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.
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
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 improves the score of Speedometer2/React-Redux by about 15%, improving the total Speedometer 2 score by 1.5%.
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.
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.