Fix InitAndRegisterGlobal in new MM (#4673)

kotlin-native/backend.native/tests/build.gradle

@@ -2865,6 +2865,10 @@ task initializers7(type: KonanLocalTest) {
     source = "runtime/basic/initializers7.kt"
 }
 
+task initializers8(type: KonanLocalTest) {
+    source = "runtime/basic/initializers8.kt"
+}
+
 task expression_as_statement(type: KonanLocalTest) {
     expectedFail = (project.testTarget == 'wasm32') // uses exceptions.
     goldValue = "Ok\n"

kotlin-native/backend.native/tests/runtime/basic/initializers8.kt

@@ -0,0 +1,14 @@
+/*
+ * Copyright 2010-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the LICENSE file.
+ */
+
+package runtime.basic.initializers8
+
+import kotlin.test.*
+
+var globalString = "abc"
+
+@Test fun runTest() {
+    assertEquals("abc", globalString)
+}

kotlin-native/runtime/src/mm/cpp/Memory.cpp

@@ -144,7 +144,10 @@ extern "C" ALWAYS_INLINE OBJ_GETTER(InitSingleton, ObjHeader** location, const T
 extern "C" RUNTIME_NOTHROW void InitAndRegisterGlobal(ObjHeader** location, const ObjHeader* initialValue) {
     auto* threadData = mm::ThreadRegistry::Instance().CurrentThreadData();
     mm::GlobalsRegistry::Instance().RegisterStorageForGlobal(threadData, location);
-    mm::SetHeapRef(location, const_cast<ObjHeader*>(initialValue));
+    // Null `initialValue` means that the appropriate value was already set by static initialization.
+    if (initialValue != nullptr) {
+        mm::SetHeapRef(location, const_cast<ObjHeader*>(initialValue));
+    }
 }
 
 extern "C" const MemoryModel CurrentMemoryModel = MemoryModel::kExperimental;