Ammo.js WebAssembly Heap Limit Exceeded

This article explains what happens when the ammo.js physics engine exceeds its allocated WebAssembly (Wasm) memory heap limit. We will explore the immediate technical consequences of running out of memory, including application crashes and silent failures, and outline how to diagnose and resolve these heap limit issues in your JavaScript and WebGL projects.

When ammo.js—which is a port of the C++ Bullet physics engine to WebAssembly—exhausts its allocated memory heap, the application will immediately crash or behave unpredictably. Because WebAssembly operates within a strictly allocated, isolated linear memory buffer, exceeding this boundary prevents the engine from allocating new physics bodies, colliders, or constraints.

WebAssembly Memory Exceptions

The most common outcome of exceeding the heap limit is a severe runtime error thrown by the browser’s WebAssembly engine. You will typically see an uncaught exception in the developer console, such as: * RuntimeError: memory access out of bounds * Cannot enlarge memory arrays

Once this error is thrown, the WebAssembly module enters a broken state. The physics loop halts, meaning your 3D scenes will freeze, and any subsequent calls to the Ammo namespace from JavaScript will fail.

Memory Leaks and Null Pointer Dereferences

In WebAssembly, memory must be managed manually. If you continuously create physics objects without destroying them using Ammo.destroy(object), the heap will eventually fill up.

When the heap is full, internal C++ allocation functions (like malloc) return null pointers. If ammo.js attempts to write data to these null pointers, it causes a silent failure or a crash. In some cases, instead of a clean browser error, the physics simulation will simply stop updating, or objects will glitch and fall through the ground because their underlying physics representations could not be instantiated.

How to Resolve the Memory Limit

To prevent ammo.js from exceeding its heap limit, you can implement the following solutions:

  1. Enable Memory Growth: If you compile your own version of ammo.js using Emscripten, ensure the -s ALLOW_MEMORY_GROWTH=1 flag is enabled. This allows the WebAssembly heap to dynamically expand when it runs out of space, though it may introduce minor performance hiccups during expansion.
  2. Increase Initial Memory: You can increase the initial heap size at startup by configuring the Emscripten settings (e.g., -s INITIAL_MEMORY=67108864 for 64MB).
  3. Strict Memory Management: Always clean up unused physics assets. Whenever you remove a rigid body, collision shape, or physics constraint from your world, you must explicitly free its memory using Ammo.destroy().