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<!doctype html><html lang=en dir=ltr class="blog-wrapper blog-post-page plugin-blog plugin-id-default" data-has-hydrated=false><meta charset=UTF-8><meta name=generator content="Docusaurus v3.6.0"><title data-rh=true>Allocations in Rust: Compiler optimizations | The Old Speice Guy</title><meta data-rh=true name=viewport content="width=device-width,initial-scale=1.0"><meta data-rh=true name=twitter:card content=summary_large_image><meta data-rh=true property=og:url content=https://speice.io/2019/02/08/compiler-optimizations><meta data-rh=true property=og:locale content=en><meta data-rh=true name=docusaurus_locale content=en><meta data-rh=true name=docusaurus_tag content=default><meta data-rh=true name=docsearch:language content=en><meta data-rh=true name=docsearch:docusaurus_tag content=default><meta data-rh=true property=og:title content="Allocations in Rust: Compiler optimizations | The Old Speice Guy"><meta data-rh=true name=description content="A lot. The answer is a lot."><meta data-rh=true property=og:description content="A lot. The answer is a lot."><meta data-rh=true property=og:type content=article><meta data-rh=true property=article:published_time content=2019-02-08T12:00:00.000Z><link data-rh=true rel=icon href=/img/favicon.ico><link data-rh=true rel=canonical href=https://speice.io/2019/02/08/compiler-optimizations><link data-rh=true rel=alternate href=https://speice.io/2019/02/08/compiler-optimizations hreflang=en><link data-rh=true rel=alternate href=https://speice.io/2019/02/08/compiler-optimizations hreflang=x-default><script data-rh=true type=application/ld+json>{"@context":"https://schema.org","@id":"https://speice.io/2019/02/08/compiler-optimizations","@type":"BlogPosting","author":[],"dateModified":"2024-11-10T02:05:00.000Z","datePublished":"2019-02-08T12:00:00.000Z","description":"A lot. The answer is a lot.","headline":"Allocations in Rust: Compiler optimizations","isPartOf":{"@id":"https://speice.io/","@type":"Blog","name":"Blog"},"keywords":[],"mainEntityOfPage":"https://speice.io/2019/02/08/compiler-optimizations","name":"Allocations in Rust: Compiler optimizations","url":"https://speice.io/2019/02/08/compiler-optimizations"}</script><link rel=alternate type=application/rss+xml href=/rss.xml title="The Old Speice Guy RSS Feed"><link rel=alternate type=application/atom+xml href=/atom.xml title="The Old Speice Guy Atom Feed"><link rel=stylesheet href=https://cdn.jsdelivr.net/npm/katex@0.13.24/dist/katex.min.css integrity=sha384-odtC+0UGzzFL/6PNoE8rX/SPcQDXBJ+uRepguP4QkPCm2LBxH3FA3y+fKSiJ+AmM crossorigin><link rel=stylesheet href=/assets/css/styles.ae6ff4a3.css><script src=/assets/js/runtime~main.751b419d.js defer></script><script src=/assets/js/main.62ce6156.js defer></script><body class=navigation-with-keyboard><script>!function(){var t,e=function(){try{return new URLSearchParams(window.location.search).get("docusaurus-theme")}catch(t){}}()||function(){try{return window.localStorage.getItem("theme")}catch(t){}}();t=null!==e?e:"light",document.documentElement.setAttribute("data-theme",t)}(),function(){try{for(var[t,e]of new URLSearchParams(window.location.search).entries())if(t.startsWith("docusaurus-data-")){var a=t.replace("docusaurus-data-","data-");document.documentElement.setAttribute(a,e)}}catch(t){}}()</script><div id=__docusaurus><div role=region aria-label="Skip to main content"><a class=skipToContent_fXgn href=#__docusaurus_skipToContent_fallback>Skip to main content</a></div><nav aria-label=Main class="navbar navbar--fixed-top"><div class=navbar__inner><div class=navbar__items><button aria-label="Toggle navigation bar" aria-expanded=false class="navbar__toggle clean-btn" type=button><svg width=30 height=30 viewBox="0 0 30 30" aria-hidden=true><path stroke=currentColor stroke-linecap=round stroke-miterlimit=10 stroke-width=2 d="M4 7h22M4 15h22M4 23h22"/></svg></button><a class=navbar__brand href=/><div class=navbar__logo><img src=/img/logo.svg alt="Sierpinski Gasket" class="themedComponent_mlkZ themedComponent--light_NVdE"><img src=/img/logo-dark.svg alt="Sierpinski Gasket" class="themedCo
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rules that are mostly right for small chunks of code. We've spent time showing how those rules work
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themselves out in practice, and become familiar with reading the assembly code needed to see each
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memory type (global, stack, heap) in action.</p>
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<p>Throughout the series so far, we've put a handicap on the code. In the name of consistent and
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understandable results, we've asked the compiler to pretty please leave the training wheels on. Now
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is the time where we throw out all the rules and take off the kid gloves. As it turns out, both the
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Rust compiler and the LLVM optimizers are incredibly sophisticated, and we'll step back and let them
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do their job.</p>
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<p>Similar to
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<a href="https://www.youtube.com/watch?v=bSkpMdDe4g4" target=_blank rel="noopener noreferrer">"What Has My Compiler Done For Me Lately?"</a>, we're
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focusing on interesting things the Rust language (and LLVM!) can do with memory management. We'll
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still be looking at assembly code to understand what's going on, but it's important to mention
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again: <strong>please use automated tools like <a href=https://crates.io/crates/alloc_counter target=_blank rel="noopener noreferrer">alloc-counter</a> to
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double-check memory behavior if it's something you care about</strong>. It's far too easy to mis-read
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assembly in large code sections, you should always verify behavior if you care about memory usage.</p>
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<p>The guiding principal as we move forward is this: <em>optimizing compilers won't produce worse programs
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than we started with.</em> There won't be any situations where stack allocations get moved to heap
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allocations. There will, however, be an opera of optimization.</p>
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<p><strong>Update 2019-02-10</strong>: When debugging a
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<a href=https://gitlab.com/sio4/code/alloc-counter/issues/1 target=_blank rel="noopener noreferrer">related issue</a>, it was discovered that the
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original code worked because LLVM optimized out the entire function, rather than just the allocation
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segments. The code has been updated with proper use of
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<a href=https://doc.rust-lang.org/std/ptr/fn.read_volatile.html target=_blank rel="noopener noreferrer"><code>read_volatile</code></a>, and a previous section
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on vector capacity has been removed.</p>
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<h2 class="anchor anchorWithStickyNavbar_LWe7" id=the-case-of-the-disappearing-box>The Case of the Disappearing Box<a href=#the-case-of-the-disappearing-box class=hash-link aria-label="Direct link to The Case of the Disappearing Box" title="Direct link to The Case of the Disappearing Box"></a></h2>
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<p>Our first optimization comes when LLVM can reason that the lifetime of an object is sufficiently
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short that heap allocations aren't necessary. In these cases, LLVM will move the allocation to the
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stack instead! The way this interacts with <code>#[inline]</code> attributes is a bit opaque, but the important
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part is that LLVM can sometimes do better than the baseline Rust language:</p>
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<div class="language-rust codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-rust codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token keyword" style="color:hsl(301, 63%, 40%)">use</span><span class="token plain"> </span><span class="token namespace">std</span><span class="token namespace punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token namespace">alloc</span><span class="token namespace punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token class-name" style="color:hsl(35, 99%, 36%)">GlobalAlloc</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Layout</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">System</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">use</span><span class="token plain"> </span><span class="token namespace">std</span><span class="token namespace punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token namespace">sync</span><span class="token namespace punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token namespace">atomic</span><span class="token namespace punctuation" style="color:hsl(119, 34%, 47%)">::</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token class-name" style="color:hsl(35, 99%, 36%)">AtomicBool</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">,</span><span class="token plain"> </span><span class="token class-name" style="color:hsl(35, 99%, 36%)">Ordering</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">pub</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">fn</span><span class="token plain"> </span><span class="token function-definition function" style="color:hsl(221, 87%, 60%)">cmp</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">(</span><span class="token plain">x</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"> </span><span class="token keyword" style="color:hsl(301, 63%, 40%)">u32</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">)</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token comment" style="color:hsl(230, 4%, 64%)">// Turn on panicking if we allocate on the heap</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> </span><span class="token constant" style="color:hsl(35, 99%, 36%)">DO_PANIC</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">.</span><
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<p>-- <a href=https://godbolt.org/z/BZ_Yp3 target=_blank rel="noopener noreferrer">Compiler Explorer</a></p>
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<p>-- <a href="https://play.rust-lang.org/?version=stable&mode=release&edition=2018&gist=4a765f753183d5b919f62c71d2109d5d" target=_blank rel="noopener noreferrer">Rust Playground</a></p>
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<h2 class="anchor anchorWithStickyNavbar_LWe7" id=dr-array-or-how-i-learned-to-love-the-optimizer>Dr. Array or: how I learned to love the optimizer<a href=#dr-array-or-how-i-learned-to-love-the-optimizer class=hash-link aria-label="Direct link to Dr. Array or: how I learned to love the optimizer" title="Direct link to Dr. Array or: how I learned to love the optimizer"></a></h2>
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<p>Finally, this isn't so much about LLVM figuring out different memory behavior, but LLVM stripping
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out code that doesn't do anything. Optimizations of this type have a lot of nuance to them; if
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you're not careful, they can make your benchmarks look
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<a href="https://www.youtube.com/watch?v=nXaxk27zwlk&feature=youtu.be&t=1199" target=_blank rel="noopener noreferrer">impossibly good</a>. In Rust, the
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<code>black_box</code> function (implemented in both
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<a href=https://doc.rust-lang.org/1.1.0/test/fn.black_box.html target=_blank rel="noopener noreferrer"><code>libtest</code></a> and
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<a href=https://docs.rs/criterion/0.2.10/criterion/fn.black_box.html target=_blank rel="noopener noreferrer"><code>criterion</code></a>) will tell the compiler
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to disable this kind of optimization. But if you let LLVM remove unnecessary code, you can end up
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running programs that previously caused errors:</p>
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<div class="language-rust codeBlockContainer_Ckt0 theme-code-block" style="--prism-background-color:hsl(230, 1%, 98%);--prism-color:hsl(230, 8%, 24%)"><div class=codeBlockContent_biex><pre tabindex=0 class="prism-code language-rust codeBlock_bY9V thin-scrollbar" style="background-color:hsl(230, 1%, 98%);color:hsl(230, 8%, 24%)"><code class=codeBlockLines_e6Vv><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token attribute attr-name" style="color:hsl(35, 99%, 36%)">#[derive(Default)]</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">struct</span><span class="token plain"> </span><span class="token type-definition class-name" style="color:hsl(35, 99%, 36%)">TwoFiftySix</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> _a</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">[</span><span class="token keyword" style="color:hsl(301, 63%, 40%)">u64</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"> </span><span class="token number" style="color:hsl(35, 99%, 36%)">32</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">]</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token attribute attr-name" style="color:hsl(35, 99%, 36%)">#[derive(Default)]</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">struct</span><span class="token plain"> </span><span class="token type-definition class-name" style="color:hsl(35, 99%, 36%)">EightK</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">{</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"> _a</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">:</span><span class="token plain"> </span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">[</span><span class="token class-name" style="color:hsl(35, 99%, 36%)">TwoFiftySix</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">;</span><span class="token plain"> </span><span class="token number" style="color:hsl(35, 99%, 36%)">32</span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">]</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token punctuation" style="color:hsl(119, 34%, 47%)">}</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain" style=display:inline-block></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token attribute attr-name" style="color:hsl(35, 99%, 36%)">#[derive(Default)]</span><span class="token plain"></span><br></span><span class=token-line style="color:hsl(230, 8%, 24%)"><span class="token plain"></span><span class="token keyword" style="color:hsl(301, 63%, 40%)">struct</span><span class="token plain"> </span><span class="token type-definition class-name" style="
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<p>-- <a href=https://godbolt.org/z/daHn7P target=_blank rel="noopener noreferrer">Compiler Explorer</a></p>
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<p>-- <a href="https://play.rust-lang.org/?version=stable&mode=release&edition=2018&gist=4c253bf26072119896ab93c6ef064dc0" target=_blank rel="noopener noreferrer">Rust Playground</a></div></article><nav class="pagination-nav docusaurus-mt-lg" aria-label="Blog post page navigation"><a class="pagination-nav__link pagination-nav__link--prev" href=/2019/02/a-heaping-helping><div class=pagination-nav__sublabel>Older post</div><div class=pagination-nav__label>Allocations in Rust: Dynamic memory</div></a><a class="pagination-nav__link pagination-nav__link--next" href=/2019/02/summary><div class=pagination-nav__sublabel>Newer post</div><div class=pagination-nav__label>Allocations in Rust: Summary</div></a></nav></main><div class="col col--2"><div class="tableOfContents_bqdL thin-scrollbar"><ul class="table-of-contents table-of-contents__left-border"><li><a href=#the-case-of-the-disappearing-box class="table-of-contents__link toc-highlight">The Case of the Disappearing Box</a><li><a href=#dr-array-or-how-i-learned-to-love-the-optimizer class="table-of-contents__link toc-highlight">Dr. Array or: how I learned to love the optimizer</a></ul></div></div></div></div></div><footer class=footer><div class="container container-fluid"><div class="footer__bottom text--center"><div class=footer__copyright>Copyright © 2024 Bradlee Speice</div></div></div></footer></div>
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