Merge pull request swiftlang#71926 from mikeash/instancestart-size-no-shrink

[Runtime] Don't shrink class InstanceStart when initing field offset vectors.

Author: mikeash

stdlib/public/runtime/Metadata.cpp

@@ -3514,9 +3514,14 @@ static void initClassFieldOffsetVector(ClassMetadata *self,
   //
   // The rodata may be in read-only memory if the compiler knows that the size
   // it generates is already definitely correct. Don't write to this value
-  // unless it's necessary.
-  if (rodata->InstanceStart != size)
+  // unless it's necessary. We'll grow the space for the superclass if needed,
+  // but not shrink it, as the compiler may write an unaligned size that's less
+  // than our aligned InstanceStart.
+  if (rodata->InstanceStart < size)
     rodata->InstanceStart = size;
+  else
+    size = rodata->InstanceStart;
+
 #endif
 
   // Okay, now do layout.

test/Runtime/subclass_instance_start_adjustment.swift

@@ -0,0 +1,61 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift %s -o %t/subclass_instance_start_adjustment
+// RUN: %target-codesign %t/subclass_instance_start_adjustment
+// RUN: %target-run %t/subclass_instance_start_adjustment | %FileCheck %s
+
+// REQUIRES: executable_test
+// UNSUPPORTED: use_os_stdlib
+// UNSUPPORTED: back_deployment_runtime
+
+// Make sure we get the InstanceStart adjustment right for concrete subclasses
+// of generic classes with different sizes, and especially with an unaligned
+// size.
+
+class GenericSuperclassWithAlignedSize<T> {
+  var field = 42
+}
+
+class SubclassOfGenericSuperclassWithAlignedSize: GenericSuperclassWithAlignedSize<Int> {
+  var subfield = 43
+}
+
+do {
+  let obj = SubclassOfGenericSuperclassWithAlignedSize()
+  print(obj, obj.field, obj.subfield)
+  // CHECK: SubclassOfGenericSuperclassWithAlignedSize 42 43
+}
+
+class GenericSuperclassWithMisalignedSize<T> {
+  var field = true
+}
+
+class SubclassOfGenericSuperclassWithMisalignedSize: GenericSuperclassWithMisalignedSize<Int> {
+  var subfield = 44
+}
+
+do {
+  let obj = SubclassOfGenericSuperclassWithMisalignedSize()
+  print(obj, obj.field, obj.subfield)
+  // CHECK: SubclassOfGenericSuperclassWithMisalignedSize true 44
+}
+
+#if canImport(Foundation)
+import Foundation
+
+class GenericSuperclassWithURLField<T> {
+  var field: URL?
+}
+
+class SubclassOfGenericSuperclassWithURLField: GenericSuperclassWithURLField<Int> {
+  var subfield = 45
+}
+
+do {
+  let obj = SubclassOfGenericSuperclassWithURLField()
+  print(obj, obj.field as Any, obj.subfield)
+  // CHECK: SubclassOfGenericSuperclassWithURLField nil 45
+}
+#else
+// Simulate the expected output.
+print("SubclassOfGenericSuperclassWithURLField nil 45")
+#endif