Don't generate useless string checks for string adds.

src/hydrogen.cc

 // Copyright 2013 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 1255 matching lines @@
     ASSERT(continuation->predecessors()->length() == 0);
   }
 }
 
 
 HValue* HGraphBuilder::BuildCheckMap(HValue* obj, Handle<Map> map) {
   return Add<HCheckMaps>(obj, map, top_info());
 }
 
 
+HValue* HGraphBuilder::BuildCheckString(
+    HValue* object, const char* failure_reason) {
+  if (!object->type().IsString()) {
+    ASSERT(!object->IsConstant() ||
+           !HConstant::cast(object)->HasStringValue());
+    IfBuilder if_isstring(this);
+    if_isstring.If<HIsStringAndBranch>(object);
+    if_isstring.Then();
+    if_isstring.ElseDeopt(failure_reason);
+  }
+  return object;
+}
+
+
 HValue* HGraphBuilder::BuildWrapReceiver(HValue* object, HValue* function) {
   if (object->type().IsJSObject()) return object;
   return Add<HWrapReceiver>(object, function);
 }
 
 
 HValue* HGraphBuilder::BuildCheckForCapacityGrow(HValue* object,
                                                  HValue* elements,
                                                  ElementsKind kind,
                                                  HValue* length,
@@ skipping 7338 matching lines @@
   } else {
     if (!maybe_string_add) right = TruncateToNumber(right, &right_type);
     right_rep = Representation::FromType(right_type);
   }
 
   // Special case for string addition here.
   if (op == Token::ADD &&
       (left_type->Is(Type::String()) || right_type->Is(Type::String()))) {
     // Validate type feedback for left argument.
     if (left_type->Is(Type::String())) {
-      IfBuilder if_isstring(this);
-      if_isstring.If<HIsStringAndBranch>(left);
-      if_isstring.Then();
-      if_isstring.ElseDeopt("Expected string for LHS of binary operation");
+      left = BuildCheckString(
+          left, "Expected string for LHS of binary operation");
     }
 
     // Validate type feedback for right argument.
     if (right_type->Is(Type::String())) {
-      IfBuilder if_isstring(this);
-      if_isstring.If<HIsStringAndBranch>(right);
-      if_isstring.Then();
-      if_isstring.ElseDeopt("Expected string for RHS of binary operation");
+      right = BuildCheckString(
+          right, "Expected string for RHS of binary operation");
     }
 
     // Convert left argument as necessary.
     if (left_type->Is(Type::Number())) {
       ASSERT(right_type->Is(Type::String()));
       left = BuildNumberToString(left, left_type);
     } else if (!left_type->Is(Type::String())) {
       ASSERT(right_type->Is(Type::String()));
       HValue* function = AddLoadJSBuiltin(Builtins::STRING_ADD_RIGHT);
       Add<HPushArgument>(left);
@@ skipping 1977 matching lines @@
   if (ShouldProduceTraceOutput()) {
     isolate()->GetHTracer()->TraceHydrogen(name(), graph_);
   }
 
 #ifdef DEBUG
   graph_->Verify(false);  // No full verify.
 #endif
 }
 
 } }  // namespace v8::internal