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Simplify full-queue check to use single margin #4426

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Simplify full-queue check to use single margin #4426

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5 changes: 2 additions & 3 deletions kotlinx-coroutines-core/common/src/internal/LockFreeTaskQueue.kt
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Original file line number Diff line number Diff line change
Expand Up @@ -102,9 +102,8 @@ internal class LockFreeTaskQueueCore<E : Any>(
if (state and (FROZEN_MASK or CLOSED_MASK) != 0L) return state.addFailReason() // cannot add
state.withState { head, tail ->
val mask = this.mask // manually move instance field to local for performance
// If queue is Single-Consumer then there could be one element beyond head that we cannot overwrite,
// so we check for full queue with an extra margin of one element
if ((tail + 2) and mask == head and mask) return ADD_FROZEN // overfull, so do freeze & copy
// Reserve one margin element to determine when the queue is full
if ((tail + 1) and mask == head and mask) return ADD_FROZEN // overfull, so do freeze & copy
// If queue is Multi-Consumer then the consumer could still have not cleared element
// despite the above check for one free slot.
if (!singleConsumer && array[tail and mask].value != null) {
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