V8 release v7.1

Published · tagged with release

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 7.1, which is in beta until its release in coordination with Chrome 71 Stable in several weeks. V8 v7.1 is filled with all sorts of developer-facing goodies. This post provides a preview of some of the highlights in anticipation of the release.


Following the work in v6.9/v7.0 to embed builtins directly into the binary, bytecode handlers for the interpreter are now also embedded into the binary. This saves around 200 KB on average per Isolate.


The escape analysis in TurboFan, which performs scalar replacement for objects that are local to an optimization unit, was improved to also handle local function contexts for higher-order functions when variables from the surrounding context escape to a local closure. Consider the following example:

function mapAdd(a, x) {
return a.map(y => y + x);

Note that x is a free variable of the local closure y => y + x. V8 v7.1 can now fully elide the context allocation of x, yielding an improvement of up to 40% in some cases.

Performance improvement with new escape analysis (lower is better)

The escape analysis is now also able to eliminate some cases of variable index access to local arrays. Here’s an example:

function sum(...args) {
let total = 0;
for (let i = 0; i < args.length; ++i)
total += args[i];
return total;

function sum2(x, y) {
return sum(x, y);

Note that the args are local to sum2 (assuming that sum is inlined into sum2). in V8 v7.1, TurboFan can now eliminate the allocation of args completely and replace the variable index access args[i] with a ternary operation of the form i === 0 ? x : y. This yields a ~2% improvement on the JetStream/EarleyBoyer benchmark. We might extend this optimization for arrays with more than two elements in the future.

Structured cloning of Wasm modules

Finally, postMessage is supported for Wasm modules. WebAssembly.Module objects can now be postMessage'd to web workers. To clarify, this is scoped to just web workers (same process, different thread), and not extended to cross-process scenarios (such as cross-origin postMessage or shared web workers).

JavaScript language features

The Intl.RelativeTimeFormat API enables localized formatting of relative times (e.g. “yesterday”, “42 seconds ago”, or “in 3 months”) without sacrificing performance. Here's an example:

// Create a relative time formatter for the English language that does
// not always have to use numeric value in the output.
const rtf = new Intl.RelativeTimeFormat('en', { numeric: 'auto' });

rtf.format(-1, 'day');
// → 'yesterday'

rtf.format(0, 'day');
// → 'today'

rtf.format(1, 'day');
// → 'tomorrow'

rtf.format(-1, 'week');
// → 'last week'

rtf.format(0, 'week');
// → 'this week'

rtf.format(1, 'week');
// → 'next week'

Read the Web Fundamentals article on Intl.RelativeTimeFormat for more information.

V8 v7.1 also adds support for the globalThis proposal, enabling a universal mechanism to access the global object even in strict functions or modules regardless of the platform.


Please use git log branch-heads/7.0..branch-heads/7.1 include/v8.h to get a list of the API changes.

Developers with an active V8 checkout can use git checkout -b 7.1 -t branch-heads/7.1 to experiment with the new features in V8 v7.1. Alternatively you can subscribe to Chrome’s Beta channel and try the new features out yourself soon.