WebAssembly Debugging

Why Debugging WebAssembly Is Important?

1. Error Resolution:
   • Debugging helps identify and fix runtime errors, such as memory access violations or type mismatches.
2. Performance Optimization:
   • Debugging reveals performance bottlenecks in your WebAssembly code.
3. Improved Understanding:
   • Debugging allows developers to understand how their WebAssembly module interacts with the host environment.
4. Seamless Integration:
   • It ensures that WebAssembly modules work smoothly with JavaScript or other host programming languages.

Challenges in Debugging WebAssembly

1. Binary Format:
   • WebAssembly modules are stored in a compact binary format, making them hard to interpret directly.
2. Minimal Native Debugging Support:
   • WebAssembly itself doesn’t include built-in debugging tools.
3. Interfacing with the Host:
   • Debugging often involves the interplay between WebAssembly and the host environment (e.g., JavaScript).

Debugging Tools for WebAssembly

Modern browsers and tools provide robust debugging support for WebAssembly. Below are some widely used options:

1. Browser Developer Tools

• Most modern browsers like Chrome, Firefox and Edge offer built-in debugging features for WebAssembly.

Key Features:

• View and step through WebAssembly instructions.
• Examine memory and stack states.
• Inspect variables and function calls.

Example in Chrome DevTools:

1. Open the browser’s DevTools.
2. Go to the Sources tab.
3. Find your WebAssembly module under file:// or the server path.
4. Debug by placing breakpoints in the .wasm file or the source map if available.

2. Source Maps

Source maps link WebAssembly binary code back to the original high-level source code (e.g., C, C++, Rust).

Steps:

1. Generate source maps during compilation. > emcc source.c -o output.wasm –source-map
2. Load the .wasm module in the browser with the source map.
3. Use DevTools to debug in your original source language.

3. Command-Line Debugging Tools

For server-side debugging or headless environments, use command-line tools like:

• LLDB (with Wasmtime for debugging compiled Wasm modules).
• GDB for C/C++ generated WebAssembly.

Example with LLDB:

wasmtime run --debug program.wasm

4. WABT (WebAssembly Binary Toolkit)

The WABT toolkit allows you to:

• Decompile .wasm files to human-readable .wat files.
• Inspect WebAssembly text format for debugging.

Example:

Decompile a .wasm file:

wat2wasm module.wat -o module.wasm
wasm2wat module.wasm -o debug.wat

You can then debug the .wat file manually.

5. Debugging in Emscripten

Emscripten is a popular toolchain for compiling C/C++ to WebAssembly and includes debugging utilities.

Enabling Debugging:

1. Compile with debugging symbols > emcc program.c -o program.wasm -g
2. Open the .wasm in the browser with source maps enabled.

Practical Debugging Techniques

1. Viewing Function Calls

Use browser DevTools to view and debug WebAssembly function calls. If debugging in JavaScript:

try {
  wasmModule.instance.exports.someFunction();
} catch (error) {
  console.error("Error in WebAssembly function:", error);
}

2. Inspecting Memory

View WebAssembly memory as an ArrayBuffer and debug it.

Example:

const memory = wasmModule.instance.exports.memory.buffer;
const view = new Uint8Array(memory);

console.log("Memory contents:", view);

3. Setting Breakpoints

In Chrome DevTools, set breakpoints directly in the .wasm file. This allows you to step through WebAssembly instructions and inspect state.

4. Log Debugging

Add custom logging to WebAssembly functions by exporting debug information to JavaScript.

Example:

(module
  (import "env" "log" (func $log (param i32)))
  (func $testFunction (param $x i32)
    local.get $x
    call $log ;; Call the imported log function
  )
  (export "testFunction" (func $testFunction))
)

In JavaScript:

const imports = {
  env: {
    log: (value) => console.log("Wasm log:", value),
  },
};

Debugging Example: Finding Memory Errors

Problem:

You attempt to write beyond the allocated memory.

Example WebAssembly Code:

(module
  (memory 1) ;; 1 page = 64 KB
  (func (export "writeToMemory")
    i32.const 65540 ;; Beyond the memory limit
    i32.const 42
    i32.store ;; Store operation
  )
)

JavaScript Debugging:

(async () => {
  const memory = new WebAssembly.Memory({ initial: 1 });
  const importObject = { env: { memory } };

  try {
    const wasm = await WebAssembly.instantiateStreaming(fetch("module.wasm"), importObject);
    wasm.instance.exports.writeToMemory();
  } catch (error) {
    console.error("Memory error:", error.message);
  }
})();

Expected output:

Memory error: Out of bounds memory access.

Best Practices for Debugging WebAssembly

1. Compile with Debugging Symbols:
   • Use -g or equivalent flags during compilation to enable debugging.
2. Use Logging Extensively:
   • Print intermediate values and states to track execution flow.
3. Test in a Controlled Environment:
   • Debug smaller modules before integrating them into larger applications.
4. Understand Traps:
   • WebAssembly traps (e.g., division by zero) must be handled at the host level.
5. Use Source Maps:
   • Debug in the original source language for easier troubleshooting.