AtomicRef<Int>.compareAndSet always fails when value >= 128 on macOS and iOS

[paxbun]

I'm using kotlinx-atomicfu 0.22.0. I was having a problem where my program halts at AtomicRef<Int>.getAndUpdate, and I found that AtomicRef<Int>.compareAndSet always fails when the value >= 128 on macOS and iOS, both for JVM and native. For example:

for (i in 0 until 1024) {
    val a = kotlinx.atomicfu.atomic<Int>(i)
    print("${i}: ${a.compareAndSet(i, 8192)}")
}

run result:

0 true
...
127 true
128 false
...
1023 false

This does not happen when I use AtomicInt instead of AtomicRef<Int>.

for (i in 0 until 1024) {
    val a = kotlinx.atomicfu.atomic(i)
    println("${i}: ${a.compareAndSet(i, 8192)}")
}

run result:

0 true
...
127 true
128 true
...
1023 true

[paxbun]

This is the original issue: https://gitlab.com/trixnity/uniffi-kotlin-multiplatform-bindings/-/issues/24

[paxbun]

Found that kotlin.concurrent.AtomicReference<Int> (not kotlinx) has the same problem. Create a fresh multiplatform project, set the Kotlin version to 1.9.20, run the following, and you'll get the same result.

import kotlin.concurrent.AtomicReference

fun main() {
    repeat(129) {
        val a = AtomicReference(it)
        val result = a.compareAndSet(it, 8192)
        println("$it: $result")
    }
}

[paxbun]

Is it an expected behavior and an imitation of JVM's Integer pooling? I also found that compareAndSet always fails when value < -128.

[qwwdfsad]

It seems like we need a proper inspection or compiler diagnostic here.
AtomicRef<Int> is as dangerous and tricky as Java's AtomicReference<Integer> -- it leaks the fact that boxed integer values may or may not be cached, and atomic ref (rightfully) uses identity equals and fails here.

So I'd say the behaviour is as intended, but we need a proper diagnostic to prevent such a bug (note: it's also reproducible with other primitives including booleans)

[qwwdfsad]

Posted the solution internally, re-posting here:

public fun <T> atomic(initial: T): AtomicRef<T> = atomic(initial, TraceBase.None)
public fun atomic(initial: Int): AtomicInt = atomic(initial, TraceBase.None)
@JvmName("atomicInt")
@Suppress("FINAL_UPPER_BOUND")
public fun <T : Int> atomic(initial: T): AtomicInt = atomic(initial.toInt(), TraceBase.None)

private val atomic = atomic<Int>(0)

@Test
fun foo() {
    atomic.value = 129
    println(atomic.compareAndSet(129, 130)) // true
}