| <!DOCTYPE HTML> |
| <html lang="en" class="light sidebar-visible" dir="ltr"> |
| <head> |
| <!-- Book generated using mdBook --> |
| <meta charset="UTF-8"> |
| <title>Incremental compilation - Rust Compiler Development Guide</title> |
| |
| |
| <!-- Custom HTML head --> |
| |
| <meta name="description" content="A guide to developing the Rust compiler (rustc)"> |
| <meta name="viewport" content="width=device-width, initial-scale=1"> |
| <meta name="theme-color" content="#ffffff"> |
| |
| <link rel="icon" href="../favicon-de23e50b.svg"> |
| <link rel="shortcut icon" href="../favicon-8114d1fc.png"> |
| <link rel="stylesheet" href="../css/variables-8adf115d.css"> |
| <link rel="stylesheet" href="../css/general-2459343d.css"> |
| <link rel="stylesheet" href="../css/chrome-ae938929.css"> |
| <link rel="stylesheet" href="../css/print-9e4910d8.css" media="print"> |
| |
| <!-- Fonts --> |
| <link rel="stylesheet" href="../fonts/fonts-9644e21d.css"> |
| |
| <!-- Highlight.js Stylesheets --> |
| <link rel="stylesheet" id="mdbook-highlight-css" href="../highlight-493f70e1.css"> |
| <link rel="stylesheet" id="mdbook-tomorrow-night-css" href="../tomorrow-night-4c0ae647.css"> |
| <link rel="stylesheet" id="mdbook-ayu-highlight-css" href="../ayu-highlight-3fdfc3ac.css"> |
| |
| <!-- Custom theme stylesheets --> |
| |
| |
| <!-- Provide site root and default themes to javascript --> |
| <script> |
| const path_to_root = "../"; |
| const default_light_theme = "light"; |
| const default_dark_theme = "navy"; |
| window.path_to_searchindex_js = "../searchindex-e3b7aa8d.js"; |
| </script> |
| <!-- Start loading toc.js asap --> |
| <script src="../toc-190e7008.js"></script> |
| </head> |
| <body> |
| <div id="mdbook-help-container"> |
| <div id="mdbook-help-popup"> |
| <h2 class="mdbook-help-title">Keyboard shortcuts</h2> |
| <div> |
| <p>Press <kbd>←</kbd> or <kbd>→</kbd> to navigate between chapters</p> |
| <p>Press <kbd>S</kbd> or <kbd>/</kbd> to search in the book</p> |
| <p>Press <kbd>?</kbd> to show this help</p> |
| <p>Press <kbd>Esc</kbd> to hide this help</p> |
| </div> |
| </div> |
| </div> |
| <div id="mdbook-body-container"> |
| <!-- Work around some values being stored in localStorage wrapped in quotes --> |
| <script> |
| try { |
| let theme = localStorage.getItem('mdbook-theme'); |
| let sidebar = localStorage.getItem('mdbook-sidebar'); |
| |
| if (theme.startsWith('"') && theme.endsWith('"')) { |
| localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1)); |
| } |
| |
| if (sidebar.startsWith('"') && sidebar.endsWith('"')) { |
| localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1)); |
| } |
| } catch (e) { } |
| </script> |
| |
| <!-- Set the theme before any content is loaded, prevents flash --> |
| <script> |
| const default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? default_dark_theme : default_light_theme; |
| let theme; |
| try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { } |
| if (theme === null || theme === undefined) { theme = default_theme; } |
| const html = document.documentElement; |
| html.classList.remove('light') |
| html.classList.add(theme); |
| html.classList.add("js"); |
| </script> |
| |
| <input type="checkbox" id="mdbook-sidebar-toggle-anchor" class="hidden"> |
| |
| <!-- Hide / unhide sidebar before it is displayed --> |
| <script> |
| let sidebar = null; |
| const sidebar_toggle = document.getElementById("mdbook-sidebar-toggle-anchor"); |
| if (document.body.clientWidth >= 1080) { |
| try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { } |
| sidebar = sidebar || 'visible'; |
| } else { |
| sidebar = 'hidden'; |
| sidebar_toggle.checked = false; |
| } |
| if (sidebar === 'visible') { |
| sidebar_toggle.checked = true; |
| } else { |
| html.classList.remove('sidebar-visible'); |
| } |
| </script> |
| |
| <nav id="mdbook-sidebar" class="sidebar" aria-label="Table of contents"> |
| <!-- populated by js --> |
| <mdbook-sidebar-scrollbox class="sidebar-scrollbox"></mdbook-sidebar-scrollbox> |
| <noscript> |
| <iframe class="sidebar-iframe-outer" src="../toc.html"></iframe> |
| </noscript> |
| <div id="mdbook-sidebar-resize-handle" class="sidebar-resize-handle"> |
| <div class="sidebar-resize-indicator"></div> |
| </div> |
| </nav> |
| |
| <div id="mdbook-page-wrapper" class="page-wrapper"> |
| |
| <div class="page"> |
| <div id="mdbook-menu-bar-hover-placeholder"></div> |
| <div id="mdbook-menu-bar" class="menu-bar sticky"> |
| <div class="left-buttons"> |
| <label id="mdbook-sidebar-toggle" class="icon-button" for="mdbook-sidebar-toggle-anchor" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="mdbook-sidebar"> |
| <span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 448 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M0 96C0 78.3 14.3 64 32 64H416c17.7 0 32 14.3 32 32s-14.3 32-32 32H32C14.3 128 0 113.7 0 96zM0 256c0-17.7 14.3-32 32-32H416c17.7 0 32 14.3 32 32s-14.3 32-32 32H32c-17.7 0-32-14.3-32-32zM448 416c0 17.7-14.3 32-32 32H32c-17.7 0-32-14.3-32-32s14.3-32 32-32H416c17.7 0 32 14.3 32 32z"/></svg></span> |
| </label> |
| <button id="mdbook-theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="mdbook-theme-list"> |
| <span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 576 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M371.3 367.1c27.3-3.9 51.9-19.4 67.2-42.9L600.2 74.1c12.6-19.5 9.4-45.3-7.6-61.2S549.7-4.4 531.1 9.6L294.4 187.2c-24 18-38.2 46.1-38.4 76.1L371.3 367.1zm-19.6 25.4l-116-104.4C175.9 290.3 128 339.6 128 400c0 3.9 .2 7.8 .6 11.6c1.8 17.5-10.2 36.4-27.8 36.4H96c-17.7 0-32 14.3-32 32s14.3 32 32 32H240c61.9 0 112-50.1 112-112c0-2.5-.1-5-.2-7.5z"/></svg></span> |
| </button> |
| <ul id="mdbook-theme-list" class="theme-popup" aria-label="Themes" role="menu"> |
| <li role="none"><button role="menuitem" class="theme" id="mdbook-theme-default_theme">Auto</button></li> |
| <li role="none"><button role="menuitem" class="theme" id="mdbook-theme-light">Light</button></li> |
| <li role="none"><button role="menuitem" class="theme" id="mdbook-theme-rust">Rust</button></li> |
| <li role="none"><button role="menuitem" class="theme" id="mdbook-theme-coal">Coal</button></li> |
| <li role="none"><button role="menuitem" class="theme" id="mdbook-theme-navy">Navy</button></li> |
| <li role="none"><button role="menuitem" class="theme" id="mdbook-theme-ayu">Ayu</button></li> |
| </ul> |
| <button id="mdbook-search-toggle" class="icon-button" type="button" title="Search (`/`)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="/ s" aria-controls="mdbook-searchbar"> |
| <span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M416 208c0 45.9-14.9 88.3-40 122.7L502.6 457.4c12.5 12.5 12.5 32.8 0 45.3s-32.8 12.5-45.3 0L330.7 376c-34.4 25.2-76.8 40-122.7 40C93.1 416 0 322.9 0 208S93.1 0 208 0S416 93.1 416 208zM208 352c79.5 0 144-64.5 144-144s-64.5-144-144-144S64 128.5 64 208s64.5 144 144 144z"/></svg></span> |
| </button> |
| </div> |
| |
| <h1 class="menu-title">Rust Compiler Development Guide</h1> |
| |
| <div class="right-buttons"> |
| <a href="../print.html" title="Print this book" aria-label="Print this book"> |
| <span class=fa-svg id="print-button"><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M128 0C92.7 0 64 28.7 64 64v96h64V64H354.7L384 93.3V160h64V93.3c0-17-6.7-33.3-18.7-45.3L400 18.7C388 6.7 371.7 0 354.7 0H128zM384 352v32 64H128V384 368 352H384zm64 32h32c17.7 0 32-14.3 32-32V256c0-35.3-28.7-64-64-64H64c-35.3 0-64 28.7-64 64v96c0 17.7 14.3 32 32 32H64v64c0 35.3 28.7 64 64 64H384c35.3 0 64-28.7 64-64V384zm-16-88c-13.3 0-24-10.7-24-24s10.7-24 24-24s24 10.7 24 24s-10.7 24-24 24z"/></svg></span> |
| </a> |
| <a href="https://github.com/rust-lang/rustc-dev-guide" title="Git repository" aria-label="Git repository"> |
| <span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 496 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M165.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6zm-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3zm44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9zM244.8 8C106.1 8 0 113.3 0 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.7 34.7 5.2 61.4 2.6 67.9 16 17.7 25.8 31.5 25.8 58.9 0 96.5-58.9 104.2-114.8 110.5 9.2 7.9 17 22.9 17 46.4 0 33.7-.3 75.4-.3 83.6 0 6.5 4.6 14.4 17.3 12.1C428.2 457.8 496 362.9 496 252 496 113.3 383.5 8 244.8 8zM97.2 352.9c-1.3 1-1 3.3.7 5.2 1.6 1.6 3.9 2.3 5.2 1 1.3-1 1-3.3-.7-5.2-1.6-1.6-3.9-2.3-5.2-1zm-10.8-8.1c-.7 1.3.3 2.9 2.3 3.9 1.6 1 3.6.7 4.3-.7.7-1.3-.3-2.9-2.3-3.9-2-.6-3.6-.3-4.3.7zm32.4 35.6c-1.6 1.3-1 4.3 1.3 6.2 2.3 2.3 5.2 2.6 6.5 1 1.3-1.3.7-4.3-1.3-6.2-2.2-2.3-5.2-2.6-6.5-1zm-11.4-14.7c-1.6 1-1.6 3.6 0 5.9 1.6 2.3 4.3 3.3 5.6 2.3 1.6-1.3 1.6-3.9 0-6.2-1.4-2.3-4-3.3-5.6-2z"/></svg></span> |
| </a> |
| <a href="https://github.com/rust-lang/rustc-dev-guide/edit/main/src/queries/incremental-compilation.md" title="Suggest an edit" aria-label="Suggest an edit" rel="edit"> |
| <span class=fa-svg id="git-edit-button"><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M421.7 220.3l-11.3 11.3-22.6 22.6-205 205c-6.6 6.6-14.8 11.5-23.8 14.1L30.8 511c-8.4 2.5-17.5 .2-23.7-6.1S-1.5 489.7 1 481.2L38.7 353.1c2.6-9 7.5-17.2 14.1-23.8l205-205 22.6-22.6 11.3-11.3 33.9 33.9 62.1 62.1 33.9 33.9zM96 353.9l-9.3 9.3c-.9 .9-1.6 2.1-2 3.4l-25.3 86 86-25.3c1.3-.4 2.5-1.1 3.4-2l9.3-9.3H112c-8.8 0-16-7.2-16-16V353.9zM453.3 19.3l39.4 39.4c25 25 25 65.5 0 90.5l-14.5 14.5-22.6 22.6-11.3 11.3-33.9-33.9-62.1-62.1L314.3 67.7l11.3-11.3 22.6-22.6 14.5-14.5c25-25 65.5-25 90.5 0z"/></svg></span> |
| </a> |
| |
| </div> |
| </div> |
| |
| <div id="mdbook-search-wrapper" class="hidden"> |
| <form id="mdbook-searchbar-outer" class="searchbar-outer"> |
| <div class="search-wrapper"> |
| <input type="search" id="mdbook-searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="mdbook-searchresults-outer" aria-describedby="searchresults-header"> |
| <div class="spinner-wrapper"> |
| <span class=fa-svg id="fa-spin"><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M304 48c0-26.5-21.5-48-48-48s-48 21.5-48 48s21.5 48 48 48s48-21.5 48-48zm0 416c0-26.5-21.5-48-48-48s-48 21.5-48 48s21.5 48 48 48s48-21.5 48-48zM48 304c26.5 0 48-21.5 48-48s-21.5-48-48-48s-48 21.5-48 48s21.5 48 48 48zm464-48c0-26.5-21.5-48-48-48s-48 21.5-48 48s21.5 48 48 48s48-21.5 48-48zM142.9 437c18.7-18.7 18.7-49.1 0-67.9s-49.1-18.7-67.9 0s-18.7 49.1 0 67.9s49.1 18.7 67.9 0zm0-294.2c18.7-18.7 18.7-49.1 0-67.9S93.7 56.2 75 75s-18.7 49.1 0 67.9s49.1 18.7 67.9 0zM369.1 437c18.7 18.7 49.1 18.7 67.9 0s18.7-49.1 0-67.9s-49.1-18.7-67.9 0s-18.7 49.1 0 67.9z"/></svg></span> |
| </div> |
| </div> |
| </form> |
| <div id="mdbook-searchresults-outer" class="searchresults-outer hidden"> |
| <div id="mdbook-searchresults-header" class="searchresults-header"></div> |
| <ul id="mdbook-searchresults"> |
| </ul> |
| </div> |
| </div> |
| |
| <!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM --> |
| <script> |
| document.getElementById('mdbook-sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible'); |
| document.getElementById('mdbook-sidebar').setAttribute('aria-hidden', sidebar !== 'visible'); |
| Array.from(document.querySelectorAll('#mdbook-sidebar a')).forEach(function(link) { |
| link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1); |
| }); |
| </script> |
| |
| <div id="mdbook-content" class="content"> |
| <main> |
| <h1 id="incremental-compilation"><a class="header" href="#incremental-compilation">Incremental compilation</a></h1> |
| <p>The incremental compilation scheme is, in essence, a surprisingly |
| simple extension to the overall query system. We’ll start by describing |
| a slightly simplified variant of the real thing – the “basic algorithm” – |
| and then describe some possible improvements.</p> |
| <h2 id="the-basic-algorithm"><a class="header" href="#the-basic-algorithm">The basic algorithm</a></h2> |
| <p>The basic algorithm is |
| called the <strong>red-green</strong> algorithm<sup class="footnote-reference" id="fr-salsa-1"><a href="#footnote-salsa">1</a></sup>. The high-level idea is |
| that, after each run of the compiler, we will save the results of all |
| the queries that we do, as well as the <strong>query DAG</strong>. The |
| <strong>query DAG</strong> is a <a href="https://en.wikipedia.org/wiki/Directed_acyclic_graph">DAG</a> that indexes which queries executed which |
| other queries. So, for example, there would be an <a href="https://en.wikipedia.org/wiki/Glossary_of_graph_theory_terms#edge">edge</a> from a query Q1 |
| to another query Q2 if computing Q1 required computing Q2 (note that |
| because queries cannot depend on themselves, this results in a DAG and |
| not a general graph).</p> |
| <blockquote> |
| <p><strong>NOTE</strong>: You might think of a query as simply the definition of a query. |
| A thing that you can invoke, a bit like a function, |
| and which either returns a cached result or actually executes the code.</p> |
| <p>If that’s the way you think about queries, |
| it’s good to know that in the following text, queries will be said to have colours. |
| Keep in mind though, that here the word query also refers to a certain invocation of |
| the query for a certain input. As you will read later, queries are fingerprinted based |
| on their arguments. The result of a query might change when we give it one argument |
| and be coloured red, while it stays the same for another argument and is thus green.</p> |
| <p>In short, the word query is here not just used to mean the definition of a query, |
| but also for a specific instance of that query with given arguments.</p> |
| </blockquote> |
| <p>On the next run of the compiler, then, we can sometimes reuse these |
| query results to avoid re-executing a query. We do this by assigning |
| every query a <strong>color</strong>:</p> |
| <ul> |
| <li>If a query is colored <strong>red</strong>, that means that its result during |
| this compilation has <strong>changed</strong> from the previous compilation.</li> |
| <li>If a query is colored <strong>green</strong>, that means that its result is |
| the <strong>same</strong> as the previous compilation.</li> |
| </ul> |
| <p>There are two key insights here:</p> |
| <ul> |
| <li>First, if all the inputs to query Q are colored green, then the |
| query Q <strong>must</strong> result in the same value as last time and hence |
| need not be re-executed (or else the compiler is not deterministic).</li> |
| <li>Second, even if some inputs to a query changes, it may be that it |
| <strong>still</strong> produces the same result as the previous compilation. In |
| particular, the query may only use part of its input. |
| <ul> |
| <li>Therefore, after executing a query, we always check whether it |
| produced the same result as the previous time. <strong>If it did,</strong> we |
| can still mark the query as green, and hence avoid re-executing |
| dependent queries.</li> |
| </ul> |
| </li> |
| </ul> |
| <h3 id="the-try-mark-green-algorithm"><a class="header" href="#the-try-mark-green-algorithm">The try-mark-green algorithm</a></h3> |
| <p>At the core of incremental compilation is an algorithm called |
| “try-mark-green”. It has the job of determining the color of a given |
| query Q (which must not have yet been executed). In cases where Q has |
| red inputs, determining Q’s color may involve re-executing Q so that |
| we can compare its output, but if all of Q’s inputs are green, then we |
| can conclude that Q must be green without re-executing it or inspecting |
| its value at all. In the compiler, this allows us to avoid |
| deserializing the result from disk when we don’t need it, and in fact |
| enables us to sometimes skip <em>serializing</em> the result as well |
| (see the refinements section below).</p> |
| <p>Try-mark-green works as follows:</p> |
| <ul> |
| <li>First check if the query Q was executed during the previous compilation. |
| <ul> |
| <li>If not, we can just re-execute the query as normal, and assign it the |
| color of red.</li> |
| </ul> |
| </li> |
| <li>If yes, then load the ‘dependent queries’ of Q.</li> |
| <li>If there is a saved result, then we load the <code>reads(Q)</code> vector from the |
| query DAG. The “reads” is the set of queries that Q executed during |
| its execution. |
| <ul> |
| <li>For each query R in <code>reads(Q)</code>, we recursively demand the color |
| of R using try-mark-green. |
| <ul> |
| <li>Note: it is important that we visit each node in <code>reads(Q)</code> in same order |
| as they occurred in the original compilation. See <a href="#dag">the section on the |
| query DAG below</a>.</li> |
| <li>If <strong>any</strong> of the nodes in <code>reads(Q)</code> wind up colored <strong>red</strong>, then Q is |
| dirty. |
| <ul> |
| <li>We re-execute Q and compare the hash of its result to the hash of the |
| result from the previous compilation.</li> |
| <li>If the hash has not changed, we can mark Q as <strong>green</strong> and return.</li> |
| </ul> |
| </li> |
| <li>Otherwise, <strong>all</strong> of the nodes in <code>reads(Q)</code> must be <strong>green</strong>. In that |
| case, we can color Q as <strong>green</strong> and return.</li> |
| </ul> |
| </li> |
| </ul> |
| </li> |
| </ul> |
| <p><a id="dag"></a></p> |
| <h3 id="the-query-dag"><a class="header" href="#the-query-dag">The query DAG</a></h3> |
| <p>The query DAG code is stored in |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/dep_graph/index.html"><code>compiler/rustc_middle/src/dep_graph</code></a>. Construction of the DAG is done |
| by instrumenting the query execution.</p> |
| <p>One key point is that the query DAG also tracks ordering; that is, for |
| each query Q, we not only track the queries that Q reads, we track the |
| <strong>order</strong> in which they were read. This allows try-mark-green to walk |
| those queries back in the same order. This is important because once a |
| subquery comes back as red, we can no longer be sure that Q will continue |
| along the same path as before. That is, imagine a query like this:</p> |
| <pre><code class="language-rust ignore">fn main_query(tcx) { |
| if tcx.subquery1() { |
| tcx.subquery2() |
| } else { |
| tcx.subquery3() |
| } |
| }</code></pre> |
| <p>Now imagine that in the first compilation, <code>main_query</code> starts by |
| executing <code>subquery1</code>, and this returns true. In that case, the next |
| query <code>main_query</code> executes will be <code>subquery2</code>, and <code>subquery3</code> will |
| not be executed at all.</p> |
| <p>But now imagine that in the <strong>next</strong> compilation, the input has |
| changed such that <code>subquery1</code> returns <strong>false</strong>. In this case, <code>subquery2</code> |
| would never execute. If try-mark-green were to visit <code>reads(main_query)</code> out |
| of order, however, it might visit <code>subquery2</code> before <code>subquery1</code>, and hence |
| execute it. |
| This can lead to ICEs and other problems in the compiler.</p> |
| <h2 id="improvements-to-the-basic-algorithm"><a class="header" href="#improvements-to-the-basic-algorithm">Improvements to the basic algorithm</a></h2> |
| <p>In the description of the basic algorithm, we said that at the end of |
| compilation we would save the results of all the queries that were |
| performed. In practice, this can be quite wasteful – many of those |
| results are very cheap to recompute, and serializing and deserializing |
| them is not a particular win. In practice, what we would do is to save |
| <strong>the hashes</strong> of all the subqueries that we performed. Then, in select cases, |
| we <strong>also</strong> save the results.</p> |
| <p>This is why the incremental algorithm separates computing the |
| <strong>color</strong> of a node, which often does not require its value, from |
| computing the <strong>result</strong> of a node. Computing the result is done via a simple |
| algorithm like so:</p> |
| <ul> |
| <li>Check if a saved result for Q is available. If so, compute the color of Q. |
| If Q is green, deserialize and return the saved result.</li> |
| <li>Otherwise, execute Q. |
| <ul> |
| <li>We can then compare the hash of the result and color Q as green if |
| it did not change.</li> |
| </ul> |
| </li> |
| </ul> |
| <h2 id="resources"><a class="header" href="#resources">Resources</a></h2> |
| <p>The initial design document can be found <a href="https://github.com/nikomatsakis/rustc-on-demand-incremental-design-doc/blob/master/0000-rustc-on-demand-and-incremental.md">here</a>, which expands |
| on the memoization details, provides more high-level overview and motivation |
| for this system.</p> |
| <h1 id="footnotes"><a class="header" href="#footnotes">Footnotes</a></h1> |
| <hr> |
| <ol class="footnote-definition"> |
| <li id="footnote-salsa"> |
| <p>I have long wanted to rename it to the Salsa algorithm, but it never caught on. -@nikomatsakis <a href="#fr-salsa-1">↩</a></p> |
| </li> |
| </ol> |
| |
| </main> |
| |
| <nav class="nav-wrapper" aria-label="Page navigation"> |
| <!-- Mobile navigation buttons --> |
| <a rel="prev" href="../queries/query-evaluation-model-in-detail.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> |
| <span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 320 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M41.4 233.4c-12.5 12.5-12.5 32.8 0 45.3l160 160c12.5 12.5 32.8 12.5 45.3 0s12.5-32.8 0-45.3L109.3 256 246.6 118.6c12.5-12.5 12.5-32.8 0-45.3s-32.8-12.5-45.3 0l-160 160z"/></svg></span> |
| </a> |
| |
| <a rel="next prefetch" href="../queries/incremental-compilation-in-detail.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> |
| <span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 320 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M278.6 233.4c12.5 12.5 12.5 32.8 0 45.3l-160 160c-12.5 12.5-32.8 12.5-45.3 0s-12.5-32.8 0-45.3L210.7 256 73.4 118.6c-12.5-12.5-12.5-32.8 0-45.3s32.8-12.5 45.3 0l160 160z"/></svg></span> |
| </a> |
| |
| <div style="clear: both"></div> |
| </nav> |
| </div> |
| </div> |
| |
| <nav class="nav-wide-wrapper" aria-label="Page navigation"> |
| <a rel="prev" href="../queries/query-evaluation-model-in-detail.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> |
| <span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 320 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M41.4 233.4c-12.5 12.5-12.5 32.8 0 45.3l160 160c12.5 12.5 32.8 12.5 45.3 0s12.5-32.8 0-45.3L109.3 256 246.6 118.6c12.5-12.5 12.5-32.8 0-45.3s-32.8-12.5-45.3 0l-160 160z"/></svg></span> |
| </a> |
| |
| <a rel="next prefetch" href="../queries/incremental-compilation-in-detail.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> |
| <span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 320 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M278.6 233.4c12.5 12.5 12.5 32.8 0 45.3l-160 160c-12.5 12.5-32.8 12.5-45.3 0s-12.5-32.8 0-45.3L210.7 256 73.4 118.6c-12.5-12.5-12.5-32.8 0-45.3s32.8-12.5 45.3 0l160 160z"/></svg></span> |
| </a> |
| </nav> |
| |
| </div> |
| |
| <template id=fa-eye><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 576 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M288 32c-80.8 0-145.5 36.8-192.6 80.6C48.6 156 17.3 208 2.5 243.7c-3.3 7.9-3.3 16.7 0 24.6C17.3 304 48.6 356 95.4 399.4C142.5 443.2 207.2 480 288 480s145.5-36.8 192.6-80.6c46.8-43.5 78.1-95.4 93-131.1c3.3-7.9 3.3-16.7 0-24.6c-14.9-35.7-46.2-87.7-93-131.1C433.5 68.8 368.8 32 288 32zM432 256c0 79.5-64.5 144-144 144s-144-64.5-144-144s64.5-144 144-144s144 64.5 144 144zM288 192c0 35.3-28.7 64-64 64c-11.5 0-22.3-3-31.6-8.4c-.2 2.8-.4 5.5-.4 8.4c0 53 43 96 96 96s96-43 96-96s-43-96-96-96c-2.8 0-5.6 .1-8.4 .4c5.3 9.3 8.4 20.1 8.4 31.6z"/></svg></span></template> |
| <template id=fa-eye-slash><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 640 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M38.8 5.1C28.4-3.1 13.3-1.2 5.1 9.2S-1.2 34.7 9.2 42.9l592 464c10.4 8.2 25.5 6.3 33.7-4.1s6.3-25.5-4.1-33.7L525.6 386.7c39.6-40.6 66.4-86.1 79.9-118.4c3.3-7.9 3.3-16.7 0-24.6c-14.9-35.7-46.2-87.7-93-131.1C465.5 68.8 400.8 32 320 32c-68.2 0-125 26.3-169.3 60.8L38.8 5.1zM223.1 149.5C248.6 126.2 282.7 112 320 112c79.5 0 144 64.5 144 144c0 24.9-6.3 48.3-17.4 68.7L408 294.5c5.2-11.8 8-24.8 8-38.5c0-53-43-96-96-96c-2.8 0-5.6 .1-8.4 .4c5.3 9.3 8.4 20.1 8.4 31.6c0 10.2-2.4 19.8-6.6 28.3l-90.3-70.8zm223.1 298L373 389.9c-16.4 6.5-34.3 10.1-53 10.1c-79.5 0-144-64.5-144-144c0-6.9 .5-13.6 1.4-20.2L83.1 161.5C60.3 191.2 44 220.8 34.5 243.7c-3.3 7.9-3.3 16.7 0 24.6c14.9 35.7 46.2 87.7 93 131.1C174.5 443.2 239.2 480 320 480c47.8 0 89.9-12.9 126.2-32.5z"/></svg></span></template> |
| <template id=fa-copy><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M502.6 70.63l-61.25-61.25C435.4 3.371 427.2 0 418.7 0H255.1c-35.35 0-64 28.66-64 64l.0195 256C192 355.4 220.7 384 256 384h192c35.2 0 64-28.8 64-64V93.25C512 84.77 508.6 76.63 502.6 70.63zM464 320c0 8.836-7.164 16-16 16H255.1c-8.838 0-16-7.164-16-16L239.1 64.13c0-8.836 7.164-16 16-16h128L384 96c0 17.67 14.33 32 32 32h47.1V320zM272 448c0 8.836-7.164 16-16 16H63.1c-8.838 0-16-7.164-16-16L47.98 192.1c0-8.836 7.164-16 16-16H160V128H63.99c-35.35 0-64 28.65-64 64l.0098 256C.002 483.3 28.66 512 64 512h192c35.2 0 64-28.8 64-64v-32h-47.1L272 448z"/></svg></span></template> |
| <template id=fa-play><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 384 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M73 39c-14.8-9.1-33.4-9.4-48.5-.9S0 62.6 0 80V432c0 17.4 9.4 33.4 24.5 41.9s33.7 8.1 48.5-.9L361 297c14.3-8.7 23-24.2 23-41s-8.7-32.2-23-41L73 39z"/></svg></span></template> |
| <template id=fa-clock-rotate-left><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M75 75L41 41C25.9 25.9 0 36.6 0 57.9V168c0 13.3 10.7 24 24 24H134.1c21.4 0 32.1-25.9 17-41l-30.8-30.8C155 85.5 203 64 256 64c106 0 192 86 192 192s-86 192-192 192c-40.8 0-78.6-12.7-109.7-34.4c-14.5-10.1-34.4-6.6-44.6 7.9s-6.6 34.4 7.9 44.6C151.2 495 201.7 512 256 512c141.4 0 256-114.6 256-256S397.4 0 256 0C185.3 0 121.3 28.7 75 75zm181 53c-13.3 0-24 10.7-24 24V256c0 6.4 2.5 12.5 7 17l72 72c9.4 9.4 24.6 9.4 33.9 0s9.4-24.6 0-33.9l-65-65V152c0-13.3-10.7-24-24-24z"/></svg></span></template> |
| |
| |
| |
| <script> |
| window.playground_copyable = true; |
| </script> |
| |
| |
| <script src="../elasticlunr-ef4e11c1.min.js"></script> |
| <script src="../mark-09e88c2c.min.js"></script> |
| <script src="../searcher-c2a407aa.js"></script> |
| |
| <script src="../clipboard-1626706a.min.js"></script> |
| <script src="../highlight-abc7f01d.js"></script> |
| <script src="../book-a0b12cfe.js"></script> |
| |
| <!-- Custom JS scripts --> |
| <script src="../mermaid-cc85ecea.min.js"></script> |
| <script src="../mermaid-init-4533fb11.js"></script> |
| |
| |
| |
| </div> |
| </body> |
| </html> |
