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 a27fae70fb27b4ba1bc890c65b583ed54abdaab5..0f3fd22bc554ce68b7d8428a134ad6450e776651 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -1982,11 +1982,33 @@ 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 kUnknownLength the length has not been determined.
+  // If length is kVariableLength this field has multiple list lengths

[srdjan, 2013/08/16 22:26:01]

kVariableLength?

[Cutch, 2013/08/19 21:52:01]

Done.

+  // 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::kUnknownLength;
+    ASSERT((r && is_final()) || !r);

[srdjan, 2013/08/16 22:26:01]

This can be simpler: !r || is_final()

[Cutch, 2013/08/19 21:52:01]

Done.

+    return r;
+  }
 
   static bool IsExternalizableCid(intptr_t cid) {
     return (cid == kOneByteStringCid) || (cid == kTwoByteStringCid);
   }
 
+  enum {
+    kUnknownLength = -1,
+    kNoLength = -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
@@ -2003,8 +2025,12 @@ 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.