SpiderMonkey is the JavaScript and WebAssembly engine in Firefox. Cretonne is designed to be used in SpiderMonkey with the goal of enabling better code generation for ARM's 32-bit and 64-bit architectures, and building a framework for improved low-level code optimizations in the future.
SpiderMonkey currently has two WebAssembly compilers: The tier 1 baseline compiler (not shown below) and the tier 2 compiler using the IonMonkey JavaScript compiler's optimizations and register allocation.
In phase 1, Cretonne aims to replace the IonMonkey-based tier 2 compiler for WebAssembly only. It will still be orchestrated by the BaldrMonkey engine and compile WebAssembly modules on multiple threads. Cretonne translates binary wasm functions directly into its own intermediate representation, and it generates binary machine code without depending on SpiderMonkey's macro assembler.
The IonMonkey JIT compiler is designed to compile JavaScript code. It uses two separate intermediate representations to do that:
- MIR is used for optimizations that are specific to JavaScript JIT compilation. It has good support for JS types and the special tricks needed to make JS fast.
- LIR is used for register allocation.
Cretonne has its own register allocator, so the LIR representation can be skipped when using Cretonne as a backend for IonMonkey.