Try convert polymorphic stores into a single monomorphic store.

src/hydrogen.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 3776 matching lines @@
   } while (false)
 
 
 #define CHECK_ALIVE(call)                                       \
   do {                                                          \
     call;                                                       \
     if (HasStackOverflow() || current_block() == NULL) return;  \
   } while (false)
 
 
+#define CHECK_ALIVE_OR_RETURN(call, value)                            \
+  do {                                                                \
+    call;                                                             \
+    if (HasStackOverflow() || current_block() == NULL) return value;  \
+  } while (false)
+
+
 void HOptimizedGraphBuilder::Bailout(const char* reason) {
   current_info()->set_bailout_reason(reason);
   SetStackOverflow();
 }
 
 
 void HOptimizedGraphBuilder::VisitForEffect(Expression* expr) {
   EffectContext for_effect(this);
   Visit(expr);
 }
@@ skipping 2482 matching lines @@
 }
 
 
 HInstruction* HOptimizedGraphBuilder::TryLoadPolymorphicAsMonomorphic(
     Property* expr,
     HValue* object,
     SmallMapList* types,
     Handle<String> name) {
   // Use monomorphic load if property lookup results in the same field index
   // for all maps. Requires special map check on the set of all handled maps.
+  if (types->length() > kMaxLoadPolymorphism) return NULL;
+
   LookupResult lookup(isolate());
   int count;
   Representation representation = Representation::None();
   HObjectAccess access = HObjectAccess::ForMap();  // initial value unused.
-  for (count = 0;
-       count < types->length() && count < kMaxLoadPolymorphism;
-       ++count) {
+  for (count = 0; count < types->length(); ++count) {
     Handle<Map> map = types->at(count);
     if (!ComputeLoadStoreField(map, name, &lookup, false)) break;
 
     HObjectAccess new_access = HObjectAccess::ForField(map, &lookup, name);
     Representation new_representation =
         ComputeLoadStoreRepresentation(map, &lookup);
 
     if (count == 0) {
       // First time through the loop; set access and representation.
       access = new_access;
-      representation = new_representation;
     } else if (!representation.IsCompatibleForLoad(new_representation)) {
       // Representations did not match.
       break;
     } else if (access.offset() != new_access.offset()) {
       // Offsets did not match.
       break;
     } else if (access.IsInobject() != new_access.IsInobject()) {
       // In-objectness did not match.
       break;
     }
+    representation = representation.generalize(new_representation);
   }
 
   if (count != types->length()) return NULL;
 
   // Everything matched; can use monomorphic load.
   BuildCheckNonSmi(object);
   AddInstruction(HCheckMaps::New(object, types, zone()));
   return BuildLoadNamedField(object, access, representation);
 }
 
@@ skipping 11 matching lines @@
     HValue* context = environment()->LookupContext();
     instr = new(zone()) HLoadNamedFieldPolymorphic(
         context, object, types, name, zone());
   }
 
   instr->set_position(expr->position());
   return ast_context()->ReturnInstruction(instr, expr->id());
 }
 
 
+bool HOptimizedGraphBuilder::TryStorePolymorphicAsMonomorphic(
+    Assignment* expr,
+    HValue* object,
+    HValue* value,
+    SmallMapList* types,
+    Handle<String> name) {
+  // Use monomorphic store if property lookup results in the same field index
+  // for all maps. Requires special map check on the set of all handled maps.
+  if (types->length() > kMaxStorePolymorphism) return false;
+
+  // TODO(verwaest): Merge the checking logic with the code in
+  // TryLoadPolymorphicAsMonomorphic.
+  LookupResult lookup(isolate());
+  int count;
+  Representation representation = Representation::None();
+  HObjectAccess access = HObjectAccess::ForMap();  // initial value unused.
+  for (count = 0; count < types->length(); ++count) {
+    Handle<Map> map = types->at(count);
+    // Pass false to ignore transitions.
+    if (!ComputeLoadStoreField(map, name, &lookup, false)) break;
+
+    HObjectAccess new_access = HObjectAccess::ForField(map, &lookup, name);
+    Representation new_representation =
+        ComputeLoadStoreRepresentation(map, &lookup);
+
+    if (count == 0) {
+      // First time through the loop; set access and representation.
+      access = new_access;
+      representation = new_representation;
+    } else if (!representation.IsCompatibleForStore(new_representation)) {
+      // Representations did not match.
+      break;
+    } else if (access.offset() != new_access.offset()) {
+      // Offsets did not match.
+      break;
+    } else if (access.IsInobject() != new_access.IsInobject()) {
+      // In-objectness did not match.
+      break;
+    }
+  }
+
+  if (count != types->length()) return false;
+
+  // Everything matched; can use monomorphic store.
+  BuildCheckNonSmi(object);
+  AddInstruction(HCheckMaps::New(object, types, zone()));
+  HInstruction* store;
+  CHECK_ALIVE_OR_RETURN(
+      store = BuildStoreNamedField(object, name, value, types->at(0), &lookup),
+      true);
+  Push(value);
+  store->set_position(expr->position());
+  AddInstruction(store);
+  AddSimulate(expr->AssignmentId());
+  ast_context()->ReturnValue(Pop());
+  return true;
+}
+
+
 void HOptimizedGraphBuilder::HandlePolymorphicStoreNamedField(
     Assignment* expr,
     HValue* object,
     HValue* value,
     SmallMapList* types,
     Handle<String> name) {
+  if (TryStorePolymorphicAsMonomorphic(expr, object, value, types, name)) {
+    return;
+  }
+
   // TODO(ager): We should recognize when the prototype chains for different
   // maps are identical. In that case we can avoid repeatedly generating the
   // same prototype map checks.
   int count = 0;
   HBasicBlock* join = NULL;
   for (int i = 0; i < types->length() && count < kMaxStorePolymorphism; ++i) {
     Handle<Map> map = types->at(i);
     LookupResult lookup(isolate());
     if (ComputeLoadStoreField(map, name, &lookup, true)) {
       if (count == 0) {
         BuildCheckNonSmi(object);
         join = graph()->CreateBasicBlock();
       }
       ++count;
       HBasicBlock* if_true = graph()->CreateBasicBlock();
       HBasicBlock* if_false = graph()->CreateBasicBlock();
       HCompareMap* compare =
           new(zone()) HCompareMap(object, map, if_true, if_false);
       current_block()->Finish(compare);
 
       set_current_block(if_true);
       HInstruction* instr;
-      CHECK_ALIVE(instr =
-          BuildStoreNamedField(object, name, value, map, &lookup));
+      CHECK_ALIVE(
+          instr = BuildStoreNamedField(object, name, value, map, &lookup));
       instr->set_position(expr->position());
       // Goto will add the HSimulate for the store.
       AddInstruction(instr);
       if (!ast_context()->IsEffect()) Push(value);
       current_block()->Goto(join);
 
       set_current_block(if_false);
     }
   }
 
@@ skipping 23 matching lines @@
           Drop(1);
         }
       }
       return ast_context()->ReturnValue(value);
     }
   }
 
   ASSERT(join != NULL);
   join->SetJoinId(expr->id());
   set_current_block(join);
-  if (!ast_context()->IsEffect()) return ast_context()->ReturnValue(Pop());
+  if (!ast_context()->IsEffect()) ast_context()->ReturnValue(Pop());
 }
 
 
 void HOptimizedGraphBuilder::HandlePropertyAssignment(Assignment* expr) {
   Property* prop = expr->target()->AsProperty();
   ASSERT(prop != NULL);
   CHECK_ALIVE(VisitForValue(prop->obj()));
 
   if (prop->key()->IsPropertyName()) {
     // Named store.
@@ skipping 5129 matching lines @@
     }
   }
 
 #ifdef DEBUG
   if (graph_ != NULL) graph_->Verify(false);  // No full verify.
   if (allocator_ != NULL) allocator_->Verify();
 #endif
 }
 
 } }  // namespace v8::internal