WebAssembly GC: Compiling Java and C# to Wasm

The WebAssembly Garbage Collection (Wasm GC) proposal is a major milestone that redefines how managed languages like Java and C# are compiled for the web. Historically, running these languages in the browser required shipping a bulky runtime and custom garbage collector compiled to WebAssembly. With the native Wasm GC proposal, compilers can now target the browser’s host garbage collector directly, resulting in significantly smaller file sizes, faster startup times, and improved performance.

The Old Way: Shipping the Runtime

Before the Wasm GC proposal, WebAssembly only understood simple data types like integers and floats, operating within a single flat array of memory (linear memory). Because Wasm had no concept of objects, structs, or garbage collection, compilers for languages like Java and C# had to compile the language’s entire runtime—including its memory allocator and garbage collector—into the final WebAssembly binary.

For Java and C#, this meant compiling virtual machine components (like a mini-JVM or .NET CLR) into Wasm. This approach suffered from several major drawbacks: * Bloated File Sizes: Even a simple “Hello World” application required megabytes of data to ship the runtime. * Slow Startup Times: Browsers had to download, parse, and compile massive binaries before the application could run. * Memory Isolation: The compiled garbage collector could not easily see or clean up JavaScript objects, leading to complex reference-tracking workarounds and potential memory leaks.

The New Way: Utilizing Host Garbage Collection

The Wasm GC proposal changes this by introducing high-level, garbage-collected types—specifically structs and arrays—directly into the WebAssembly instruction set. Instead of managing a private block of linear memory, compiled WebAssembly code can now allocate objects directly in the host environment’s (the browser’s) garbage-collected heap.

This shifts the responsibility of memory management from the compiled application binary to the browser’s native engine (such as V8 in Chrome, SpiderMonkey in Firefox, or JavaScriptCore in Safari).

Key Impacts on Java and C# Compilation

1. Drastic Reduction in Binary Sizes

Compilers no longer need to package a complex garbage collector or a heavy runtime memory manager. When targeting Wasm GC, tools like TeaVM (for Java) or the .NET compiler pipeline can emit native Wasm GC instructions. This eliminates megabytes of redundant runtime code, resulting in lightweight binaries that load and run almost instantly.

2. Seamless JavaScript Interoperability

Previously, passing complex object graphs between JavaScript and WebAssembly required serializing data into linear memory, copying it, and deserializing it. With Wasm GC, Java and C# objects can be passed directly to JavaScript as garbage-collected references. Both languages can share the same heap, allowing the browser to clean up cycles of mutually referencing JS and Wasm objects without memory leaks.

3. Better Performance and Optimization

Modern browser engines have highly optimized, generational garbage collectors developed over decades. By leveraging Wasm GC, compiled Java and C# applications automatically benefit from these advanced garbage collection algorithms, including concurrent marking and sweeping, without any additional overhead in the application code.

4. Simplified Compiler Architecture

Compiler developers no longer need to write custom memory allocators or port GC algorithms to WebAssembly. Instead, they can focus on mapping the source language’s object model (such as Java classes or C# classes and structs) directly to WebAssembly’s native struct and array types. This simplifies the creation of ahead-of-time (AOT) compilers for managed languages targeting the web.