Fix HGraphBuilder::BuildAddStringLengths

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 1776 matching lines @@
   }
 
   return object;
 }
 
 
 HValue* HGraphBuilder::BuildAddStringLengths(HValue* left_length,
                                              HValue* right_length) {
   // Compute the combined string length and check against max string length.
   HValue* length = AddUncasted<HAdd>(left_length, right_length);
-  HValue* max_length = Add<HConstant>(String::kMaxLength);
+  // Check that length <= kMaxLength <=> length < MaxLength + 1.
+  HValue* max_length = Add<HConstant>(String::kMaxLength + 1);
   Add<HBoundsCheck>(length, max_length);
   return length;
 }
 
 
 HValue* HGraphBuilder::BuildCreateConsString(
     HValue* length,
     HValue* left,
     HValue* right,
     HAllocationMode allocation_mode) {
@@ skipping 9692 matching lines @@
   if (ShouldProduceTraceOutput()) {
     isolate()->GetHTracer()->TraceHydrogen(name(), graph_);
   }
 
 #ifdef DEBUG
   graph_->Verify(false);  // No full verify.
 #endif
 }
 
 } }  // namespace v8::internal