Initial support for length guards on final fields.

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index 2c543b5041fde5f2a6c86bd2b653f1b80dc0f50b..20c508bee71380a29fc043910f66a69a2612638e 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -1987,11 +1987,33 @@ class Field : public Object {
   static intptr_t guarded_cid_offset() {
     return OFFSET_OF(RawField, guarded_cid_);
   }
+  // Return the list length that any list stored in this field is guaranteed
+  // to have. If length is kUnknownFixedLength the length has not
+  // been determined. If length is kNoFixedLength this field has multiple
+  // list lengths associated with it and cannot be predicted.
+  intptr_t guarded_list_length() const {
+    return raw_ptr()->guarded_list_length_;
+  }
+  void set_guarded_list_length(intptr_t list_length) const {
+    raw_ptr()->guarded_list_length_ = list_length;
+  }
+  static intptr_t guarded_list_length_offset() {
+    return OFFSET_OF(RawField, guarded_list_length_);
+  }
+  bool needs_length_check() const {
+    const bool r = guarded_list_length() >= Field::kUnknownFixedLength;
+    ASSERT(!r || is_final());
+    return r;
+  }
 
   static bool IsExternalizableCid(intptr_t cid) {
     return (cid == kOneByteStringCid) || (cid == kTwoByteStringCid);
   }
 
+  enum {
+    kUnknownFixedLength = -1,
+    kNoFixedLength = -2,
+  };
   // Returns false if any value read from this field is guaranteed to be
   // not null.
   // Internally we is_nullable_ field contains either kNullCid (nullable) or
@@ -2008,8 +2030,12 @@ class Field : public Object {
   }
 
   // Update guarded class id and nullability of the field to reflect assignment
-  // of the value with the given class id to this field.
+  // of the value with the given class id to this field. May trigger
+  // deoptimization of dependent code.
   void UpdateCid(intptr_t cid) const;
+  // Update guarded class length of the field to reflect assignment of the
+  // value with the given length. May trigger deoptimization of dependent code.
+  void UpdateLength(intptr_t length) const;
 
   // Return the list of optimized code objects that were optimized under
   // assumptions about guarded class id and nullability of this field.