IR: threadLocal

To be used for per-file state in global structures during parallel
lowering.

compiler/util/src/org/jetbrains/kotlin/utils/threadLocal.kt

@@ -0,0 +1,26 @@
+/*
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.utils
+
+import java.util.concurrent.ConcurrentHashMap
+import kotlin.properties.ReadWriteProperty
+import kotlin.reflect.KProperty
+
+fun <T> threadLocal(initializer: () -> T): ReadWriteProperty<Any?, T> = ThreadLocalDelegate(initializer)
+
+private class ThreadLocalDelegate<T>(private val initializer: () -> T) : ReadWriteProperty<Any?, T> {
+    private val map = ConcurrentHashMap<Thread, T>()
+
+    override operator fun getValue(thisRef: Any?, property: KProperty<*>): T {
+        return map.getOrPut(Thread.currentThread()) {
+            initializer()
+        }
+    }
+
+    override operator fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {
+        map[Thread.currentThread()] = value
+    }
+}