implement immutability of UnwrappedPlainAttributes

2 files changed, 45 insertions, 3 deletions

diff --git a/pypy/objspace/std/mapdict.py b/pypy/objspace/std/mapdict.py
index 74c153145c..9bde356768 100644
--- a/pypy/objspace/std/mapdict.py
+++ b/pypy/objspace/std/mapdict.py
@@ -526,13 +526,20 @@ class UnboxedPlainAttribute(PlainAttribute):
         return self._box(unerase_unboxed(obj._mapdict_read_storage(self.storageindex))[self.listindex])
 
     def _pure_direct_read(self, obj):
-        XXX # difficult!
-        return self._direct_read(obj)
+        # somewhat tricky! note that _direct_read isn't really elidable (it has
+        # potential side effects, and the boxes aren't always the same)
+        # but _pure_unboxed_read is elidable, and we can let the jit see the
+        # boxing
+        return self._box(self._pure_unboxed_read(obj))
+
+    @jit.elidable
+    def _pure_unboxed_read(self, obj):
+        return unerase_unboxed(obj._mapdict_read_storage(self.storageindex))[self.listindex]
 
     def _direct_write(self, obj, w_value):
         if type(w_value) is self.typ:
             val = self._unbox(w_value)
-            unboxed = erase_unboxed(obj._mapdict_read_storage(self.storageindex))
+            unboxed = unerase_unboxed(obj._mapdict_read_storage(self.storageindex))
             unboxed[self.listindex] = val
             return
         # type change not supposed to happen. according to the principle
diff --git a/pypy/objspace/std/test/test_mapdict.py b/pypy/objspace/std/test/test_mapdict.py
index 45ed1615c1..05bed07164 100644
--- a/pypy/objspace/std/test/test_mapdict.py
+++ b/pypy/objspace/std/test/test_mapdict.py
@@ -641,6 +641,41 @@ def test_unboxed_type_change_other_object():
     # now it's switched
     assert type(w_obj2.map) is PlainAttribute
 
+def test_unboxed_attr_immutability(monkeypatch):
+    cls = Class(allow_unboxing=True)
+    obj = cls.instantiate()
+    obj.setdictvalue(space, "a", 10)
+    obj.setdictvalue(space, "b", 20)
+    obj.setdictvalue(space, "b", 30)
+    assert obj.map.ever_mutated == True
+    assert obj.map.back.ever_mutated == False
+
+    indices = []
+
+    def _pure_unboxed_read(obj):
+        indices.append(0)
+        return longlong2float(10)
+
+    obj.map.back._pure_unboxed_read = _pure_unboxed_read
+    monkeypatch.setattr(jit, "isconstant", lambda c: True)
+
+    assert obj.getdictvalue(space, "a") == 10
+    assert obj.getdictvalue(space, "b") == 30
+    assert obj.getdictvalue(space, "a") == 10
+    assert indices == [0, 0]
+
+    obj2 = cls.instantiate()
+    obj2.setdictvalue(space, "a", 15)
+    obj2.setdictvalue(space, "b", 25)
+    assert obj2.map is obj.map
+    assert obj2.map.ever_mutated == True
+    assert obj2.map.back.ever_mutated == False
+
+    # mutating obj2 changes the map
+    obj2.setdictvalue(space, "a", 50)
+    assert obj2.map.back.ever_mutated == True
+    assert obj2.map is obj.map
+
 # ___________________________________________________________
 # dict tests