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<ol class="chapter"><li class="chapter-item expanded "><a href="../intro/index.html"><strong aria-hidden="true">1.</strong> 简介</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../intro/hardware.html"><strong aria-hidden="true">1.1.</strong> 硬件</a></li><li class="chapter-item expanded "><a href="../intro/no-std.html"><strong aria-hidden="true">1.2.</strong> no_std</a></li><li class="chapter-item expanded "><a href="../intro/tooling.html"><strong aria-hidden="true">1.3.</strong> 工具</a></li><li class="chapter-item expanded "><a href="../intro/install.html"><strong aria-hidden="true">1.4.</strong> 安装</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../intro/install/linux.html"><strong aria-hidden="true">1.4.1.</strong> Linux</a></li><li class="chapter-item expanded "><a href="../intro/install/macos.html"><strong aria-hidden="true">1.4.2.</strong> MacOS</a></li><li class="chapter-item expanded "><a href="../intro/install/windows.html"><strong aria-hidden="true">1.4.3.</strong> Windows</a></li><li class="chapter-item expanded "><a href="../intro/install/verify.html"><strong aria-hidden="true">1.4.4.</strong> 验证安装</a></li></ol></li></ol></li><li class="chapter-item expanded "><a href="../start/index.html"><strong aria-hidden="true">2.</strong> 入门</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../start/qemu.html"><strong aria-hidden="true">2.1.</strong> QEMU</a></li><li class="chapter-item expanded "><a href="../start/hardware.html"><strong aria-hidden="true">2.2.</strong> 硬件</a></li><li class="chapter-item expanded "><a href="../start/registers.html"><strong aria-hidden="true">2.3.</strong> 内存映射寄存器</a></li><li class="chapter-item expanded "><a href="../start/semihosting.html"><strong aria-hidden="true">2.4.</strong> 半主机</a></li><li class="chapter-item expanded "><a href="../start/panicking.html"><strong aria-hidden="true">2.5.</strong> 恐慌</a></li><li class="chapter-item expanded "><a href="../start/exceptions.html"><strong aria-hidden="true">2.6.</strong> 异常</a></li><li class="chapter-item expanded "><a href="../start/interrupts.html"><strong aria-hidden="true">2.7.</strong> 中断</a></li><li class="chapter-item expanded "><a href="../start/io.html"><strong aria-hidden="true">2.8.</strong> IO</a></li></ol></li><li class="chapter-item expanded "><a href="../peripherals/index.html"><strong aria-hidden="true">3.</strong> 外设</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../peripherals/a-first-attempt.html"><strong aria-hidden="true">3.1.</strong> 初试Rust</a></li><li class="chapter-item expanded "><a href="../peripherals/borrowck.html"><strong aria-hidden="true">3.2.</strong> 借用检查器</a></li><li class="chapter-item expanded "><a href="../peripherals/singletons.html"><strong aria-hidden="true">3.3.</strong> 单例</a></li></ol></li><li class="chapter-item expanded "><a href="../static-guarantees/index.html"><strong aria-hidden="true">4.</strong> 静态保证</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../static-guarantees/typestate-programming.html"><strong aria-hidden="true">4.1.</strong> 类型状态机编程</a></li><li class="chapter-item expanded "><a href="../static-guarantees/state-machines.html"><strong aria-hidden="true">4.2.</strong> 外设作为状态机</a></li><li class="chapter-item expanded "><a href="../static-guarantees/design-contracts.html"><strong aria-hidden="true">4.3.</strong> 设计合约</a></li><li class="chapter-item expanded "><a href="../static-guarantees/zero-cost-abstractions.html"><strong aria-hidden="true">4.4.</strong> 零成本抽象</a></li></ol></li><li class="chapter-item expanded "><a href="../portability/index.html"><strong aria-hidden="true">5.</strong> 可移植性</a></li><li class="chapter-item expanded "><a href="../concurrency/index.html"><strong aria-hidden="true">6.</strong> 并发</a></li><li class="chapter-item expanded "><a href="../collections/index.html"><strong aria-hidden="true">7.</strong> 容器</a></li><li class="chapter-item expanded "><a href="../c-tips/index.html"><strong aria-hidden="true">8.</strong> 嵌入式C开发人员的技巧</a></li><li class="chapter-item expanded "><a href="../interoperability/index.html"><strong aria-hidden="true">9.</strong> 互操作性</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../interoperability/c-with-rust.html"><strong aria-hidden="true">9.1.</strong> Rust中使用C代码</a></li><li class="chapter-item expanded "><a href="../interoperability/rust-with-c.html"><strong aria-hidden="true">9.2.</strong> C中使用Rust代码</a></li></ol></li><li class="chapter-item expanded "><a href="../unsorted/index.html"><strong aria-hidden="true">10.</strong> 其他主题</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../unsorted/speed-vs-size.html" class="active"><strong aria-hidden="true">10.1.</strong> 优化：速度大小的权衡</a></li><li class="spacer"></li></ol></li><li class="chapter-item expanded "><a href="../appendix/glossary.html">Appendix A: Glossary</a></li></ol>
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<h1><a class="header" href="#优化速度大小的权衡" id="优化速度大小的权衡">优化：速度大小的权衡</a></h1>
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<p>每个人都希望他们的程序超快，超小，但通常不可能兼具这两个特性。本节讨论<code>rustc</code> 提供的不同优化级别，以及它们如何影响程序的执行时间和二进制大小。</p>
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<h2><a class="header" href="#没有优化" id="没有优化">没有优化</a></h2>
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<p>这是默认值。当您调用<code>cargo build</code>时，可以使用开发(也就是<code>dev</code>)配置文件。这个配置文件针对调试进行了优化，因此它启用调试信息并且<em>不</em>启用任何优化，即它使用<code>-C opt-level = 0</code>。</p>
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<p>至少对于裸机开发而言，debuginfo的成本为零，因为它不会占用Flash/ROM中的空间，因此我们建议您在发行配置文件中启用debuginfo(默认情况下处于禁用状态)。这样您就可以在调试发行版时使用断点。</p>
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<pre><code class="language-toml">[profile.release]
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# symbols are nice and they don't increase the size on Flash
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debug = true
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</code></pre>
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<p>禁用优化对调试很有效，因为单步执行代码就像逐行执行代码一样，而且可以在GDB中“打印”堆栈变量和函数参数。优化代码后，尝试打印变量将导致打印<code>$0 = <value optimized out></code>。</p>
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<p>dev配置文件的最大缺点是，生成的二进制文件会很大且很慢。大小通常是一个更大的问题，因为未优化的二进制文件可能会占用数十KiB的Flash，而目标设备可能没有这么大空间，这会导致：未优化的二进制文件不适合您的设备！</p>
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<p>我们可以使用较小的，调试器友好的二进制文件吗？是的，有个窍门。</p>
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<h3><a class="header" href="#优化依赖" id="优化依赖">优化依赖</a></h3>
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<p>有一个名为<a href="https://doc.rust-lang.org/cargo/reference/profiles.html#overrides"><code>profile-overrides</code></a>的cargo特性，可让您覆盖依赖的crate的优化级别。您可以使用该功能优化所有依赖的crate的大小，同时保持项目自身不优化和对调试器的友好性。</p>
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<p>这是一个例子：</p>
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<pre><code class="language-toml"># Cargo.toml
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[package]
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name = "app"
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# ..
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[profile.dev.package."*"] # +
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opt-level = "z" # +
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</code></pre>
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<p>没有覆盖默认优化时：</p>
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<pre><code class="language-console">$ cargo size --bin app -- -A
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app :
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section size addr
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.vector_table 1024 0x8000000
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.text 9060 0x8000400
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.rodata 1708 0x8002780
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.data 0 0x20000000
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.bss 4 0x20000000
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</code></pre>
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<p>使用覆盖：</p>
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<pre><code class="language-console">$ cargo size --bin app -- -A
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app :
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section size addr
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.vector_table 1024 0x8000000
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.text 3490 0x8000400
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.rodata 1100 0x80011c0
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.data 0 0x20000000
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.bss 4 0x20000000
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</code></pre>
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<p>闪存使用量减少了6 KiB，而项目自身的可调试性没有任何损失。如果在调试时,您进入依赖项，您将再次看到那些<code><value Optimized out></code>消息，但是通常情况下，您要调试的是项目自身而不是依赖项。而且如果您需要调试依赖项，则可以使用<code>profile-overrides</code>特性来排除特定的依赖项，以使其不被优化。请参见下面的示例：</p>
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<pre><code class="language-toml"># ..
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# don't optimize the `cortex-m-rt` crate
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[profile.dev.package.cortex-m-rt] # +
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opt-level = 0 # +
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# but do optimize all the other dependencies
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[profile.dev.package."*"]
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codegen-units = 1 # better optimizations
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opt-level = "z"
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</code></pre>
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<p>现在，项目自身和<code>cortex-m-rt</code>都对调试器友好了！</p>
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<h2><a class="header" href="#优化速度" id="优化速度">优化速度</a></h2>
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<p>自2018年9月18日起，rustc支持三种“优化速度”级别<code>opt-level = 1,2,3</code>。当您运行<code>cargo build --release</code>时，您使用的是发布配置文件，默认为<code>opt-level = 3</code>。</p>
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<p><code>opt-level = 2</code> 和<code>3</code>都针对速度进行了优化，但以二进制大小为代价，3级比2级进行了更多的矢量化和内联。特别是，您将看到在opt-level等于或大于2的情况下，LLVM可能会展开循环。就Flash/ROM而言，循环展开具有相当高的成本(例如，对于一个将数组清零的循环，大小可能从26个字节到增大到194个字节)，但在合适的条件下(例如迭代次数足够大)，也可以将执行时间减半。</p>
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<p>当前没有办法在 <code>opt-level = 2,3</code>时禁用循环展开，因此，如果您负担不起其成本，则应针对大小优化程序。</p>
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<h2><a class="header" href="#优化尺寸" id="优化尺寸">优化尺寸</a></h2>
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<p>从2018年9月18日开始，rustc支持两个“大小优化”级别：<code>opt-level = s,z</code>。这些名称是从clang/LLVM继承的，所以描述性不太强，“z”的含义是产生比“s”更小的二进制文件。</p>
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<p>如果您希望优化发布二进制文件的大小，请按如下所示,在Cargo.toml中更改profile.release.opt-level设置。</p>
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<pre><code class="language-toml">[profile.release]
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# or "z"
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opt-level = "s"
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</code></pre>
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<p>这两个优化级别大大降低了LLVM的内联阈值，该阈值用于确定是否内联函数。 Rust原则之一是零成本抽象。这些抽象倾向于使用大量的新类型和小的函数来保存不变式(例如，诸如“ deref”，“as_ref”之类的借用内部值的函数)，因此低的内联阈值会使LLVM错过优化机会(例如，消除无效分支，闭包的内联)。</p>
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||
<p>在优化大小时，您可能想尝试增加内联阈值，以查看这是否对二进制大小有影响。推荐的更改内联阈值的方法是将<code>-C inline-threshold</code> 参数附加到<code>.cargo/config</code>中的rustflags。</p>
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||
<pre><code class="language-toml"># .cargo/config
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# this assumes that you are using the cortex-m-quickstart template
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[target.'cfg(all(target_arch = "arm", target_os = "none"))']
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rustflags = [
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# ..
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"-C", "inline-threshold=123", # +
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]
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</code></pre>
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<p>内联阈值使用什么值合适？从1.29.0开始，下面是不同优化级别使用的[内联阈值]:</p>
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<ul>
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<li><code>opt-level = 3</code> 使用275</li>
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<li><code>opt-level = 2</code> 使用225</li>
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<li><code>opt-level =“ s”</code> 使用75</li>
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<li><code>opt-level =“ z” </code>使用25</li>
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</ul>
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<p>在优化大小时，应尝试使用较大的内联阈值比如“225”和“275”。</p>
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