Make empty array handling more efficient, also fixes KT-21758 (#1285)

* Make empty array handling more efficient, also fixes KT-21758

* Fix WebAssembly.

runtime/src/main/cpp/Arrays.cpp

@@ -39,6 +39,14 @@ static inline void copyImpl(KConstRef thiz, KInt fromIndex,
           count * sizeof(T));
 }
 
+namespace {
+
+const ArrayHeader anEmptyArray = {
+  theArrayTypeInfo, /* permanent object */ 0, /* element count */ 0
+};
+
+}  // namespace
+
 extern "C" {
 
 // TODO: those must be compiler intrinsics afterwards.
@@ -98,6 +106,10 @@ void Kotlin_Array_copyImpl(KConstRef thiz, KInt fromIndex,
 }
 
 // Arrays.kt
+OBJ_GETTER0(Kotlin_emptyArray) {
+  RETURN_OBJ(const_cast<ObjHeader*>(anEmptyArray.obj()));
+}
+
 KByte Kotlin_ByteArray_get(KConstRef thiz, KInt index) {
   const ArrayHeader* array = thiz->array();
   if (static_cast<uint32_t>(index) >= array->count_) {

runtime/src/main/cpp/Types.h

@@ -71,7 +71,6 @@ extern "C" {
 #endif
 
 extern const TypeInfo* theAnyTypeInfo;
-extern const TypeInfo* theCloneableTypeInfo;
 extern const TypeInfo* theArrayTypeInfo;
 extern const TypeInfo* theByteArrayTypeInfo;
 extern const TypeInfo* theCharArrayTypeInfo;

runtime/src/main/kotlin/kotlin/Arrays.kt

@@ -12667,10 +12667,8 @@ public inline fun <reified T> Collection<T>.toTypedArray(): Array<T> {
 @Suppress("UNCHECKED_CAST")
 public inline fun <reified @PureReifiable T> arrayOf(vararg elements: T): Array<T> = elements as Array<T>
 
-private val kEmptyArray = arrayOf<Any>()
-
-@Suppress("UNCHECKED_CAST")
-public fun <T> emptyArray() = kEmptyArray as Array<T>
+@SymbolName("Kotlin_emptyArray")
+external public fun <T> emptyArray(): Array<T>
 
 /**
  * Returns an array containing the specified [Double] numbers.