Dict change: scan manually instead of conservatively

microbenchmarks/string_key_ubench.py

+# Test the speed of using strings as dictionary keys.
+# We internally optimize many things to not use dictionaries,
+# but there are some times that we can't.
+
+def f():
+    d = {'a':1}
+    for i in xrange(3000000):
+        d['a']
+        d['a']
+        d['a']
+        d['a']
+        d['a']
+        d['a']
+        d['a']
+        d['a']
+        d['a']
+        d['a']
+f()

src/runtime/dict.cpp

@@ -689,14 +689,10 @@ void BoxedDict::gcHandler(GCVisitor* v, Box* b) {
 
     BoxedDict* d = (BoxedDict*)b;
 
-    // This feels like a cludge, but we need to find anything that
-    // the unordered_map might have allocated.
-    // Another way to handle this would be to rt_alloc the unordered_map
-    // as well, though that incurs extra memory dereferences which would
-    // be nice to avoid.
-    void** start = (void**)&d->d;
-    void** end = start + (sizeof(d->d) / 8);
-    v->visitPotentialRange(start, end);
+    for (auto p : d->d) {
+        v->visit(p.first);
+        v->visit(p.second);
+    }
 }
 
 void BoxedDictIterator::gcHandler(GCVisitor* v, Box* b) {

src/runtime/types.h

@@ -661,7 +661,7 @@ struct PyLt {
 
 class BoxedDict : public Box {
 public:
-    typedef std::unordered_map<Box*, Box*, PyHasher, PyEq, StlCompatAllocator<std::pair<Box*, Box*>>> DictMap;
+    typedef std::unordered_map<Box*, Box*, PyHasher, PyEq> DictMap;
 
     DictMap d;