[vm/ffi] Union transform fix offsets

TEST=tests/ffi/regress_45988_test.dart

Closes: #45988

Change-Id: Ie8bf76bb2cc1781a4b59e508d0e5f2ada85d22a4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/199422
Reviewed-by: Clement Skau <cskau@google.com>
Commit-Queue: Daco Harkes <dacoharkes@google.com>

pkg/vm/lib/transformations/ffi_definitions.dart

@@ -1271,8 +1271,7 @@ class UnionNativeTypeCfe extends CompoundNativeTypeCfe {
       unionAlignment = math.max(unionAlignment, alignment);
     }
     final int size = _alignOffset(unionSize, unionAlignment);
-    return CompoundLayout(
-        size, unionAlignment, List.filled(Abi.values.length, 0));
+    return CompoundLayout(size, unionAlignment, List.filled(types.length, 0));
   }
 }
 

tests/ffi/regress_45988_test.dart

@@ -0,0 +1,21 @@
+// Copyright (c) 2021, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:ffi';
+
+import "package:expect/expect.dart";
+import "package:ffi/ffi.dart";
+
+class DartValueFields extends Union {
+  external Pointer p1;
+  external Pointer p2;
+  external Pointer p3;
+  external Pointer p4;
+}
+
+void main() {
+  final p = calloc<DartValueFields>();
+  Expect.equals(0, p.ref.p4.address);
+  calloc.free(p);
+}

tests/ffi_2/regress_45988_test.dart

@@ -0,0 +1,21 @@
+// Copyright (c) 2021, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:ffi';
+
+import "package:expect/expect.dart";
+import "package:ffi/ffi.dart";
+
+class DartValueFields extends Union {
+  Pointer p1;
+  Pointer p2;
+  Pointer p3;
+  Pointer p4;
+}
+
+void main() {
+  final p = calloc<DartValueFields>();
+  Expect.equals(0, p.ref.p4.address);
+  calloc.free(p);
+}