Support string add in crankshaft:

src/hydrogen.cc

 // Copyright 2011 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 4870 matching lines @@
   HStringLength* length = new(zone()) HStringLength(string);
   AddInstruction(length);
   AddInstruction(new(zone()) HBoundsCheck(index, length));
   return new(zone()) HStringCharCodeAt(string, index);
 }
 
 
 HInstruction* HGraphBuilder::BuildBinaryOperation(BinaryOperation* expr,
                                                   HValue* left,
                                                   HValue* right) {
+  TypeInfo info = oracle()->BinaryType(expr);
   HInstruction* instr = NULL;
   switch (expr->op()) {
     case Token::ADD:
-      instr = new(zone()) HAdd(left, right);
+      if (info.IsString()) {
+        AddInstruction(new(zone()) HCheckNonSmi(left));
+        AddInstruction(new(zone()) HCheckInstanceType(
+            left, FIRST_STRING_TYPE, LAST_STRING_TYPE));
+        AddInstruction(new(zone()) HCheckNonSmi(right));
+        AddInstruction(new(zone()) HCheckInstanceType(
+            right, FIRST_STRING_TYPE, LAST_STRING_TYPE));
+        instr = new(zone()) HStringAdd(left, right);
+      } else {
+        instr = new(zone()) HAdd(left, right);
+      }
       break;
     case Token::SUB:
       instr = new(zone()) HSub(left, right);
       break;
     case Token::MUL:
       instr = new(zone()) HMul(left, right);
       break;
     case Token::MOD:
       instr = new(zone()) HMod(left, right);
       break;
@@ skipping 14 matching lines @@
       break;
     case Token::SHR:
       instr = new(zone()) HShr(left, right);
       break;
     case Token::SHL:
       instr = new(zone()) HShl(left, right);
       break;
     default:
       UNREACHABLE();
   }
-  TypeInfo info = oracle()->BinaryType(expr);
   // If we hit an uninitialized binary op stub we will get type info
   // for a smi operation. If one of the operands is a constant string
   // do not generate code assuming it is a smi operation.
   if (info.IsSmi() &&
       ((left->IsConstant() && HConstant::cast(left)->HasStringValue()) ||
        (right->IsConstant() && HConstant::cast(right)->HasStringValue()))) {
     return instr;
   }
   if (FLAG_trace_representation) {
     PrintF("Info: %s/%s\n", info.ToString(), ToRepresentation(info).Mnemonic());
@@ skipping 1186 matching lines @@
     }
   }
 
 #ifdef DEBUG
   if (graph_ != NULL) graph_->Verify();
   if (allocator_ != NULL) allocator_->Verify();
 #endif
 }
 
 } }  // namespace v8::internal