Swift 6.4 Bridges WebAssembly Up to 40 Times Faster

Swift 6.4 shipped Sept. 15, and its most measurable change is not a language feature: JavaScriptKit's "safe" bridging into WebAssembly is now up to 40 times faster than the library's earlier dynamic bridging, according to the Swift.org release announcement. The post is credited to Joe Heck of Apple's Open Source Program Office and Holly Borla, engineering manager of the Swift language team at Apple and a member of the Swift Core Team.

What changes for a Swift Package Manager build

Swift Package Manager now uses Swift Build as its default build system rather than the older build engine, so projects build the same way on Linux, macOS and Windows for the first time, the announcement said. SwiftPM 6.4 also adds Software Bill of Materials generation in either SPDX or CycloneDX format, letting a team export a dependency manifest without a third-party tool.

Subprocess reaches 1.0, a year after its first release

The Subprocess library, originally proposed as SF-0007 and shipped as an 0.1 preview in 2025, reached version 1.0 in 6.4. It gives Swift a cross-platform way to run and interact with other programs, built on Swift concurrency rather than Foundation's older Process API, according to the announcement.

Component	Status before 6.4	Status in 6.4
Subprocess library	0.1 preview (2025)	1.0
Swift Build in SwiftPM	opt-in	default on Linux, macOS, Windows
JavaScriptKit Wasm bridging	dynamic bridging only	safe bridging up to 40x faster

Where interoperability reaches further

Swift's Span type now bridges directly with C++20's std::span. That interop step was still in beta as of June 28, when InfoQ's Sergio De Simone also detailed a new @C attribute for exposing Swift functions and imported C structs to C code, plus a new @diagnose attribute that lets a project silence a specific deprecation warning or promote a future error early, function by function. De Simone reported up to 4 times faster URL parsing after Swift Foundation moved off legacy Objective-C code, a figure that held from beta into the final release. Android support moves to NDK 30 LTS, and Swift Build now supports Android in SwiftPM without the post-install script earlier releases needed, per the Swift.org post.

Embedded Swift, aimed at microcontroller-class targets with no operating system, gains generalized existential types and a new set of EmbeddedRestrictions warnings that flag code relying on features the embedded runtime cannot support, rather than failing silently at link time.

What the release notes leave out

Neither the Swift.org announcement nor InfoQ's beta coverage says what share of existing JavaScriptKit code uses the older dynamic-bridging path the 40-times figure is measured against, so a project already on safe bridging elsewhere may not see anything close to that number. The release lands on the same Apple developer platform where iOS 27's code points at swapping in a different model provider for Siri and where Rosetta is being retired as of macOS 27, both signs of how much is moving across Apple's toolchain in the same stretch of releases. The next test of the Wasm number comes when JavaScriptKit projects built against 6.4 start shipping outside Apple's own benchmarks.