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For 392 | the avoidance of doubt, this paragraph does not form part of the public 393 | licenses. 394 | 395 | Creative Commons may be contacted at creativecommons.org. 396 | -------------------------------------------------------------------------------- /LICENSE.md: -------------------------------------------------------------------------------- 1 | All Reports in this Repository are licensed by Contributors 2 | under the 3 | [W3C Software and Document License](https://www.w3.org/copyright/software-license/). 4 | 5 | Contributions to Specifications are made under the 6 | [W3C CLA](https://www.w3.org/community/about/agreements/cla/). 7 | 8 | Contributions to Test Suites are made under the 9 | [W3C 3-clause BSD License](https://www.w3.org/copyright/3-clause-bsd-license-2008/) 10 | 11 | -------------------------------------------------------------------------------- /README.md: -------------------------------------------------------------------------------- 1 | # Explainer for Explicit JavaScript Compile Hints 2 | 3 | This proposal has two parts, the per-file explicit compile hints and the per-function explicit compile hints. 4 | 5 | The per-file part is currently enabled in default in Chromium-based browsers. 6 | 7 | The per-function part is a design sketch by the V8 team to describe the problem below and solicit feedback on the proposed solution. It has not been approved to ship in Chrome. 8 | 9 | ## Proponents 10 | 11 | - V8 team / Google 12 | 13 | ## Participate 14 | - https://github.com/explainers-by-googlers/explicit-javascript-compile-hints-file-based/issues 15 | 16 | ## Spec draft for the per-file version 17 | 18 | [Spec draft](https://wicg.github.io/explicit-javascript-compile-hints-file-based/) 19 | 20 | ## Spec draft for the per-function version 21 | 22 | TBD 23 | 24 | ## Table of contents 25 | 26 | 27 | 28 | 29 | 30 | - [Introduction](#introduction) 31 | - [On JavaScript parsing and compilation](#on-javascript-parsing-and-compilation) 32 | - [The PIFE heuristic](#the-pife-heuristic) 33 | - [Goals](#goals) 34 | - [Use cases](#use-cases) 35 | - [Use case 1: loading websites](#use-case-1-loading-websites) 36 | - [Use case 2: interaction](#use-case-2-interaction) 37 | - [Potential solution: Magic comment in JavaScript files](#potential-solution-magic-comment-in-javascript-files) 38 | - [Possible browser implementations](#possible-browser-implementations) 39 | - [How this solution would solve the use cases](#how-this-solution-would-solve-the-use-cases) 40 | - [Detailed design discussion](#detailed-design-discussion) 41 | - [Per-file or per-function?](#per-file-or-per-function) 42 | - [Alternatives considered](#alternatives-considered) 43 | - [Alternative: Top-level magic comment with per-function data as payload](#alternative-top-level-magic-comment-with-per-function-data-as-payload) 44 | - [Alternative: Top-level "use eager" directive and per-function "use eager" directive](#alternative-top-level-use-eager-directive-and-per-function-use-eager-directive) 45 | - [Alternative: per-function magic comment](#alternative-per-function-magic-comment) 46 | - [Alternative: compile hint data in the script tag](#alternative-compile-hint-data-in-the-script-tag) 47 | - [Alternative: compile hint data in an HTTP header](#alternative-compile-hint-data-in-an-http-header) 48 | - [Alternative: do nothing / recommend using the PIFE heuristic for triggering eager compilation](#alternative-do-nothing--recommend-using-the-pife-heuristic-for-triggering-eager-compilation) 49 | - [Alternative: do nothing / no solution for web developers to control eager compilation](#alternative-do-nothing--no-solution-for-web-developers-to-control-eager-compilation) 50 | - [Risks and mitigations](#risks-and-mitigations) 51 | - [Stakeholder feedback / opposition](#stakeholder-feedback--opposition) 52 | - [FAQ](#faq) 53 | - [Q: Why are you not pursuing standardizing the feature via TC39?](#q-why-are-you-not-pursuing-standardizing-the-feature-via-tc39) 54 | - [References & acknowledgements](#references--acknowledgements) 55 | 56 | 57 | 58 | ## Introduction 59 | 60 | This proposal introduces a new magic comment that signals to browsers that some or all the functions in a JavaScript file are likely to be needed by the website. This allows the browser to parse and compile them eagerly, which can improve page load times. 61 | 62 | In this example, the magic comment is used for triggering the eager compilation of the two JavaScript functions in the file: 63 | ```JavaScript 64 | //# allFunctionsCalledOnLoad 65 | 66 | function foo() { ... } // will be eagerly parsed and compiled 67 | function bar() { ... } // will be eagerly parsed and compiled 68 | ``` 69 | 70 | Chromium-based browsers currently support the file-based compile hints. 71 | 72 | Triggering eager compilations for individual functions is under active development: 73 | ```JavaScript 74 | //# functionsCalledOnLoad= 75 | // Experimental!!! 76 | 77 | function foo() { ... } // will be eagerly parsed and compiled, if the binary data instructs so 78 | function bar() { ... } // will be eagerly parsed and compiled, if the binary data instructs so 79 | ``` 80 | 81 | ### On JavaScript parsing and compilation 82 | 83 | Knowing which JavaScript functions to parse and compile during the initial script compilation can speed up web page loading. 84 | 85 | When processing a script we are loading from the network, we have a choice for each function; either we parse and compile it right away ("eagerly"), or we don't. If the function is later called and it was not compiled yet, we need to parse and compile it at that point. The main thread is waiting for the function to be called and cannot proceed until the function is compiled. 86 | 87 | If a JavaScript function ends up being called during page load, parsing and compiling eagerly is beneficial, because: 88 | - During the initial parsing, we anyway need to do at least a lightweight parse to find the function end. In JavaScript, finding the function end requires parsing the full syntax (there are no shortcuts where we could count the curly braces - the grammar is too complex for them to work). Doing the lightweight parsing and after that the actual parsing is duplicate work. 89 | - The eager parsing might happen on a background thread instead of the main thread. When we need to compile the function because it's being called, it's too late to parallelize work. 90 | 91 | Based on initial experiments, Google Workspace products (such as Google Docs) report 5-7% improvement in their userland page load metrics with our prototype implementation, when selecting the core JS file for eager compilation. 92 | 93 | ### The PIFE heuristic 94 | 95 | Currently, Chromium and Firefox use the [PIFE heuristic](https://v8.dev/blog/preparser#pife) to direct which functions to compile. Safari doesn't follow the heuristic. 96 | 97 | The PIFE heuristic has existed for a long time and is well known to web developers - some websites (e.g., Facebook) use it or have used it for triggering eager compilation. 98 | 99 | Using PIFEs for triggering eager compilation has downsides, though. Especially: 100 | - using it forces using function expressions instead of function declarations. The semantics of function expressions mandate doing the assignment, so they're generally less performant than function declarations. For browsers which don't follow the PIFE hint there's no upside. 101 | - it cannot be applied to ES6 class methods 102 | 103 | Thus, we'd like to specify a more elegant way for triggering eager compilation. 104 | 105 | ## Goals 106 | 107 | The goal of this proposal is to improve intial web page load speed and reduce interaction delays by allowing web developers to control which JavaScript functions are parsed and compiled eagerly. 108 | 109 | ## Use cases 110 | 111 | ### Use case 1: loading websites 112 | 113 | When users load websites, they often encounter delays as the browser parses and compiles necessary scripts. A part of the delays are due to "lazy function compilation", where we haven't compiled a JavaScript function before it is called, and we need to compile it now. 114 | 115 | ### Use case 2: interaction 116 | 117 | When users interact with websites, there are delays in how quickly the website responds to the interaction. Likewise, a part of these delays are due to "lazy function compilation". 118 | 119 | ## Potential solution: Magic comment in JavaScript files 120 | 121 | We propse adding the following magic comment to trigger eager compilation of all functions in the JavaScript file: 122 | 123 | ```JavaScript 124 | //# allFunctionsCalledOnLoad 125 | ``` 126 | and the following comment to trigger eager compilation of individual functions in the JavaScript file: 127 | 128 | ```JavaScript 129 | //# functionsCalledOnLoad= 130 | ``` 131 | 132 | The magic comment is intended as a hint to the browser. It signals the functions in this JS file should be treated as "high priority" - for example, compile them immediately when processing the script, as opposed to when a function is called. 133 | 134 | The magic comment doesn't change the semantics of the JavaScript file. The browser is allowed to ignore the hint. 135 | 136 | The overall format for the magic comment is similar to the [Source Map magic comment](https://sourcemaps.info/spec.html). 137 | 138 | The per-file magic comment must be at the top of the file, preceeded only by other single-line or multiline comments. The per-function magic comment can be anywhere, but it can only refer to functions after it. It can also be overridden by another per-function magic comment. 139 | 140 | Web developers should consider using the file-based explicit compile hints for files that contain important functions which are likely to be needed by the website early on. For example, they might use explicit compile hints for a file that contains the main entry point for the application, or for a file that contains a critical library. 141 | 142 | The per-function compile hints should be inserted based on data about which functions are called during the web page load or important user interactions. Here, a profile-guided optimization (PGO) approach is useful. Tools can first instrument the JavaScript source code to track when functions are called. This can be done by adding a statement which creates a log entry containing a suitable function identifier (or simply prints it out with `console.log`). Then the instrumented web page is started up, and exercised by loading a web page and potentially executing some user interactions. Then the logs are inspected to find out which functions got called, and those functions are then encoded in the per-function compile hint. 143 | 144 | ### Possible browser implementations 145 | 146 | Different browsers may handle the magic comment differently, based on their design and available resources. 147 | 148 | The following examples describe possible and valid actions when encountering a file with the magic comment: 149 | 150 | Implementation 1: Background-parse the JavaScript file while downloading it. Mark the targeted functions for compilation, potentially by separate background compilation tasks (possibly up to a quota). 151 | 152 | Implementation 2: Parse the JavaScript file and compile the targeted functions eagerly on the main thread (possibly up to a quota). 153 | 154 | Implementation 3: Ignore the hint when initially compiling the file. When a code cache for the file is created (e.g., when a user visits the same website often enough that cache creation is deemed useful), create a code cache containing the targeted functions in the file. 155 | 156 | Implementation 4: Like Implementation 1/2 but compile the functions with a higher tier compiler right away. 157 | 158 | Implementation 5: Ignore the hint. 159 | 160 | The implementation of the per-file compile hints and the experimental implementation of the per-function compile hints in Chromium both follow Option 1. There are currently no non-Chromium implementations. 161 | 162 | ### How this solution would solve the use cases 163 | 164 | The solution of explicit compile hints addresses the identified use cases by providing developers with a mechanism to prioritize the parsing and compilation of critical JavaScript functions, thereby speeding up the initial page load. This results in smoother browsing experiences, especially on mobile devices with limited resources. 165 | 166 | ## Design discussion 167 | 168 | ### Per-file or per-function? 169 | 170 | If compile hints apply to the full file, web developers can manually select a "core file" of their website for eager compilation. Selecting individual functions for eager compilation would need to be done in an automatic fashion - modern websites have tens of thousands of JavaScript functions, and improving the page load time requires selecting thoudands of them for eager compilation. 171 | 172 | Selecting a whole file for eager compilation might overshoot: if some functions are not needed, compiling them takes CPU time, and storing the compiled code takes up memory (although, unused code may eventually be garbage collected). 173 | 174 | The per-file version of this feature is currently shipping in Chromium-based browsers. The per-function version is under active development. 175 | 176 | ### Require the per-file magic comment to be at the top of the file? 177 | 178 | We must spec the per-file magic comment so it only applies to functions after it. This way JavaScript engines can make the eager vs lazy decision right after seeing the function declaration, without waiting to see whether there's a magic comment somewhere later in the file. 179 | 180 | For this, we have two options. Option 1: the comment must be at the top of the file and Option 2: the comment can be anywhere, but only applies to functions after it. We chose Option 1 because it's easier to spec and avoids the potential confusion that web developers might think that the comment also applies to functions before it. 181 | 182 | ### Concatenability of JavaScript files with per-function comments 183 | 184 | We want to design the per-function magic comment so that the following property holds: If you concatenate multiple JavaScript files which have their own per-function magic comments, the result will be a JavaScript file with valid per-function magic comments, and the same functions will be marked for eager compilation as would be in individual files. This makes it easier for tools to process JavaScript files which contain the per-function magic comment. 185 | 186 | This property is achieved by making the per-function magic comment encode function positions relative to the comment end. 187 | 188 | ### Several tiers or priorities? 189 | 190 | We might want to mark some functions "more important" than other functions. For example, functions used during the web page load are more important than functions used during specific user interactions, and functions called many times are more important than functions called only a few times. 191 | 192 | Depending on the meaning of "more important", the browser could then compile "more important" functions earlier (if it's about those functions being called ealier in time) or compile them with a higher compiler tier (if it's about those functions being called more times). 193 | 194 | This area needs more experimentation and is left as a possible future extension. 195 | 196 | ## Format for the per-function magic comment 197 | 198 | The exact format of the per-function magic comment is under active development. The initial approach will be to encode a list of function positions, where each position is relative to the comment end. 199 | 200 | ## Alternatives considered 201 | 202 | ### Alternative: Top-level "use eager" directive and per-function "use eager" directive 203 | 204 | Example / top level "use eager" directive: 205 | ``` 206 | "use eager"; // All functions in this file will be eager 207 | 208 | function foo() { ... } 209 | function bar() { ... } 210 | class C { 211 | m() { ... } 212 | } 213 | ``` 214 | 215 | Example / per function "use eager" directive: 216 | 217 | ``` 218 | function foo() { "use eager"; ... } 219 | class C { 220 | m() { "use eager"; ... } 221 | } 222 | ``` 223 | 224 | The top-level "use eager" directive is very similar to the top level magic comment we're proposing. The downside is that the per-function "use eager" directive would bloat the source code size. This is unoptimal; although compression would alleviate the transmitted file size overhead, parsing the magic comments still adds processing overhead which we think is unnecessary. 225 | 226 | In addition, the per-function compile hint data should be automatically generated by the web development toolchains, so it's unnecessary to have it in the source code next to the function. We don't expect humans to look at or edit the data when working with the source code. 227 | 228 | We'd like to propose a solution which we can later extend with per-function information in a lean way, and thus decided against this alternative. 229 | 230 | ### Alternative: per-function magic comment 231 | 232 | Example: 233 | ``` 234 | /*calledOnLoad*/ function foo() { ... } 235 | class C { 236 | /*calledOnLoad*/ m() { ... } 237 | } 238 | ``` 239 | 240 | Similarily to per-functon "use eager" directives, this would bloat the source code size and introduce unnecessary parsing overhead. 241 | 242 | ### Alternative: compile hint data in the script tag 243 | 244 | We could also add compile hint data (either the per-file version, or per-function data) into the script tag: 245 | 246 | Example / per-file compile hint in script tag: 247 | ``` 248 |