When adding constant string with something unknown, assume it's a string.

src/hydrogen.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/hydrogen.h"
 
 #include <sstream>
 
 #include "src/v8.h"
 
@@ skipping 10592 matching lines @@
 HValue* HGraphBuilder::BuildBinaryOperation(
     Token::Value op,
     HValue* left,
     HValue* right,
     Type* left_type,
     Type* right_type,
     Type* result_type,
     Maybe<int> fixed_right_arg,
     HAllocationMode allocation_mode,
     LanguageMode language_mode) {
+  bool maybe_string_add = false;
+  if (op == Token::ADD) {
+    // If we are adding constant string with something for which we don't have
+    // a feedback yet, assume that it's also going to be a string and don't
+    // generate deopt instructions.
+    if (!left_type->IsInhabited() && right->IsConstant() &&
+        HConstant::cast(right)->HasStringValue()) {
+      left_type = Type::String();
+    }
+
+    if (!right_type->IsInhabited() && left->IsConstant() &&
+        HConstant::cast(left)->HasStringValue()) {
+      right_type = Type::String();
+    }
+
+    maybe_string_add = (left_type->Maybe(Type::String()) ||
+                        left_type->Maybe(Type::Receiver()) ||
+                        right_type->Maybe(Type::String()) ||
+                        right_type->Maybe(Type::Receiver()));
+  }
+
   Representation left_rep = RepresentationFor(left_type);
   Representation right_rep = RepresentationFor(right_type);
 
-  bool maybe_string_add = op == Token::ADD &&
-                          (left_type->Maybe(Type::String()) ||
-                           left_type->Maybe(Type::Receiver()) ||
-                           right_type->Maybe(Type::String()) ||
-                           right_type->Maybe(Type::Receiver()));
-
   if (!left_type->IsInhabited()) {
     Add<HDeoptimize>(
         Deoptimizer::kInsufficientTypeFeedbackForLHSOfBinaryOperation,
         is_strong(language_mode) ? Deoptimizer::EAGER : Deoptimizer::SOFT);
     left_type = Type::Any(zone());
     left_rep = RepresentationFor(left_type);
     maybe_string_add = op == Token::ADD;
   }
 
   if (!right_type->IsInhabited()) {
@@ skipping 2497 matching lines @@
   if (ShouldProduceTraceOutput()) {
     isolate()->GetHTracer()->TraceHydrogen(name(), graph_);
   }
 
 #ifdef DEBUG
   graph_->Verify(false);  // No full verify.
 #endif
 }
 
 } }  // namespace v8::internal