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cranelift/spidermonkey.md

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+Cranelift in SpiderMonkey
+=========================
+
+[SpiderMonkey](https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey)
+is the JavaScript and WebAssembly engine in Firefox. Cranelift 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.
+
+Phase 1: WebAssembly
+--------------------
+
+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.
+
+![Cranelift in SpiderMonkey phase 1](media/spidermonkey1.png)
+
+In phase 1, Cranelift 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.
+Cranelift translates binary wasm functions directly into its own
+intermediate representation, and it generates binary machine code
+without depending on SpiderMonkey's macro assembler.
+
+Phase 2: IonMonkey
+------------------
+
+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.
+
+![Cranelift in SpiderMonkey phase 2](media/spidermonkey2.png)
+
+Cranelift has its own register allocator, so the LIR representation can
+be skipped when using Cranelift as a backend for IonMonkey.