ECMAScript® 2024 Language Specification

Draft ECMA-262 / February 15, 2024

29.1 Memory Model Fundamentals

Shared memory accesses (reads and writes) are divided into two groups, atomic accesses and data accesses, defined below. Atomic accesses are sequentially consistent, i.e., there is a strict total ordering of events agreed upon by all agents in an agent cluster. Non-atomic accesses do not have a strict total ordering agreed upon by all agents, i.e., unordered.

Note 1

No orderings weaker than sequentially consistent and stronger than unordered, such as release-acquire, are supported.

A Shared Data Block event is either a ReadSharedMemory, WriteSharedMemory, or ReadModifyWriteSharedMemory Record.

Table 91: ReadSharedMemory Event Fields
Field Name Value Meaning
[[Order]] seq-cst or unordered The weakest ordering guaranteed by the memory model for the event.
[[NoTear]] a Boolean Whether this event is allowed to read from multiple write events with equal range as this event.
[[Block]] a Shared Data Block The block the event operates on.
[[ByteIndex]] a non-negative integer The byte address of the read in [[Block]].
[[ElementSize]] a non-negative integer The size of the read.
Table 92: WriteSharedMemory Event Fields
Field Name Value Meaning
[[Order]] seq-cst, unordered, or init The weakest ordering guaranteed by the memory model for the event.
[[NoTear]] a Boolean Whether this event is allowed to be read from multiple read events with equal range as this event.
[[Block]] a Shared Data Block The block the event operates on.
[[ByteIndex]] a non-negative integer The byte address of the write in [[Block]].
[[ElementSize]] a non-negative integer The size of the write.
[[Payload]] a List of byte values The List of byte values to be read by other events.
Table 93: ReadModifyWriteSharedMemory Event Fields
Field Name Value Meaning
[[Order]] seq-cst Read-modify-write events are always sequentially consistent.
[[NoTear]] true Read-modify-write events cannot tear.
[[Block]] a Shared Data Block The block the event operates on.
[[ByteIndex]] a non-negative integer The byte address of the read-modify-write in [[Block]].
[[ElementSize]] a non-negative integer The size of the read-modify-write.
[[Payload]] a List of byte values The List of byte values to be passed to [[ModifyOp]].
[[ModifyOp]] a read-modify-write modification function An abstract closure that returns a modified List of byte values from a read List of byte values and [[Payload]].

These events are introduced by abstract operations or by methods on the Atomics object.

Some operations may also introduce Synchronize events. A Synchronize event has no fields, and exists purely to directly constrain the permitted orderings of other events.

In addition to Shared Data Block and Synchronize events, there are host-specific events.

Let the range of a ReadSharedMemory, WriteSharedMemory, or ReadModifyWriteSharedMemory event be the Set of contiguous integers from its [[ByteIndex]] to [[ByteIndex]] + [[ElementSize]] - 1. Two events' ranges are equal when the events have the same [[Block]], and the ranges are element-wise equal. Two events' ranges are overlapping when the events have the same [[Block]], the ranges are not equal and their intersection is non-empty. Two events' ranges are disjoint when the events do not have the same [[Block]] or their ranges are neither equal nor overlapping.

Note 2

Examples of host-specific synchronizing events that should be accounted for are: sending a SharedArrayBuffer from one agent to another (e.g., by postMessage in a browser), starting and stopping agents, and communicating within the agent cluster via channels other than shared memory. It is assumed those events are appended to agent-order during evaluation like the other SharedArrayBuffer events.

Events are ordered within candidate executions by the relations defined below.