[vm] Fix Instance::IdentityHashCode for strings

This method should return the same value
as `String::Hash` to not be misaligned
with how `identityHashCode()` is implemented.

On 64bit platforms we also use the same
location in the object header to store
identity hash and string hash so these two
can't be misaligned.

Fixes https://github.com/dart-lang/sdk/issues/56749

TEST=vm/cc/String,vm/dart/regress_56749
R=rmacnak@google.com

Change-Id: I221cc8ad85f7ffb7f7d2bcdd0f4341c3ecf25663
Fixed: 56749
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/385860
Commit-Queue: Slava Egorov <vegorov@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>

runtime/tests/vm/dart/regress_56749_test.dart

@@ -0,0 +1,19 @@
+// Copyright (c) 2024, 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 "package:expect/expect.dart";
+
+void main() {
+  String a = "Hello, world!";
+  String b = a.substring(0, 5) + a.substring(5);
+
+  // `Closure::ComputeHash` will invoke `Instance::IdentityHashCode` on
+  // the receiver (b). Previously this function did not handle String
+  // specially and used a random hash code instead of the expected
+  // content-based hash code.
+  print(b.toString.hashCode);
+
+  Expect.equals(a.hashCode, b.hashCode);
+  Expect.equals(a, b);
+}

runtime/vm/object.cc

@@ -20727,6 +20727,9 @@ ObjectPtr Instance::HashCode() const {
 // Keep in sync with AsmIntrinsifier::Object_getHash.
 IntegerPtr Instance::IdentityHashCode(Thread* thread) const {
   if (IsInteger()) return Integer::Cast(*this).ptr();
+  if (IsString()) {
+    return Smi::New(String::Cast(*this).Hash());
+  }
 
 #if defined(HASH_IN_OBJECT_HEADER)
   intptr_t hash = Object::GetCachedHash(ptr());

runtime/vm/object_test.cc

@@ -824,6 +824,17 @@ ISOLATE_UNIT_TEST_CASE(String) {
     EXPECT_EQ(0x7FFF, str16.CharAt(1));
     EXPECT_EQ(0xFFFF, str16.CharAt(2));
   }
+
+  // Check that String's identity hash and hashCode are the same.
+  {
+    for (auto str : {"Hello", "Hell\xC3\x98"}) {
+      const String& s = String::Handle(String::New(str));
+      const String& s2 = String::Handle(String::New(str));
+      EXPECT_EQ(s.Hash(),
+                static_cast<uword>(
+                    Integer::Handle(s2.IdentityHashCode(thread)).Value()));
+    }
+  }
 }
 
 ISOLATE_UNIT_TEST_CASE(StringFormat) {