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| <h1 id="optimizations-the-speed-size-tradeoff"><a class="header" href="#optimizations-the-speed-size-tradeoff">Optimizations: the speed size tradeoff</a></h1> |
| <p>Everyone wants their program to be super fast and super small but it's usually |
| not possible to have both characteristics. This section discusses the |
| different optimization levels that <code>rustc</code> provides and how they affect the |
| execution time and binary size of a program.</p> |
| <h2 id="no-optimizations"><a class="header" href="#no-optimizations">No optimizations</a></h2> |
| <p>This is the default. When you call <code>cargo build</code> you use the development (AKA |
| <code>dev</code>) profile. This profile is optimized for debugging so it enables debug |
| information and does <em>not</em> enable any optimizations, i.e. it uses <code>-C opt-level = 0</code>.</p> |
| <p>At least for bare metal development, debuginfo is zero cost in the sense that it |
| won't occupy space in Flash / ROM so we actually recommend that you enable |
| debuginfo in the release profile -- it is disabled by default. That will let you |
| use breakpoints when debugging release builds.</p> |
| <pre><code class="language-toml">[profile.release] |
| # symbols are nice and they don't increase the size on Flash |
| debug = true |
| </code></pre> |
| <p>No optimizations is great for debugging because stepping through the code feels |
| like you are executing the program statement by statement, plus you can <code>print</code> |
| stack variables and function arguments in GDB. When the code is optimized, trying |
| to print variables results in <code>$0 = <value optimized out></code> being printed.</p> |
| <p>The biggest downside of the <code>dev</code> profile is that the resulting binary will be |
| huge and slow. The size is usually more of a problem because unoptimized |
| binaries can occupy dozens of KiB of Flash, which your target device may not |
| have -- the result: your unoptimized binary doesn't fit in your device!</p> |
| <p>Can we have smaller, debugger friendly binaries? Yes, there's a trick.</p> |
| <h3 id="optimizing-dependencies"><a class="header" href="#optimizing-dependencies">Optimizing dependencies</a></h3> |
| <p>There's a Cargo feature named <a href="https://doc.rust-lang.org/cargo/reference/profiles.html#overrides"><code>profile-overrides</code></a> that lets you |
| override the optimization level of dependencies. You can use that feature to |
| optimize all dependencies for size while keeping the top crate unoptimized and |
| debugger friendly.</p> |
| <p>Beware that generic code can sometimes be optimized alongside the crate where it |
| is instantiated, rather than the crate where it is defined. If you create an |
| instance of a generic struct in your application and find that it pulls in code |
| with a large footprint, it may be that increasing the optimisation level of the |
| relevant dependencies has no effect.</p> |
| <p>Here's an example:</p> |
| <pre><code class="language-toml"># Cargo.toml |
| [package] |
| name = "app" |
| # .. |
| |
| [profile.dev.package."*"] # + |
| opt-level = "z" # + |
| </code></pre> |
| <p>Without the override:</p> |
| <pre><code class="language-text">$ cargo size --bin app -- -A |
| app : |
| section size addr |
| .vector_table 1024 0x8000000 |
| .text 9060 0x8000400 |
| .rodata 1708 0x8002780 |
| .data 0 0x20000000 |
| .bss 4 0x20000000 |
| </code></pre> |
| <p>With the override:</p> |
| <pre><code class="language-text">$ cargo size --bin app -- -A |
| app : |
| section size addr |
| .vector_table 1024 0x8000000 |
| .text 3490 0x8000400 |
| .rodata 1100 0x80011c0 |
| .data 0 0x20000000 |
| .bss 4 0x20000000 |
| </code></pre> |
| <p>That's a 6 KiB reduction in Flash usage without any loss in the debuggability of |
| the top crate. If you step into a dependency then you'll start seeing those |
| <code><value optimized out></code> messages again but it's usually the case that you want |
| to debug the top crate and not the dependencies. And if you <em>do</em> need to debug a |
| dependency then you can use the <code>profile-overrides</code> feature to exclude a |
| particular dependency from being optimized. See example below:</p> |
| <pre><code class="language-toml"># .. |
| |
| # don't optimize the `cortex-m-rt` crate |
| [profile.dev.package.cortex-m-rt] # + |
| opt-level = 0 # + |
| |
| # but do optimize all the other dependencies |
| [profile.dev.package."*"] |
| codegen-units = 1 # better optimizations |
| opt-level = "z" |
| </code></pre> |
| <p>Now the top crate and <code>cortex-m-rt</code> are debugger friendly!</p> |
| <h2 id="optimize-for-speed"><a class="header" href="#optimize-for-speed">Optimize for speed</a></h2> |
| <p>As of 2018-09-18 <code>rustc</code> supports three "optimize for speed" levels: <code>opt-level = 1</code>, <code>2</code> and <code>3</code>. When you run <code>cargo build --release</code> you are using the release |
| profile which defaults to <code>opt-level = 3</code>.</p> |
| <p>Both <code>opt-level = 2</code> and <code>3</code> optimize for speed at the expense of binary size, |
| but level <code>3</code> does more vectorization and inlining than level <code>2</code>. In |
| particular, you'll see that at <code>opt-level</code> equal to or greater than <code>2</code> LLVM will |
| unroll loops. Loop unrolling has a rather high cost in terms of Flash / ROM |
| (e.g. from 26 bytes to 194 for a zero this array loop) but can also halve the |
| execution time given the right conditions (e.g. number of iterations is big |
| enough).</p> |
| <p>Currently there's no way to disable loop unrolling in <code>opt-level = 2</code> and <code>3</code> so |
| if you can't afford its cost you should optimize your program for size.</p> |
| <h2 id="optimize-for-size"><a class="header" href="#optimize-for-size">Optimize for size</a></h2> |
| <p>As of 2018-09-18 <code>rustc</code> supports two "optimize for size" levels: <code>opt-level = "s"</code> and <code>"z"</code>. These names were inherited from clang / LLVM and are not too |
| descriptive but <code>"z"</code> is meant to give the idea that it produces smaller |
| binaries than <code>"s"</code>.</p> |
| <p>If you want your release binaries to be optimized for size then change the |
| <code>profile.release.opt-level</code> setting in <code>Cargo.toml</code> as shown below.</p> |
| <pre><code class="language-toml">[profile.release] |
| # or "z" |
| opt-level = "s" |
| </code></pre> |
| <p>These two optimization levels greatly reduce LLVM's inline threshold, a metric |
| used to decide whether to inline a function or not. One of Rust principles are |
| zero cost abstractions; these abstractions tend to use a lot of newtypes and |
| small functions to hold invariants (e.g. functions that borrow an inner value |
| like <code>deref</code>, <code>as_ref</code>) so a low inline threshold can make LLVM miss |
| optimization opportunities (e.g. eliminate dead branches, inline calls to |
| closures).</p> |
| <p>When optimizing for size you may want to try increasing the inline threshold to |
| see if that has any effect on the binary size. The recommended way to change the |
| inline threshold is to append the <code>-C inline-threshold</code> flag to the other |
| rustflags in <code>.cargo/config.toml</code>.</p> |
| <pre><code class="language-toml"># .cargo/config.toml |
| # this assumes that you are using the cortex-m-quickstart template |
| [target.'cfg(all(target_arch = "arm", target_os = "none"))'] |
| rustflags = [ |
| # .. |
| "-C", "inline-threshold=123", # + |
| ] |
| </code></pre> |
| <p>What value to use? <a href="https://github.com/rust-lang/rust/blob/1.29.0/src/librustc_codegen_llvm/back/write.rs#L2105-L2122">As of 1.29.0 these are the inline thresholds that the |
| different optimization levels use</a>:</p> |
| <ul> |
| <li><code>opt-level = 3</code> uses 275</li> |
| <li><code>opt-level = 2</code> uses 225</li> |
| <li><code>opt-level = "s"</code> uses 75</li> |
| <li><code>opt-level = "z"</code> uses 25</li> |
| </ul> |
| <p>You should try <code>225</code> and <code>275</code> when optimizing for size.</p> |
| |
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