Optimize call sites that only ever see a single elements transition

src/ic.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1485 matching lines @@
 
 
 Handle<Code> KeyedIC::ComputeStub(Handle<JSObject> receiver,
                                   StubKind stub_kind,
                                   StrictModeFlag strict_mode,
                                   Handle<Code> generic_stub) {
   State ic_state = target()->ic_state();
   KeyedAccessGrowMode grow_mode = IsGrowStubKind(stub_kind)
       ? ALLOW_JSARRAY_GROWTH
       : DO_NOT_ALLOW_JSARRAY_GROWTH;
-  if ((ic_state == UNINITIALIZED || ic_state == PREMONOMORPHIC) &&
-      !IsTransitionStubKind(stub_kind)) {
+
+  bool monomorphic = false;
+  MapHandleList target_receiver_maps;
+  if (!IsTransitionStubKind(stub_kind)) {
+    if (ic_state == UNINITIALIZED || ic_state == PREMONOMORPHIC) {
+      monomorphic = true;
+    } else {
+      GetReceiverMapsForStub(Handle<Code>(target()), &target_receiver_maps);
+      if (ic_state == MONOMORPHIC) {
+        // The first time there's an element transition, assume the new

[Jakob Kummerow, 2012/02/13 10:50:58]

Suggestion:
s/The first time there's an element transition/The first time a receiver is seen
that is a transitioned version of the previous monomorphic receiver type/
Otherwise it sounds like the transition happens at this IC here.

+        // ElementsKind is the monomorphic type. This benefits global arrays
+        // that only transition once, and all call sites accessing them are
+        // faster if they remain monomorphic. If this optimistic assumption is
+        // not true, the IC will miss again and it will become polymorphic and
+        // support both the untransitioned and transition maps.

[Jakob Kummerow, 2012/02/13 10:50:58]

s/transition/transitioned/

+        monomorphic = IsMoreGeneralElementsKindTransition(
+            target_receiver_maps.at(0)->elements_kind(),
+            receiver->GetElementsKind());
+      }
+    }
+  }
+
+  if (monomorphic) {
     return ComputeMonomorphicStub(
         receiver, stub_kind, strict_mode, generic_stub);
   }
   ASSERT(target() != *generic_stub);
 
   // Don't handle megamorphic property accesses for INTERCEPTORS or CALLBACKS
   // via megamorphic stubs, since they don't have a map in their relocation info
   // and so the stubs can't be harvested for the object needed for a map check.
   if (target()->type() != NORMAL) {
     TRACE_GENERIC_IC("KeyedIC", "non-NORMAL target type");
     return generic_stub;
   }
 
   // Determine the list of receiver maps that this call site has seen,
   // adding the map that was just encountered.
-  MapHandleList target_receiver_maps;
   Handle<Map> receiver_map(receiver->map());
   if (ic_state == UNINITIALIZED || ic_state == PREMONOMORPHIC) {

[Jakob Kummerow, 2012/02/13 10:50:58]

I think we don't need this if-block anymore. Note that the same thing is done
unconditionally in the next line, and the only case where not setting the
map_added flag would make a difference (UNINITIALIZED target with
non-transitioning stub_kind) is already handled above.

     target_receiver_maps.Add(receiver_map);
-  } else {
-    GetReceiverMapsForStub(Handle<Code>(target()), &target_receiver_maps);
   }
   bool map_added =
       AddOneReceiverMapIfMissing(&target_receiver_maps, receiver_map);
   if (IsTransitionStubKind(stub_kind)) {
     Handle<Map> new_map = ComputeTransitionedMap(receiver, stub_kind);
     map_added |= AddOneReceiverMapIfMissing(&target_receiver_maps, new_map);
   }
   if (!map_added) {
     // If the miss wasn't due to an unseen map, a polymorphic stub
     // won't help, use the generic stub.
@@ skipping 969 matching lines @@
 #undef ADDR
 };
 
 
 Address IC::AddressFromUtilityId(IC::UtilityId id) {
   return IC_utilities[id];
 }
 
 
 } }  // namespace v8::internal