wasm-tracing-allocator

A global allocator for Wasm that traces allocations and deallocations for debugging purposes.

wasm-tracing-allocator enables you to better debug and analyze memory leaks and invalid frees in an environment where we don't have access to the conventional tools like Valgrind. The tracing hooks are safely implemented in JS, outside the Wasm module and its linear memory, to ensure that the tracing code doesn't perturb results.

Table of Contents

• Enabling the Tracing Allocator
• Analyzing and Debugging

Enabling the Tracing Allocator

First, add wasm-tracing-allocator to your Cargo.toml's dependency list:

[dependencies]
wasm-tracing-allocator = "0.1.0"

Next, configure wasm_tracing_allocator::WasmTracingAllocator as the global allocator:

// src/lib.rs

use std::alloc::System;
use wasm_tracing_allocator::WasmTracingAllocator;

#[global_allocator]
static GLOBAL_ALLOCATOR: WasmTracingAllocator<System> = WasmTracingAllocator(System);

Finally, make the JS implementations of the tracing hooks are available for your Wasm module to import:

• On the Web, add this script before your Wasm module is instantiated:

  <script src="https://unpkg.com/[email protected]/js/hooks.js"></script>

• On Node.js, require the hooks before your Wasm module is instantiated:

  require("wasm-tracing-allocator");

Analyzing and Debugging

Use your developer tools console to invoke methods of the global WasmTracingAllocator object to get analyses about allocations and deallocations.

The output is typically rendered with console.table:

WasmTracingAllocator.dumpLiveAllocations

Dump a table of live allocations to the console.

WasmTracingAllocator.dumpLiveAllocations({
  keyLabel: String,
  valueLabel: String,
  getKey: Object => any,
  getValue: Object => Number,
});

• keyLabel: Optional. The string label used to describe the keys column in the table.

• valueLabel: Optional. The string label used to describe the values column in the table.

• getKey: Optional. Function from an allocation entry object to anything. The table will group and aggregate entries by their keys. Defaults to the stack at the time of the allocation.

• getValue: Optional. Function from an allocation entry object to a number. The values for all entries with the same key are summed. Defaults to the byte size of each allocation; a potential alternative would be to ignore the argument and return 1 to count the number of allocations instead.

WasmTracingAllocator.dumpInvalidFrees

Dump a table of invalid frees (double frees, frees of things that were never allocated, etc...) to the console.

WasmTracingAllocator.dumpInvalidFrees({
  keyLabel: String,
  valueLabel: String,
  getKey: Object => any,
  getValue: Object => Number,
});

• keyLabel: Optional. The string label used to describe the keys column in the table.

• valueLabel: Optional. The string label used to describe the values column in the table.

• getKey: Optional. Function from an invalid free entry object to anything. The table will group and aggregate entries by their keys. Defaults to the stack at the time of the deallocation.

• getValue: Optional. Function from an invalid free entry object to a number. The values for all entries with the same key are summed. Defaults to counting the number of invalid frees.