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VM Overview

Value Representation#

The VM uses a class called HermesValue to encapsulate JS values efficiently, preserving their type while still allowing them to fit in a register. NaN-tagging is used to store different types of values; we store values in the lower bits of a uint64_t. Thus, when the uint64_t is interpreted as a double, tagged NaN values can hold non-double types.


StringPrimitive is used to store immutable UTF16 encoded strings, and StringPrimitive * can be stored in HermesValue to make JS String values.

Internally, StringPrimitive can be

  • DynamicStringPrimitive (stored in the GC heap)
  • ExternalStringPrimitive (stored as a pointer outside the VM, such as into a bytecode file)


The Runtime class is the primary driver of the VM. It contains the current environment and heap, as well as the code to execute. Runtime is used to execute RuntimeModules, which are constructed from BytecodeModules using Runtime::runModule().

Runtime Module#

A RuntimeModule is the VM representation into a bytecode file. RuntimeModules are stored outside the GC heap and are constructed via new.

To allow for segmentation of bytecode files and requireing modules between separate segments, we collect RuntimeModules in a class called Domain. You may think of the Domain as the collection of bytecode files which were all compiled in the same invocation of the compiler.

Every JSFunction shares ownership of a Domain, and the Domain owns the RuntimeModules which provide those functions. In this way, when all JSFunctions which require the files in a Domain are collected, the Domain and the RuntimeModules are also collected.

Runtime Identifiers#

The Runtime contains an IdentifierTable, which is used for getting unique IDs for strings. The table is used to go from StringPrimitive to SymbolID and back. It's prepopulated with some "predefined strings", the set of strings that are required by built in functions, which can be seen in PredefinedStrings.def.

Garbage Collection#

Currently, the VM uses GenGCNC (generational non-contiguous GC). The collector allows non-contiguous heap allocation. This avoids preallocating too much memory, as well as returning memory to the OS. The garbage collector is precise (it knows what HermesValues are valid pointers to objects in the JS heap).

See the documentation for the Hades garbage collector for details on how garbage collection works in Hermes. Hades is a garbage collector that improves pause times dramatically over the older collector, called GenGC. We recommend Hades for most applications.

The garbage collector moves objects to different place on the heap, invalidating HermesValues, so there are a couple classes which allow updating them automatically. Handle<> and Handle<T> are garbage collector-aware handles; they are moved if a collection occurs in between two successive accesses. So, to ensure correctness in the VM, use the handles instead of passing raw HermesValue between functions.

A GCScope is used to keep track of all the current HermesValue handles. Any GCScope must be constructed on the stack, whence it tracks any scoped handles that are used until it falls out of scope. The GCScope allocates space in chunks, and when it is destroyed (falls out of scope) it frees any chunks it allocated. The GCScope is used to internally generate PinnedHermesValues, which are then stored in Handle<> and Handle<T>.

We also provide PseudoHandle<T> classes which are explicitly not handles. These are used to be explicit about storage of raw pointers and HermesValue. PseudoHandle should be used as an argument in place of a raw pointer to functions which may want to turn that argument into a Handle, but in which it's not necessary to always incur the cost of handle allocation. PseudoHandle also does not have a copy constructor, and moving out of one invalidates it. This prevents the reuse of PseudoHandle after an allocating function call.

Rules for using handles#

  1. A function that can perform an allocation (even if it doesn't do it every time) or calls a function that does, must accept and return only handles (for GC-managed objects). It must also take a Runtime* as an argument.
  2. A function that accepts or returns handles is allowed (and can be assumed to) allocate more handles, but the upper bound of allocated handles must be static.
  3. The number of handles in a given GCScope should have a static upper limit.

The motivation for these rules should be self-explanatory. The practical implication of rule 2 and 3 is that recursion and loops that allocate handles in every iteration must be treated specially. In case of recursion a new GCScope should be defined in each recurrence (is that the correct term?). In case of a loop, there are a couple of possibilities:

  • in loops that are expected to be low iteration and not performance critical, a new GCScope can be defined in the body of the loop.
  • otherwise a GCScope::Marker should be used to flush the allocated handles of the previous iteration.
  • mutable handles can be used to avoid allocating a new handle on every iteration.

Object Model#

Currently the object model is a VTable-based scheme, in which all possible JS values inherit from a base garbage collector VTable. These are called "cells", and all the cells are defined in CellKinds.def. Objects have a special ObjectVTable, Callables have a CallableVTable, etc.


Each JS object is represented by Object (or a class derived from Object). JS objects have a set of name/value pairs, and some optional "indexed storage". Read more about how Object works in ObjectModel.h. The Runtime contains a global object which is used to store in global scope.


Arrays, the arguments object, etc. inherit from Object directly, but simply provide their own implementations of *OwnIndexed using the VTable.


Functions and native functions inherit from Callable. This allows them to call executeCall* to run functions using the internal API.

Boxed Primitives#

The PrimitiveBox class is used to contain Booleans, Strings, and Numbers, when they are constructed using their respective JS constructors. JSString is a PrimitiveBox that is used for String objects, etc.


The HermesVM provides a REPL in bin/hermes, which calls through to the eval() global function in the Runtime.

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