| TypeId of T |</span><br><span class="line">+--------------------------+</span><br></pre></td></tr></table></figure> +<p>在执行向下转换时，程序首先检查转换到的类型是否与虚表中给出的TypeId 所标识的类型一致。若类型检查通过，向下转换操作可以直接返回胖指针中的指针域作为转换结果。</p> +<h3 id="REFERENCE"><a href="#REFERENCE" class="headerlink" title="REFERENCE"></a>REFERENCE</h3><ol> +<li>Vtable format to support dyn upcasting coercion <a target="_blank" rel="noopener" href="https://rust-lang.github.io/dyn-upcasting-coercion-initiative/design-discussions/vtable-layout.*html">https://rust-lang.github.io/dyn-upcasting-coercion-initiative/design-discussions/vtable-layout.*html</a>*</li> +</ol> + +</div> + +<script> + window.onload = detectors(); +</script> + <div class="post-footer"> + <div class="h-line-primary"></div> + <nav class="post-nav"> + <div 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