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JS engine modularization

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The goal of this project is to incrementally tease apart JS engine source code into separate "modules" so that:

  • it is easier to understand a module in isolation
  • it is easier to get a high-level breakdown of SM code
  • module dependencies are DAG-y as opposed to clique-y or spaghetti (as it is now)

The plan isn't to do anything radical to simulate "modules" in C++. Rather, we can just say a "module" X is a triple: X.cpp, X.h, X-inl.h. Here, X.h acts as the "module interface" to the rest of the code-base. X-inl.h contains inline files that are pulled out of X.h so that:

  • X.h is more readable as a whole and
  • we can reduce average translation unit size (which right now is hard because of the dependency clique).

The granularity of modules isn't fixed; the goal is to try to have a module encapsulate a coherent hunk of interdependent logic and implementation details. This may be a single type (e.g., js::Value) or a set of related types (the stack types in vm/Stack.h).

Additionally, we can use directories to group related modules. Again, this is primarily useful to assist newcomers in developing a hierarchical understanding of the code.

This is an incremental project. As developers work on a hunk of code, things tend to be rewritten/refactored into the new SpiderMonkey C++ style. Reviewers should require new code or major alterations to follow the new style unless it would be impractical.

Plan of Record

A meta bug tracks bugs for individual pieces. This document serves to collect consensus of the direction we'd like things to head.

Preliminary directory/module structure:

  • js/src
    • JSAPI implementation (jsapi.cpp, js::Invoke, js::Execute)
    • Interpreter (ideally, if we had maximum code reuse, this would be just the one function calling lots of module (inline) functions)
    • Grotty bits and bobs that don't make sense to modularize
  • js/src/ds
    • LifoAlloc, Vector, HashMap/Set, ...
  • js/src/jit
  • js/src/vm: central, execution-mode-independent data structures: types and operations
    • Stack: execution stack (done)
    • Value: JS value representation
    • String: JS string value representation
    • Perhaps these next two need short distinctive names:
      • Low-level object representation: property tree, shapes, and raw object storage
      • General high-level objects: high level generic set/get/call operations respecting JS semantics
    • Class-specific high-level objects: statically typed interfaces for native objects of a given class
      • Global: the global object
      • Arguments: object accessed through 'arguments' keyword
      • Function: (object) perhaps also contains closely-related JSFunctino/JSScript/Bindings?
  • js/src/gc: garbage collector data structures and algorithms
  • js/src/frontend: scanning, parsing, and emitting
  • js/src/builtin: builtins specified by JS language spec (e.g., String, Array.prototype.sort)
    • String
    • Number
    • Boolean
    • Array
    • Date
    • Math
    • ...
  • js/src/devtools: code, config files and scripts that are generally useful to SM developers but not part of the official build