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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.

After we reserve the canonical quiet NaN to be used as the NaN representation in the VM, we have 51 remaining bits we can set. Even on 64-bit systems, we are able to accommodate a full pointer and type tag, since pointers are never more than 48 bits in practice.

A HermesValue can take on any of the following types of values, which are distinguished by their type tag:

  1. Empty
  2. Undefined
  3. Null
  4. Boolean
  5. Symbol
  6. Native value (used to store an int or pointer for bookkeeping)
  7. String pointer
  8. Object pointer

HermesValue is used across the VM to store and pass JS values. PinnedHermesValue is used in non-moveable memory, primarily for the register stack and other GC roots in known locations in memory. GCHermesValues are used on the GC managed heap.


When compressed pointers are enabled, we also encode some values in a compact 32-bit representation called HermesValue32. Because compressing pointers to store in this format requires additional work, we also avoid using HermesValue32 for frequently accessed values (like the register stack). Instead we selectively use HermesValue32 for objects that are known to consume a large percentage of heap memory, but that are unlikely to affect performance.

HermesValue32 supports storing all of the types that HermesValue does, except for native values, which were dropped because they are relatively rare and because we do not have a mechanism for compressing native pointers outside the GC managed heap.

Instead of using NaN boxing, it takes advantage of the 8-byte alignment of the Hermes heap to store tags, since the lowest 3 bits of a pointer are always guaranteed to be zero (even after compression).

HermesValue32 also requires special handling for doubles. It is able to store small integers up to 29 bits inline, but anything that cannot be represented in that form must be stored as a separate double object on the heap. Based on the workloads we have looked at, actual doubles are very rarely used, so the overhead of this approach is small.


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 HadesGC by default, a concurrent garbage collector aimed at dramatically lowering pause times over our previous collector GenGC. The heap in Hermes is non-contiguous, which allows us to avoid reserving large regions of address space upfront, and allows us to return memory to the OS at a finer granularity. Garbage collection in Hermes is precise, which means that the GC always knows which values contain valid pointers.

See the documentation for Hades for details on how it works.

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 VM has classes used to contain Booleans, Strings, and Numbers, when they are constructed using their respective JS constructors. JSString is a boxed String object, etc.


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