Wednesday, October 25, 2017

V8 Release 6.3

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 6.3, which is in beta until its release in coordination with Chrome 63 Stable in several weeks. V8 v6.3 is filled with all sorts of developer-facing goodies. This post provides a preview of some of the highlights in anticipation of the release.


Jank Busters III hit the shelves as part of the Orinoco project. Concurrent marking (70-80% of marking is done on a non-blocking thread) is shipped.

The parser now does not need to preparse a function a second time. This translates to a 14 % median improvement in parse time on our internal startup top25 benchmark.

string.js has been completely ported to CodeStubAssembler. Thanks a lot to @peterwmwong for his awesome contributions! As a developer this means that builtin string functions like String#trim are a lot faster starting with 6.3.'s performance is now roughly on-par with alternatives. In general, 6.3 continues the path to better the ES2015+ performance. Beside other items we boosted the speed of polymorphic access to symbols, polymorphic inlining of constructor calls and (tagged) template literals.

 V8's performance over the past six releases

Weak optimized function list is gone. More information can be found in the dedicated blog post.

The mentioned items are a non-exhaustive list of speed improvements. Lot's of other performance-related work has happened.

Memory consumption

Write barriers are switched over to using the CodeStubAssembler. This saves around 100kb of memory per isolate.

ECMAScript language features

V8 shipped the following stage 3 features: Dynamic module import via import(), Promise.prototype.finally and async iterators/generators.

With dynamic module import it is very straightforward to import modules based on runtime conditions. This comes in handy when an application should lazy load certain code modules.

Promise.prototype.finally introduces a way to easily clean up after a promise is settled.

Iterating with async functions got more ergonomic with the introduction of async iterators/generators.


In Chrome 63 block coverage is also supported in the DevTools UI. Please note that the inspector protocol already supports block coverage since V8 6.2.


Please check out our summary of API changes. This document is regularly updated a few weeks after each major release.

Developers with an active V8 checkout can use git checkout -b 6.3 -t branch-heads/6.3 to experiment with the new features in V8 6.3. Alternatively you can subscribe to Chrome’s Beta channel and try the new features out yourself soon.

Posted by the V8 team