[WIP] Improve performance of comine(Iterable<Flow>) by reusing lambdas for onReceive

Addresses #2296

Author: qwwdfsad

Diff for: benchmarks/src/jmh/kotlin/benchmarks/flow/CombineBenchmark.kt

@@ -0,0 +1,36 @@
+/*
+ * Copyright 2016-2020 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package benchmarks.flow
+
+import kotlinx.coroutines.*
+import kotlinx.coroutines.flow.*
+import org.openjdk.jmh.annotations.*
+import java.util.concurrent.*
+
+@Warmup(iterations = 5, time = 1, timeUnit = TimeUnit.SECONDS)
+@Measurement(iterations = 5, time = 1, timeUnit = TimeUnit.SECONDS)
+@Fork(value = 1)
+@BenchmarkMode(Mode.AverageTime)
+@OutputTimeUnit(TimeUnit.MILLISECONDS)
+@State(Scope.Benchmark)
+open class CombineBenchmark {
+
+    @Benchmark
+    fun measure10() = measure(10)
+
+    @Benchmark
+    fun measure100() = measure(100)
+
+    @Benchmark
+    fun measure1000() = measure(1000)
+
+    fun measure(size: Int) = runBlocking {
+        val flowList = (1..size).map { flowOf(it) }
+        val listFlow = combine(flowList) { it.toList() }
+
+        listFlow.collect {
+        }
+    }
+}

Diff for: kotlinx-coroutines-core/common/src/flow/internal/Combine.kt

@@ -56,14 +56,18 @@ internal suspend fun <R, T> FlowCollector<R>.combineInternal(
     val isClosed = Array(size) { false }
     var nonClosed = size
     var remainingNulls = size
-    // See flow.combine(other) for explanation.
-    while (nonClosed != 0) {
-        select<Unit> {
-            for (i in 0 until size) {
-                onReceive(isClosed[i], channels[i], { isClosed[i] = true; --nonClosed }) { value ->
-                    if (latestValues[i] == null) --remainingNulls
-                    latestValues[i] = value
-                    if (remainingNulls != 0) return@onReceive
+    // See flow.combine(other) for explanation of the logic
+    // Reuse receive blocks to avoid allocations on each iteration
+    val onReceiveBlocks = Array<suspend (Any?) -> Unit>(size) { i ->
+        { value ->
+            if (value === null) {
+                isClosed[i] = true;
+                --nonClosed
+            }
+            else {
+                if (latestValues[i] == null) --remainingNulls
+                latestValues[i] = value
+                if (remainingNulls == 0) {
                     val arguments = arrayFactory()
                     for (index in 0 until size) {
                         arguments[index] = NULL.unbox(latestValues[index])
@@ -73,6 +77,15 @@ internal suspend fun <R, T> FlowCollector<R>.combineInternal(
             }
         }
     }
+
+    while (nonClosed != 0) {
+        select<Unit> {
+            for (i in 0 until size) {
+                if (isClosed[i]) continue
+                channels[i].onReceiveOrNull(onReceiveBlocks[i])
+            }
+        }
+    }
 }
 
 private inline fun SelectBuilder<Unit>.onReceive(