[vm] Ensure immutable bit is preserved in aot snapshot serialization/deserialization.

Before this cl, in AOT, the immutable bit was not set on objects like Strings, so use of Strings in shared fields resulted in exception thrown due to use of non-trivially shareable values in shared fields.

TEST=run_isolate_group_run_test on aot

Change-Id: Ib3c79852a6c7c0cfa477039096b73892dd8b2ea7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/443440
Commit-Queue: Alexander Aprelev <[email protected]>
Reviewed-by: Daco Harkes <[email protected]>

Author: aam

runtime/vm/image_snapshot.cc

@@ -656,7 +656,8 @@ static constexpr uword kReadOnlyGCBits =
 
 uword ImageWriter::GetMarkedTags(classid_t cid,
                                  intptr_t size,
-                                 bool is_canonical /* = false */) {
+                                 bool is_canonical /* = false */,
+                                 bool is_immutable /* = false */) {
   // UntaggedObject::SizeTag expects a size divisible by kObjectAlignment and
   // checks this in debug mode, but the size on the target machine may not be
   // divisible by the host machine's object alignment if they differ.
@@ -676,12 +677,14 @@ uword ImageWriter::GetMarkedTags(classid_t cid,
 
   return kReadOnlyGCBits | UntaggedObject::ClassIdTag::encode(cid) |
          UntaggedObject::SizeTag::encode(adjusted_size) |
-         UntaggedObject::CanonicalBit::encode(is_canonical);
+         UntaggedObject::CanonicalBit::encode(is_canonical) |
+         UntaggedObject::ImmutableBit::encode(is_immutable);
 }
 
 uword ImageWriter::GetMarkedTags(const Object& obj) {
-  uword tags = GetMarkedTags(obj.ptr()->untag()->GetClassId(),
-                             SizeInSnapshot(obj), obj.IsCanonical());
+  uword tags =
+      GetMarkedTags(obj.ptr()->untag()->GetClassId(), SizeInSnapshot(obj),
+                    obj.IsCanonical(), obj.IsImmutable());
 #if defined(HASH_IN_OBJECT_HEADER)
   tags = UntaggedObject::HashTag::update(obj.ptr()->untag()->GetHeaderHash(),
                                          tags);

runtime/vm/image_snapshot.h

@@ -502,7 +502,8 @@ class ImageWriter : public ValueObject {
 
   static uword GetMarkedTags(classid_t cid,
                              intptr_t size,
-                             bool is_canonical = false);
+                             bool is_canonical = false,
+                             bool is_immutable = false);
   static uword GetMarkedTags(const Object& obj);
 
   void DumpInstructionStats();

tests/ffi/run_isolate_group_run_test.dart

@@ -47,6 +47,9 @@ bar() {
 @pragma('vm:shared')
 var list_length = 0;
 
+@pragma('vm:shared')
+String string_foo = "";
+
 main() {
   IsolateGroup.runSync(() {
     final l = <int>[];
@@ -144,5 +147,10 @@ main() {
     Expect.equals(42, Baz.foo);
   }
 
+  IsolateGroup.runSync(() {
+    string_foo = "foo bar";
+  });
+  Expect.equals("foo bar", string_foo);
+
   print("All tests completed :)");
 }