Tuesday, March 27, 2018

V8 release v6.6

Every six weeks, we create a new branch of V8 as part of our release process. Each version is branched from V8’s Git master immediately before a Chrome Beta milestone. Today we’re pleased to announce our newest branch, V8 version 6.6, which is in beta until its release in coordination with Chrome 66 Stable in several weeks. V8 v6.6 is filled with all sorts of developer-facing goodies. This post provides a preview of some of the highlights in anticipation of the release.

JavaScript language features

Function.prototype.toString() now returns exact slices of source code text, including whitespace and comments. Here’s an example comparing the old and the new behavior:

// Note the comment between the `function` keyword
// and the function name, as well as the space following
// the function name.
function /* a comment */ foo () {}

// Previously:
// → 'function foo() {}'
//             ^ no comment
//                ^ no space

// Now:
// → 'function /* comment */ foo () {}'

Line separator (U+2028) and paragraph separator (U+2029) symbols are now allowed in string literals, matching JSON. Previously, these symbols were treated as line terminators within string literals, and so using them resulted in a SyntaxError exception.

The catch clause of try statements can now be used without a parameter. This is useful if you don’t have a need for the exception object in the code that handles the exception.

try {
} catch { // → Look mom, no binding!

In addition to String.prototype.trim(), V8 now implements String.prototype.trimStart() and String.prototype.trimEnd(). This functionality was previously available through the non-standard trimLeft() and trimRight() methods, which remain as aliases of the new methods for backward compatibility.

const string = '  hello world  ';
// → 'hello world  '
// → '  hello world'
// → 'hello world'

The Array.prototype.values() method gives arrays the same iteration interface as the ES2015 Map and Set collections: all can now be iterated over by keys, values, or entries by calling the same-named method. This change has the potential to be incompatible with existing JavaScript code. If you discover odd or broken behavior on a website, please try to disable this feature via chrome://flags/#enable-array-prototype-values and file an issue.

Code caching after execution

The terms cold and warm load might be well-known for people concerned about loading performance. In V8, there is also the concept of a hot load. Let’s explain the different levels with Chrome embedding V8 as an example:

  • Cold load: Chrome sees the visited web page for the first time and does not have any data cached at all.
  • Warm load: Chrome remembers that the web page was already visited and can retrieve certain assets (e.g. images and script source files) from the cache. V8 recognizes that the page shipped the same script file already, and therefore caches the compiled code along with the script file in the disk cache.
  • Hot load: The third time Chrome visits the web page, when serving script file from the disk cache, it also provides V8 with the code cached during the previous load. V8 can use this cached code to avoid having to parse and compile the script from scratch.

Before V8 v6.6 we cached the generated code immediately after the top-level compile. V8 only compiles the functions that are known to be immediately executed during the top-level compile and marks other functions for lazy compilation. This meant that cached code only included top-level code, while all other functions had to be lazily compiled from scratch on each page load. Beginning with version 6.6, V8 caches the code generated after the script’s top-level execution. As we execute the script, more functions are lazily compiled and can be included in the cache. As a result, these functions don’t need to be compiled on future page loads, reducing compile and parse time in hot load scenarios by between 20–60%. The visible user change is a less congested main thread, thus a smoother and faster loading experience.

Look out for a detailed blog post on this topic soon.

Background compilation

For some time V8 has been able to parse JavaScript code on a background thread. With V8’s new Ignition bytecode interpreter that shipped last year, we were able to extend this support to also enable compilation of the JavaScript source to bytecode on a background thread. This enables embedders to perform more work off the main thread, freeing it up to execute more JavaScript and reduce jank. We enabled this feature in Chrome 66, where we see between 5% to 20% reduction on main-thread compilation time on typical websites. For more details, please see the recent blog post on this feature.

Removal of AST numbering

We have continued to reap benefits from simplifying our compilation pipeline after the Ignition and TurboFan launch last year. Our previous pipeline required a post-parsing stage called "AST Numbering", where nodes in the generated abstract syntax tree were numbered so that the various compilers using it would have a common point of reference.

Over time this post-processing pass had ballooned to include other functionality: numbering suspend point for generators and async functions, collecting inner functions for eager compilation, initializing literals or detecting unoptimizable code patterns.

With the new pipeline, the Ignition bytecode became the common point of reference, and the numbering itself was no longer required — but, the remaining functionality was still needed, and the AST numbering pass remained.

In V8 v6.6, we finally managed to move out or deprecate this remaining functionality into other passes, allowing us to remove this tree walk. This resulted in a 3-5% improvement in real-world compile time.

Asynchronous performance improvements

We managed to squeeze out some nice performance improvements for promises and async functions, and especially managed to close the gap between async functions and desugared promise chains.

In addition, the performance of async generators and async iteration was improved significantly, making them a viable option for the upcoming Node 10 LTS, which is scheduled to include V8 v6.6. As an example, consider the following Fibonacci sequence implementation:

async function* fibonacciSequence() {
  for (let a = 0, b = 1;;) {
    yield a;
    const c = a + b;
    a = b;
    b = c;

async function fibonacci(id, n) {
  for await (const value of fibonacciSequence()) {
    if (n-- === 0) return value;

We’ve measured the following improvements for this pattern, before and after Babel transpilation:

Finally, bytecode improvements to “suspendable functions” such as generators, async functions, and modules, have improved the performance of these functions while running in the interpreter, and decreased their compiled size. We’re planning on improving the performance of async functions and async generators even further with upcoming releases, so stay tuned.

Array performance improvements

The throughput performance of Array#reduce was increased by more than 10× for holey double arrays (see our blog post for an explanation what holey and packed arrays are). This widens the fast-path for cases where Array#reduce is applied to holey and packed double arrays.

Untrusted code mitigations

In V8 v6.6 we’ve landed more mitigations for side-channel vulnerabilities to prevent information leaks to untrusted JavaScript and WebAssembly code.

GYP is gone

This is the first V8 version that officially ships without GYP files. If your product needs the deleted GYP files, you need to copy them into your own source repository.

Memory profiling

Chrome’s DevTools can now trace and snapshot C++ DOM objects and display all reachable DOM objects from JavaScript with their references. This feature is one of the benefits of the new C++ tracing mechanism of the V8 garbage collector. For more information please have a look at the dedicated blog post.


Please use git log branch-heads/6.5..branch-heads/6.6 include/v8.h to get a list of the API changes.

Developers with an active V8 checkout can use git checkout -b 6.6 -t branch-heads/6.6 to experiment with the new features in V8 v6.6. Alternatively you can subscribe to Chrome’s Beta channel and try the new features out yourself soon.

Posted by the V8 team

Monday, March 26, 2018

Background compilation

TL;DR: Starting with Chrome 66, V8 compiles JavaScript source code on a background thread, reducing the amount of time spent compiling on the main thread by between 5% to 20% on typical websites.


Since version 41, Chrome has supported parsing of JavaScript source files on a background thread via V8’s StreamedSource API. This enables V8 to start parsing JavaScript source code as soon as Chrome has downloaded the first chunk of the file from the network, and to continue parsing in parallel while Chrome streams the file over the network. This can provide considerable loading time improvements since V8 can be almost finished parsing the JavaScript by the time the file has finished downloading.

However, due to limitations in V8’s original baseline compiler, V8 still needed to go back to the main thread to finalize parsing and compile the script into JIT machine code that would execute the script’s code. With the switch to our new Ignition + TurboFan pipeline, we are now able to move bytecode compilation to the background thread as well, thereby freeing up Chrome’s main-thread to deliver a smoother, more responsive web browsing experience.

Building a background thread bytecode compiler

V8’s Ignition bytecode compiler takes the abstract syntax tree (AST) produced by the parser as input and produces a stream of bytecode (BytecodeArray) along with associated meta-data which enables the Ignition interpreter to execute the JavaScript source.

Ignition’s bytecode compiler was built with multi-threading in mind, however a number of changes were required throughout the compilation pipeline to enable background compilation. One of the main changes was to prevent the compilation pipeline from accessing objects in V8’s JavaScript heap while running on the background thread. Objects in V8’s heap are not thread-safe, since Javascript is single-threaded, and might be modified by the main-thread or V8’s garbage collector during background compilation.

There were two main stages of the compilation pipeline which accessed objects on V8’s heap: AST internalization, and bytecode finalization. AST internalization is a process by which literal objects (strings, numbers, object-literal boilerplate, etc.) identified in the AST are allocated on the V8 heap, such that they can be used directly by the generated bytecode when the script is executed. This process traditionally happened immediately after the parser built the AST. As such, there were a number of steps later in the compilation pipeline that relied on the literal objects having been allocated. To enable background compilation we moved AST internalization later in the compilation pipeline, after the bytecode had been compiled. This required modifications to the later stages of the pipeline to access the raw literal values embedded in the AST instead of internalized on-heap values.

Bytecode finalization involves building the final BytecodeArray object, used to execute the function, alongside associated metadata — for example, a ConstantPoolArray which stores constants referred to by the bytecode, and a SourcePositionTable which maps the JavaScript source line and column numbers to bytecode offset. Since JavaScript is a dynamic language, these objects all need to live in the JavaScript heap to enable them to be garbage-collected if the JavaScript function associated with the bytecode is collected. Previously some of these metadata objects would be allocated and modified during bytecode compilation, which involved accessing the JavaScript heap. In order to enable background compilation, Ignition’s bytecode generator was refactored to keep track of the details of this metadata and defer allocating them on the JavaScript heap until the very final stages of compilation.

With these changes, almost all of the script’s compilation can be moved to a background thread, with only the short AST internalization and bytecode finalization steps happening on the main thread just before script execution.

Currently, only top-level script code and immediately invoked function expressions (IIFEs) are compiled on a background thread — inner functions are still compiled lazily (when first executed) on the main thread. We are hoping to extend background compilation to more situations in the future. However, even with these restrictions, background compilation leaves the main thread free for longer, enabling it to do other work such as reacting to user-interaction, rendering animations or otherwise producing a smoother more responsive experience.


We evaluated the performance of background compilation using our real-world benchmarking framework across a set of popular webpages.

The proportion of compilation that can happen on a background thread varies depending on the proportion of bytecode compiled during top-level streaming-script compilation verses being lazy compiled as inner functions are invoked (which must still occur on the main thread). As such, the proportion of time saved on the main thread varies, with most pages seeing between 5% to 20% reduction in main-thread compilation time.

Next steps

What’s better than compiling a script on a background thread? Not having to compile the script at all! Alongside background compilation we have also been working on improving V8’s code-caching system to expand the amount of code cached by V8, thereby speeding up page loading for sites you visit often. We hope to bring you updates on this front soon. Stay tuned!

Posted by Ross McIlroy, main thread defender

Thursday, March 1, 2018

Tracing from JS to the DOM and back again


Debugging memory leaks in Chrome 66 just became much easier. Chrome’s DevTools can now trace and snapshot C++ DOM objects and display all reachable DOM objects from JavaScript with their references. This feature is one of the benefits of the new C++ tracing mechanism of the V8 garbage collector.


A memory leak in a garbage collection system occurs when an unused object is not freed due to unintentional references from other objects. Memory leaks in web pages often involve interaction between JavaScript objects and DOM elements.

The following toy example shows a memory leak that happens when a programmer forgets to unregister an event listener. None of the objects referenced by the event listener can be garbage collected. In particular, the iframe window leaks together with the event listener.

// Main window:
const iframe = document.createElement('iframe');
iframe.src = 'iframe.html';
iframe.addEventListener('load', function() {
  const local_variable = iframe.contentWindow;
  function leakingListener() {
    // Do something with `local_variable`.
    if (local_variable) {}
  document.body.addEventListener('my-debug-event', leakingListener);
  // BUG: forgot to unregister `leakingListener`.

The leaking iframe window also keeps all its JavaScript objects alive.

// iframe.html:
class Leak {};
window.global_variable = new Leak();

It is important to understand the notion of retaining paths to find the root cause of a memory leak. A retaining path is a chain of objects that prevents garbage collection of the leaking object. The chain starts at a root object such as the global object of the main window. The chain ends at the leaking object. Each intermediate object in the chain has a direct reference to the next object in the chain. For example, the retaining path of the Leak object in the iframe looks as follows:

Figure 1: Retaining path of an object leaked via iframe and event listener.

Note that the retaining path crosses the JavaScript / DOM boundary (highlighted in green/red, respectively) two times. The JavaScript objects live in the V8 heap, while DOM objects are C++ objects in Chrome.

DevTools heap snapshot

We can inspect the retaining path of any object by taking a heap snapshot in DevTools. The heap snapshot precisely captures all objects on the V8 heap. Up until recently it had only approximate information about the C++ DOM objects. For instance, Chrome 65 shows an incomplete retaining path for the Leak object from the toy example:

Figure 2: Retaining path in Chrome 65.

Only the first row is precise: the Leak object is indeed stored in the global_variable of the iframe’s window object. Subsequent rows approximate the real retaining path and make debugging of the memory leak hard.

As of Chrome 66, DevTools traces through C++ DOM objects and precisely captures the objects and references between them. This is based on the powerful C++ object tracing mechanism that was introduced for cross-component garbage collection earlier. As a result, the retaining path in DevTools is actually correct now:

Figure 3: Retaining path in Chrome 66.

Under the hood: cross-component tracing

DOM objects are managed by Blink — the rendering engine of Chrome, which is responsible for translating the DOM into actual text and images on the screen. Blink and its representation of the DOM are written in C++ which means that the DOM cannot be directly exposed to JavaScript. Instead, objects in the DOM come in two halves: a V8 wrapper object available to JavaScript and a C++ object representing the node in the DOM. These objects have direct references to each other. Determining liveness and ownership of objects across multiple components, such as Blink and V8, is difficult because all involved parties need to agree on which objects are still alive and which ones can be reclaimed.

In Chrome 56 and older versions (i.e. until Mar 2017), Chrome used a mechanism called object grouping to determine liveness. Objects were assigned groups based on containment in documents. A group with all of its containing objects was kept alive as long as a single object was kept alive through some other retaining path. This made sense in the context of DOM nodes that always refer to their containing document, forming so-called DOM trees. However, this abstraction removed all of the actual retaining paths which made it hard to use for debugging as shown in Figure 2. In the case of objects that did not fit this scenario, e.g. JavaScript closures used as event listeners, this approach also became cumbersome and led to various bugs where JavaScript wrapper objects would prematurely get collected, which resulted in them being replaced by empty JS wrappers that would lose all their properties.

Starting from Chrome 57, this approach was replaced by cross-component tracing, which is a mechanism that determines liveness by tracing from JavaScript to the C++ implementation of the DOM and back. We implemented incremental tracing on the C++ side with write barriers to avoid any stop-the-world tracing jank we’ve been talking about in previous blog posts. Cross-component tracing does not only provide better latency but also approximates liveness of objects across component boundaries better and fixes several scenarios that used to cause leaks. On top of that, it allows DevTools to provide a snapshot that actually represents the DOM, as shown in Figure 3.

Try it out! We are happy to hear your feedback.

Posted by Ulan Degenbaev, Alexei Filippov, Michael Lippautz, and Hannes Payer — the fellowship of the DOM