ECMAScript® 2024 Language Specification

Draft ECMA-262 / February 15, 2024

29.5 Abstract Operations for the Memory Model

29.5.1 EventSet ( execution )

The abstract operation EventSet takes argument execution (a candidate execution) and returns a Set of events. It performs the following steps when called:

  1. Let events be an empty Set.
  2. For each Agent Events Record aer of execution.[[EventsRecords]], do
    1. For each event E of aer.[[EventList]], do
      1. Add E to events.
  3. Return events.

29.5.2 SharedDataBlockEventSet ( execution )

The abstract operation SharedDataBlockEventSet takes argument execution (a candidate execution) and returns a Set of events. It performs the following steps when called:

  1. Let events be an empty Set.
  2. For each event E of EventSet(execution), do
    1. If E is a ReadSharedMemory, WriteSharedMemory, or ReadModifyWriteSharedMemory event, add E to events.
  3. Return events.

29.5.3 HostEventSet ( execution )

The abstract operation HostEventSet takes argument execution (a candidate execution) and returns a Set of events. It performs the following steps when called:

  1. Let events be an empty Set.
  2. For each event E of EventSet(execution), do
    1. If E is not in SharedDataBlockEventSet(execution), add E to events.
  3. Return events.

29.5.4 ComposeWriteEventBytes ( execution, byteIndex, Ws )

The abstract operation ComposeWriteEventBytes takes arguments execution (a candidate execution), byteIndex (a non-negative integer), and Ws (a List of either WriteSharedMemory or ReadModifyWriteSharedMemory events) and returns a List of byte values. It performs the following steps when called:

  1. Let byteLocation be byteIndex.
  2. Let bytesRead be a new empty List.
  3. For each element W of Ws, do
    1. Assert: W has byteLocation in its range.
    2. Let payloadIndex be byteLocation - W.[[ByteIndex]].
    3. If W is a WriteSharedMemory event, then
      1. Let byte be W.[[Payload]][payloadIndex].
    4. Else,
      1. Assert: W is a ReadModifyWriteSharedMemory event.
      2. Let bytes be ValueOfReadEvent(execution, W).
      3. Let bytesModified be W.[[ModifyOp]](bytes, W.[[Payload]]).
      4. Let byte be bytesModified[payloadIndex].
    5. Append byte to bytesRead.
    6. Set byteLocation to byteLocation + 1.
  4. Return bytesRead.
 Note 1

The read-modify-write modification [[ModifyOp]] is given by the function properties on the Atomics object that introduce ReadModifyWriteSharedMemory events.

 Note 2

This abstract operation composes a List of write events into a List of byte values. It is used in the event semantics of ReadSharedMemory and ReadModifyWriteSharedMemory events.

29.5.5 ValueOfReadEvent ( execution, R )

The abstract operation ValueOfReadEvent takes arguments execution (a candidate execution) and R (a ReadSharedMemory or ReadModifyWriteSharedMemory event) and returns a List of byte values. It performs the following steps when called:

  1. Let Ws be execution.[[ReadsBytesFrom]](R).
  2. Assert: Ws is a List of WriteSharedMemory or ReadModifyWriteSharedMemory events with length equal to R.[[ElementSize]].
  3. Return ComposeWriteEventBytes(execution, R.[[ByteIndex]], Ws).