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<ol class="chapter"><li class="chapter-item affix "><a href="../../about-book.html">关于本书</a></li><li class="chapter-item affix "><a href="../../into-rust.html">进入 Rust 编程世界</a></li><li class="chapter-item affix "><a href="../../first-try/sth-you-should-not-do.html">避免从入门到放弃</a></li><li class="chapter-item affix "><a href="../../community.html">社区和锈书</a></li><li class="spacer"></li><li class="chapter-item affix "><a href="../../some-thoughts.html">Xobserve: 一切皆可观测</a></li><li class="chapter-item affix "><a href="../../beat-ai.html">BeatAI: 工程师 AI 入门圣经</a></li><li class="chapter-item affix "><li class="part-title">Rust 语言基础学习</li><li class="spacer"></li><li class="chapter-item "><a href="../../first-try/intro.html"><strong aria-hidden="true">1.</strong> 寻找牛刀，以便小试</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../first-try/installation.html"><strong aria-hidden="true">1.1.</strong> 安装 Rust 环境</a></li><li class="chapter-item "><a href="../../first-try/editor.html"><strong aria-hidden="true">1.2.</strong> 墙推 VSCode!</a></li><li class="chapter-item "><a href="../../first-try/cargo.html"><strong aria-hidden="true">1.3.</strong> 认识 Cargo</a></li><li class="chapter-item "><a href="../../first-try/hello-world.html"><strong aria-hidden="true">1.4.</strong> 不仅仅是 Hello world</a></li><li class="chapter-item "><a href="../../first-try/slowly-downloading.html"><strong aria-hidden="true">1.5.</strong> 下载依赖太慢了？</a></li></ol></li><li class="chapter-item "><a href="../../basic/intro.html"><strong aria-hidden="true">2.</strong> Rust 基础入门</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../basic/variable.html"><strong aria-hidden="true">2.1.</strong> 变量绑定与解构</a></li><li class="chapter-item "><a href="../../basic/base-type/index.html"><strong aria-hidden="true">2.2.</strong> 基本类型</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../basic/base-type/numbers.html"><strong aria-hidden="true">2.2.1.</strong> 数值类型</a></li><li class="chapter-item "><a href="../../basic/base-type/char-bool.html"><strong aria-hidden="true">2.2.2.</strong> 字符、布尔、单元类型</a></li><li class="chapter-item "><a href="../../basic/base-type/statement-expression.html"><strong aria-hidden="true">2.2.3.</strong> 语句与表达式</a></li><li class="chapter-item "><a href="../../basic/base-type/function.html"><strong aria-hidden="true">2.2.4.</strong> 函数</a></li></ol></li><li class="chapter-item "><a href="../../basic/ownership/index.html"><strong aria-hidden="true">2.3.</strong> 所有权和借用</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../basic/ownership/ownership.html"><strong aria-hidden="true">2.3.1.</strong> 所有权</a></li><li class="chapter-item "><a href="../../basic/ownership/borrowing.html"><strong aria-hidden="true">2.3.2.</strong> 引用与借用</a></li></ol></li><li class="chapter-item "><a href="../../basic/compound-type/intro.html"><strong aria-hidden="true">2.4.</strong> 复合类型</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../../basic/compound-type/string-slice.html"><strong aria-hidden="true">2.4.1.</strong> 字符串与切片</a></li><li class="chapter-item "><a href="../../basic/compound-type/tuple.html"><strong aria-hidden="true">2.4.2.</strong> 元组</a></li><li class="chapter-item "><a href="../../basic/compound-type/struct.html"><strong aria-hidden="true">2.4.3.</strong> 结构体</a></li><li class="chapter-item "><a href="../../basic/compound-type/enum.html"><strong aria-hidden="true">2.4.4.</strong> 枚举</a></li><li class="chapter-item "><a href="../../basic/compound-type/array.html"><strong aria-hidden="true">2.4.5.</strong> 数组</a></l
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<main>
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<h1 id="内联汇编"><a class="header" href="#内联汇编">内联汇编</a></h1>
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<blockquote>
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<p>本章内容对于学习 Rust 不是必须的，而且难度很高，大家简单知道有这回事就好，不必非要学会 :D</p>
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</blockquote>
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<p>Rust 提供了 <code>asm!</code> 宏，可以让大家在 Rust 代码中嵌入汇编代码，对于一些极致高性能或者底层的场景还是非常有用的，例如操作系统内核开发。但通常来说，大家并不应该在自己的项目中使用到该项技术，它为极客而生！</p>
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<p>本章的例子是基于 <code>x86/x86-64</code> 汇编的，但是其它架构也是支持的，目前支持的包括：</p>
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<ul>
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<li>x86 和 x86-64</li>
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<li>ARM</li>
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<li>AArch64</li>
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<li>RISC-V</li>
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</ul>
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<p>当使用在不支持的平台上时，编译器会给出报错。</p>
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<h2 id="基本用法"><a class="header" href="#基本用法">基本用法</a></h2>
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<p>先从一个简单例子开始：</p>
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<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>use std::arch::asm;
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unsafe {
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asm!("nop");
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}
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<span class="boring">}</span></code></pre></pre>
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<p>注意 <code>unsafe</code> 语句块依然是必不可少的，因为可能在里面插入危险的指令，最终破坏代码的安全性。</p>
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<p>上面代码将插入一个 <code>NOP</code> 指令( 空操作 ) 到编译器生成的汇编代码中，其中指令作为 <code>asm!</code> 的第一个参数传入。</p>
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<h2 id="输入和输出"><a class="header" href="#输入和输出">输入和输出</a></h2>
|
|
|
|
|
|
<p>上面的代码有够无聊的，来点实际的:</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>use std::arch::asm;
|
|
|
|
|
|
|
|
|
|
|
|
let x: u64;
|
|
|
|
|
|
unsafe {
|
|
|
|
|
|
asm!("mov {}, 5", out(reg) x);
|
|
|
|
|
|
}
|
|
|
|
|
|
assert_eq!(x, 5);
|
|
|
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
|
|
|
<p>这段代码将 <code>5</code> 赋给 <code>u64</code> 类型的变量 <code>x</code>，值得注意的是 <code>asm!</code> 的指令参数实际上是一个格式化字符串。至于传给格式化字符串的参数，看起来还是比较陌生的:</p>
|
|
|
|
|
|
<ul>
|
|
|
|
|
|
<li>首先，需要说明目标变量是作为内联汇编的输入还是输出，在本例中，是一个输出 <code>out</code></li>
|
|
|
|
|
|
<li>最后，要指定变量将要使用的寄存器，本例中使用通用寄存器 <code>reg</code>，编译器会自动选择合适的寄存器</li>
|
|
|
|
|
|
</ul>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>use std::arch::asm;
|
|
|
|
|
|
|
|
|
|
|
|
let i: u64 = 3;
|
|
|
|
|
|
let o: u64;
|
|
|
|
|
|
unsafe {
|
|
|
|
|
|
asm!(
|
|
|
|
|
|
"mov {0}, {1}",
|
|
|
|
|
|
"add {0}, 5",
|
|
|
|
|
|
out(reg) o,
|
|
|
|
|
|
in(reg) i,
|
|
|
|
|
|
);
|
|
|
|
|
|
}
|
|
|
|
|
|
assert_eq!(o, 8);
|
|
|
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
|
|
|
<p>上面的代码中进一步使用了输入 <code>in</code>，将 <code>5</code> 加到输入的变量 <code>i</code> 上，然后将结果写到输出变量 <code>o</code>。实际的操作方式是首先将 <code>i</code> 的值拷贝到输出，然后再加上 <code>5</code>。</p>
|
|
|
|
|
|
<p>上例还能看出几点：</p>
|
|
|
|
|
|
<ul>
|
|
|
|
|
|
<li><code>asm!</code> 允许使用多个格式化字符串，每一个作为单独一行汇编代码存在，看起来跟阅读真实的汇编代码类似</li>
|
|
|
|
|
|
<li>输入变量通过 <code>in</code> 来声明</li>
|
|
|
|
|
|
<li>和以前见过的格式化字符串一样，可以使用多个参数，通过 {0}, {1} 来指定，这种方式特别有用，毕竟在代码中，变量是经常复用的，而这种参数的指定方式刚好可以复用</li>
|
|
|
|
|
|
</ul>
|
|
|
|
|
|
<p>事实上，还可以进一步优化代码，去掉 <code>mov</code> 指令:</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>use std::arch::asm;
|
|
|
|
|
|
|
|
|
|
|
|
let mut x: u64 = 3;
|
|
|
|
|
|
unsafe {
|
|
|
|
|
|
asm!("add {0}, 5", inout(reg) x);
|
|
|
|
|
|
}
|
|
|
|
|
|
assert_eq!(x, 8);
|
|
|
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
|
|
|
<p>又多出一个 <code>inout</code> 关键字，但是不难猜，它说明 <code>x</code> 即是输入又是输出。与之前的分离方式还有一点很大的区别，这种方式可以保证使用同一个寄存器来完成任务。</p>
|
|
|
|
|
|
<p>当然，你可以在使用 <code>inout</code> 的情况下，指定不同的输入和输出:</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>use std::arch::asm;
|
|
|
|
|
|
|
|
|
|
|
|
let x: u64 = 3;
|
|
|
|
|
|
let y: u64;
|
|
|
|
|
|
unsafe {
|
|
|
|
|
|
asm!("add {0}, 5", inout(reg) x => y);
|
|
|
|
|
|
}
|
|
|
|
|
|
assert_eq!(y, 8);
|
|
|
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
|
|
|
<h2 id="延迟输出操作数"><a class="header" href="#延迟输出操作数">延迟输出操作数</a></h2>
|
|
|
|
|
|
<p>Rust 编译器对于操作数分配是较为保守的，它会假设 <code>out</code> 可以在任何时间被写入，因此 <code>out</code> 不会跟其它参数共享它的位置。然而为了保证最佳性能，使用尽量少的寄存器是有必要的，这样它们不必在内联汇编的代码块内保存和重加载。</p>
|
|
|
|
|
|
<p>为了达成这个目标( 共享位置或者说寄存器，以实现减少寄存器使用的性能优化 )，Rust 提供一个 <code>lateout</code> 关键字，可以用于任何只在所有输入被消费后才被写入的输出，与之类似的还有一个 <code>inlateout</code>。</p>
|
|
|
|
|
|
<p>但是 <code>inlateout</code> 在某些场景中是无法使用的，例如下面的例子：</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>use std::arch::asm;
|
|
|
|
|
|
|
|
|
|
|
|
let mut a: u64 = 4;
|
|
|
|
|
|
let b: u64 = 4;
|
|
|
|
|
|
let c: u64 = 4;
|
|
|
|
|
|
unsafe {
|
|
|
|
|
|
asm!(
|
|
|
|
|
|
"add {0}, {1}",
|
|
|
|
|
|
"add {0}, {2}",
|
|
|
|
|
|
inout(reg) a,
|
|
|
|
|
|
in(reg) b,
|
|
|
|
|
|
in(reg) c,
|
|
|
|
|
|
);
|
|
|
|
|
|
}
|
|
|
|
|
|
assert_eq!(a, 12);
|
|
|
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
|
|
|
<p>一旦用了 <code>inlateout</code> 后，上面的代码就只能运行在 <code>Debug</code> 模式下，原因是 <code>Debug</code> 并没有做任何优化，但是 <code>release</code> 发布不同，为了性能是要做很多编译优化的。</p>
|
|
|
|
|
|
<p>在编译优化时，编译器可以很容易的为输入 <code>b</code> 和 <code>c</code> 分配同样的是寄存器，因为它知道它们有同样的值。如果这里使用 <code>inlateout</code>， 那么 <code>a</code> 和 <code>c</code> 就可以被分配到相同的寄存器，在这种情况下，第一条指令将覆盖掉 <code>c</code> 的值，最终导致汇编代码产生错误的结果。</p>
|
|
|
|
|
|
<p>因此这里使用 <code>inout</code>，那么编译器就会为 <code>a</code> 分配一个独立的寄存器.</p>
|
|
|
|
|
|
<p>但是下面的代码又不同，它是可以使用 <code>inlateout</code> 的：</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>use std::arch::asm;
|
|
|
|
|
|
|
|
|
|
|
|
let mut a: u64 = 4;
|
|
|
|
|
|
let b: u64 = 4;
|
|
|
|
|
|
unsafe {
|
|
|
|
|
|
asm!("add {0}, {1}", inlateout(reg) a, in(reg) b);
|
|
|
|
|
|
}
|
|
|
|
|
|
assert_eq!(a, 8);
|
|
|
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
|
|
|
<p>原因在于输出只有在所有寄存器都被读取后，才被修改。因此，即使 <code>a</code> 和 <code>b</code> 被分配了同样的寄存器，代码也会正常工作，不存在之前的覆盖问题。</p>
|
|
|
|
|
|
<h2 id="显式指定寄存器"><a class="header" href="#显式指定寄存器">显式指定寄存器</a></h2>
|
|
|
|
|
|
<p>一些指令会要求操作数只能存在特定的寄存器中，因此 Rust 的内联汇编提供了一些限制操作符。</p>
|
|
|
|
|
|
<p>大家应该记得之前出现过的 <code>reg</code> 是适用于任何架构的通用寄存器，意味着编译器可以自己选择合适的寄存器，但是当你需要显式地指定寄存器时，很可能会变成平台相关的代码，适用移植性会差很多。例如 <code>x86</code> 下的寄存器：<code>eax</code>, <code>ebx</code>, <code>ecx</code>, <code>ebp</code>, <code>esi</code> 等等。</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>use std::arch::asm;
|
|
|
|
|
|
|
|
|
|
|
|
let cmd = 0xd1;
|
|
|
|
|
|
unsafe {
|
|
|
|
|
|
asm!("out 0x64, eax", in("eax") cmd);
|
|
|
|
|
|
}
|
|
|
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
|
|
|
<p>上面的例子调用 <code>out</code> 指令将 <code>cmd</code> 变量的值输出到 <code>0x64</code> 内存地址中。由于 <code>out</code> 指令只接收 <code>eax</code> 和它的子寄存器，因此我们需要使用 <code>eax</code> 来指定特定的寄存器。</p>
|
|
|
|
|
|
<blockquote>
|
|
|
|
|
|
<p>显式寄存器操作数无法用于格式化字符串中，例如我们之前使用的 {}，只能直接在字符串中使用 <code>eax</code>。同时，该操作数只能出现在最后，也就是在其它操作数后面出现</p>
|
|
|
|
|
|
</blockquote>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>use std::arch::asm;
|
|
|
|
|
|
|
|
|
|
|
|
fn mul(a: u64, b: u64) -> u128 {
|
|
|
|
|
|
let lo: u64;
|
|
|
|
|
|
let hi: u64;
|
|
|
|
|
|
|
|
|
|
|
|
unsafe {
|
|
|
|
|
|
asm!(
|
|
|
|
|
|
// The x86 mul instruction takes rax as an implicit input and writes
|
|
|
|
|
|
// the 128-bit result of the multiplication to rax:rdx.
|
|
|
|
|
|
"mul {}",
|
|
|
|
|
|
in(reg) a,
|
|
|
|
|
|
inlateout("rax") b => lo,
|
|
|
|
|
|
lateout("rdx") hi
|
|
|
|
|
|
);
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
((hi as u128) << 64) + lo as u128
|
|
|
|
|
|
}
|
|
|
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
|
|
|
<p>这段代码使用了 <code>mul</code> 指令，将两个 64 位的输入相乘，生成一个 128 位的结果。</p>
|
|
|
|
|
|
<p>首先将变量 <code>a</code> 的值存到寄存器 <code>reg</code> 中，其次显式使用寄存器 <code>rax</code>，它的值来源于变量 <code>b</code>。结果的低 64 位存储在 <code>rax</code> 中，然后赋给变量 <code>lo</code> ，而结果的高 64 位则存在 <code>rdx</code> 中，最后赋给 <code>hi</code>。</p>
|
|
|
|
|
|
<h2 id="clobbered-寄存器"><a class="header" href="#clobbered-寄存器">Clobbered 寄存器</a></h2>
|
|
|
|
|
|
<p>在很多情况下，无需作为输出的状态都会被内联汇编修改，这个状态被称之为 "clobbered"。</p>
|
|
|
|
|
|
<p>我们需要告诉编译器相关的情况，因为编译器需要在内联汇编语句块的附近存储和恢复这种状态。</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">use std::arch::asm;
|
|
|
|
|
|
|
|
|
|
|
|
fn main() {
|
|
|
|
|
|
// three entries of four bytes each
|
|
|
|
|
|
let mut name_buf = [0_u8; 12];
|
|
|
|
|
|
// String is stored as ascii in ebx, edx, ecx in order
|
|
|
|
|
|
// Because ebx is reserved, the asm needs to preserve the value of it.
|
|
|
|
|
|
// So we push and pop it around the main asm.
|
|
|
|
|
|
// (in 64 bit mode for 64 bit processors, 32 bit processors would use ebx)
|
|
|
|
|
|
|
|
|
|
|
|
unsafe {
|
|
|
|
|
|
asm!(
|
|
|
|
|
|
"push rbx",
|
|
|
|
|
|
"cpuid",
|
|
|
|
|
|
"mov [rdi], ebx",
|
|
|
|
|
|
"mov [rdi + 4], edx",
|
|
|
|
|
|
"mov [rdi + 8], ecx",
|
|
|
|
|
|
"pop rbx",
|
|
|
|
|
|
// We use a pointer to an array for storing the values to simplify
|
|
|
|
|
|
// the Rust code at the cost of a couple more asm instructions
|
|
|
|
|
|
// This is more explicit with how the asm works however, as opposed
|
|
|
|
|
|
// to explicit register outputs such as `out("ecx") val`
|
|
|
|
|
|
// The *pointer itself* is only an input even though it's written behind
|
|
|
|
|
|
in("rdi") name_buf.as_mut_ptr(),
|
|
|
|
|
|
// select cpuid 0, also specify eax as clobbered
|
|
|
|
|
|
inout("eax") 0 => _,
|
|
|
|
|
|
// cpuid clobbers these registers too
|
|
|
|
|
|
out("ecx") _,
|
|
|
|
|
|
out("edx") _,
|
|
|
|
|
|
);
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
let name = core::str::from_utf8(&name_buf).unwrap();
|
|
|
|
|
|
println!("CPU Manufacturer ID: {}", name);
|
|
|
|
|
|
}</code></pre></pre>
|
|
|
|
|
|
<p>例子中，我们使用 <code>cpuid</code> 指令来读取 CPU ID，该指令会将值写入到 <code>eax</code> 、<code>edx</code> 和 <code>ecx</code> 中。</p>
|
|
|
|
|
|
<p>即使 <code>eax</code> 从没有被读取，我们依然需要告知编译器这个寄存器被修改过，这样编译器就可以在汇编代码之前存储寄存器中的值。这个需要通过将输出声明为 <code>_</code> 而不是一个具体的变量名，代表着该输出值被丢弃。</p>
|
|
|
|
|
|
<p>这段代码也会绕过一个限制： <code>ebx</code> 是一个 LLVM 保留寄存器，意味着 LLVM 会假设它拥有寄存器的全部控制权，并在汇编代码块结束时将寄存器的状态恢复到最开始的状态。由于这个限制，该寄存器无法被用于输入或者输出，除非编译器使用该寄存器的满足一个通用寄存器的需求(例如 <code>in(reg)</code> )。 但这样使用后， <code>reg</code> 操作数就在使用保留寄存器时变得危险起来，原因是我们可能会无意识的破坏输入或者输出，毕竟它们共享同一个寄存器。</p>
|
|
|
|
|
|
<p>为了解决这个问题，我们使用 <code>rdi</code> 来存储指向输出数组的指针，通过 <code>push</code> 的方式存储 <code>ebx</code>：在汇编代码块的内部读取 <code>ebx</code> 的值，然后写入到输出数组。后面再可以通过 <code>pop</code> 的方式来回复 <code>ebx</code> 到初始的状态。</p>
|
|
|
|
|
|
<p><code>push</code> 和 <code>pop</code> 使用完成的 64 位 <code>rbx</code> 寄存器，来确保整个寄存器的内容都被保存。如果是在 32 位机器上，代码将使用 <code>ebx</code> 替代。</p>
|
|
|
|
|
|
<p>还还可以在汇编代码内部使用通用寄存器:</p>
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<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>use std::arch::asm;
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// Multiply x by 6 using shifts and adds
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let mut x: u64 = 4;
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unsafe {
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asm!(
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"mov {tmp}, {x}",
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"shl {tmp}, 1",
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"shl {x}, 2",
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"add {x}, {tmp}",
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x = inout(reg) x,
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tmp = out(reg) _,
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);
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}
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assert_eq!(x, 4 * 6);
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<span class="boring">}</span></code></pre></pre>
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<h2 id="总结"><a class="header" href="#总结">总结</a></h2>
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<p>由于这块儿内容过于专业，本书毕竟是通用的 Rust 学习书籍，因此关于内联汇编就不再赘述。事实上，如果你要真的写出可用的汇编代码，要学习的还很多...</p>
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<p>感兴趣的同学可以看看如下英文资料: <a href="https://doc.rust-lang.org/reference/inline-assembly.html">Rust Reference</a> 和 <a href="https://doc.rust-lang.org/rust-by-example/unsafe/asm.html#clobbered-registers">Rust By Example</a>。</p>
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