Thursday, June 21, 2018

V8 release v6.8

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 6.8, which is in beta until its release in coordination with Chrome 68 Stable in several weeks. V8 v6.8 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 functions unnecessarily kept outer functions and their metadata (known as SharedFunctionInfo or SFI) alive. Especially in function-heavy code that relies on short-lived IIFEs, this could lead to spurious memory leaks. Before this change, an active Context (i.e. an on-heap representation of a function activation) kept the SFI alive of the function that created the context:

By letting the Context point to a ScopeInfo object which contains the stripped-down information necessary for debugging, we can break the dependency on the SFI.

We’ve already observed 3% V8 memory improvements on mobile devices over a set of top 10 pages.

In parallel we have reduced the memory consumption of SFIs themselves, removing unnecessary fields or compressing them where possible, and decreased their size by ~25%, with further reductions coming in future releases. We’ve observed SFIs taking up 2–6% of V8 memory on typical websites even after detaching them from the context, so you should see memory improvements on code with a large number of functions.


Array destructuring improvements

The optimizing compiler did not generate ideal code for array destructuring. For example, swapping variables using [a, b] = [b, a] used to be twice as slow as const tmp = a; a = b; b = tmp. Once we unblocked escape analysis to eliminate all temporary allocation, array destructuring with a temporary array is as fast as a sequence of assignments.

Object.assign improvements

So far Object.assign had a fast path written in C++. That meant that the JavaScript-to-C++ boundary had to be crossed for each Object.assign call. An obvious way to improve the builtin performance was to implement a fast path on the JavaScript side. We had two options: either implement it as an native JS builtin (which would come with some unnecessary overhead in this case), or implement it using CodeStubAssembler technology (which provides more flexibility). We went with the latter solution. The new implementation of Object.assign improves the score of Speedometer2/React-Redux by about 15%, improving the total Speedometer 2 score by 1.5%.

TypedArray.prototype.sort improvements

TypedArray.prototype.sort has two paths: a fast path, used when the user does not provide a comparison function, and a slow path for everything else. Until now, the slow path reused the implementation for Array.prototype.sort, which does a lot more than is necessary for sorting TypedArrays. V8 v6.8 replaces the slow path with an implementation in CodeStubAssembler. (Not directly CodeStubAssembler but a domain-specific language that is built on top of CodeStubAssembler).

Performance for sorting TypedArrays without a comparison function stays the same while there is a speedup of up to 2.5× when sorting using a comparison function.


In V8 v6.8 you can start using trap-based bounds checking on Linux x64 platforms. This memory management optimization considerably improves WebAssembly’s execution speed. It’s already used in Chrome 68, and in the future more platforms will be supported incrementally.


Please use git log branch-heads/6.7..branch-heads/6.8 include/v8.h to get a list of the API changes.

Developers with an active V8 checkout can use git checkout -b 6.8 -t branch-heads/6.8 to experiment with the new features in V8 v6.8. Alternatively you can subscribe to Chrome’s Beta channel and try the new features out yourself soon.

Posted by the V8 team

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