============ SpiderMonkey ============ *SpiderMonkey* is the *JavaScript* and *WebAssembly* implementation library of the *Mozilla Firefox* web browser. The implementation behaviour is defined by the `ECMAScript `_ and `WebAssembly `_ specifications. Much of the internal technical documentation of the engine can be found throughout the source files themselves by looking for comments labelled with `[SMDOC]`_. Information about the team, our processes, and about embedding *SpiderMonkey* in your own projects can be found at https://spidermonkey.dev. Specific documentation on a few topics is available at: .. toctree:: :maxdepth: 1 build test Debugger/index SavedFrame/index feature_checklist bytecode_checklist Components of SpiderMonkey ########################## ๐Ÿงน Garbage Collector ********************* .. toctree:: :maxdepth: 2 :hidden: Overview Rooting Hazard Analysis Running the Analysis *JavaScript* is a garbage collected language and at the core of *SpiderMonkey* we manage a garbage-collected memory heap. Elements of this heap have a base C++ type of `gc::Cell`_. Each round of garbage collection will free up any *Cell* that is not referenced by a *root* or another live *Cell* in turn. See :doc:`GC overview` for more details. ๐Ÿ“ฆ JS::Value and JSObject ************************** *JavaScript* values are divided into either objects or primitives (*Undefined*, *Null*, *Boolean*, *Number*, *BigInt*, *String*, or *Symbol*). Values are represented with the `JS::Value`_ type which may in turn point to an object that extends from the `JSObject`_ type. Objects include both plain *JavaScript* objects and exotic objects representing various things from functions to *ArrayBuffers* to *HTML Elements* and more. Most objects extend ``NativeObject`` (which is a subtype of ``JSObject``) which provides a way to store properties as key-value pairs similar to a hash table. These objects hold their *values* and point to a *Shape* that represents the set of *keys*. Similar objects point to the same *Shape* which saves memory and allows the JITs to quickly work with objects similar to ones it has seen before. See the `[SMDOC] Shapes`_ comment for more details. C++ (and Rust) code may create and manipulate these objects using the collection of interfaces we traditionally call the **JSAPI**. ๐Ÿ—ƒ๏ธ JavaScript Parser ********************* In order to evaluate script text, we parse it using the *Parser* into an `Abstract Syntax Tree`_ (AST) temporarily and then run the *BytecodeEmitter* (BCE) to generate `Bytecode`_ and associated metadata. We refer to this resulting format as `Stencil`_ and it has the helpful characteristic that it does not utilize the Garbage Collector. The *Stencil* can then be instantiated into a series of GC *Cells* that can be mutated and understood by the execution engines described below. Each function as well as the top-level itself generates a distinct script. This is the unit of execution granularity since functions may be set as callbacks that the host runs at a later time. There are both ``ScriptStencil`` and ``js::BaseScript`` forms of scripts. By default, the parser runs in a mode called *syntax* or *lazy* parsing where we avoid generating full bytecode for functions within the source that we are parsing. This lazy parsing is still required to check for all *early errors* that the specification describes. When such a lazily compiled inner function is first executed, we recompile just that function in a process called *delazification*. Lazy parsing avoids allocating the AST and bytecode which saves both CPU time and memory. In practice, many functions are never executed during a given load of a webpage so this delayed parsing can be quite beneficial. โš™๏ธ JavaScript Interpreter ************************** The *bytecode* generated by the parser may be executed by an interpreter written in C++ that manipulates objects in the GC heap and invokes native code of the host (eg. web browser). See `[SMDOC] Bytecode Definitions`_ for descriptions of each bytecode opcode and ``js/src/vm/Interpreter.cpp`` for their implementation. โšก JavaScript JITs ******************* .. toctree:: :maxdepth: 1 :hidden: MIR-optimizations/index In order to speed up execution of *bytecode*, we use a series of Just-In-Time (JIT) compilers to generate specialized machine code (eg. x86, ARM, etc) tailored to the *JavaScript* that is run and the data that is processed. As an individual script runs more times (or has a loop that runs many times) we describe it as getting *hotter* and at certain thresholds we *tier-up* by JIT-compiling it. Each subsequent JIT tier spends more time compiling but aims for better execution performance. Baseline Interpreter -------------------- The *Baseline Interpreter* is a hybrid interpreter/JIT that interprets the *bytecode* one opcode at a time, but attaches small fragments of code called *Inline Caches* (ICs) that rapidly speed-up executing the same opcode the next time (if the data is similar enough). See the `[SMDOC] JIT Inline Caches`_ comment for more details. Baseline Compiler ----------------- The *Baseline Compiler* use the same *Inline Caches* mechanism from the *Baseline Interpreter* but additionally translates the entire bytecode to native machine code. This removes dispatch overhead and does minor local optimizations. This machine code still calls back into C++ for complex operations. The translation is very fast but the ``BaselineScript`` uses memory and requires ``mprotect`` and flushing CPU caches. WarpMonkey ---------- The *WarpMonkey* JIT replaces the former *IonMonkey* engine and is the highest level of optimization for the most frequently run scripts. It is able to inline other scripts and specialize code based on the data and arguments being processed. We translate the *bytecode* and *Inline Cache* data into a Mid-level `Intermediate Representation`_ (Ion MIR) representation. This graph is transformed and optimized before being *lowered* to a Low-level Intermediate Representation (Ion LIR). This *LIR* performs register allocation and then generates native machine code in a process called *Code Generation*. See `MIR Optimizations`_ for an overview of MIR optimizations. The optimizations here assume that a script continues to see data similar what has been seen before. The *Baseline* JITs are essential to success here because they generate *ICs* that match observed data. If after a script is compiled with *Warp*, it encounters data that it is not prepared to handle it performs a *bailout*. The *bailout* mechanism reconstructs the native machine stack frame to match the layout used by the *Baseline Interpreter* and then branches to that interpreter as though we were running it all along. Building this stack frame may use special side-table saved by *Warp* to reconstruct values that are not otherwise available. ๐ŸŸช WebAssembly *************** In addition to *JavaScript*, the engine is also able to execute *WebAssembly* (WASM) sources. WASM-Baseline (RabaldrMonkey) ----------------------------- This engine performs fast translation to machine code in order to minimize latency to first execution. WASM-Ion (BaldrMonkey) ---------------------- This engine translates the WASM input into same *MIR* form that *WarpMonkey* uses and uses the *IonBackend* to optimize. These optimizations (and in particular, the register allocation) generate very fast native machine code. .. _gc::Cell: https://searchfox.org/mozilla-central/search?q=[SMDOC]+GC+Cell .. _JSObject: https://searchfox.org/mozilla-central/search?q=[SMDOC]+JSObject+layout .. _JS::Value: https://searchfox.org/mozilla-central/search?q=[SMDOC]+JS%3A%3AValue+type&path=js%2F .. _[SMDOC]: https://searchfox.org/mozilla-central/search?q=[SMDOC]&path=js%2F .. _[SMDOC] Shapes: https://searchfox.org/mozilla-central/search?q=[SMDOC]+Shapes .. _[SMDOC] Bytecode Definitions: https://searchfox.org/mozilla-central/search?q=[SMDOC]+Bytecode+Definitions&path=js%2F .. _[SMDOC] JIT Inline Caches: https://searchfox.org/mozilla-central/search?q=[SMDOC]+JIT+Inline+Caches .. _Stencil: https://searchfox.org/mozilla-central/search?q=[SMDOC]+Script+Stencil .. _Bytecode: https://en.wikipedia.org/wiki/Bytecode .. _Abstract Syntax Tree: https://en.wikipedia.org/wiki/Abstract_syntax_tree .. _Intermediate Representation: https://en.wikipedia.org/wiki/Intermediate_representation .. _MIR Optimizations: ./MIR-optimizations/index.html