Fix HConstant's hash function for smis on x64.

src/hydrogen-instructions.h

 // 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 2439 matching lines @@
     ASSERT(HasNumberValue());
     if (has_int32_value_) return int32_value_;
     return DoubleToInt32(double_value_);
   }
   bool HasStringValue() const { return handle_->IsString(); }
 
   bool ToBoolean() const;
 
   virtual intptr_t Hashcode() {
     ASSERT(!HEAP->allow_allocation(false));
-    return reinterpret_cast<intptr_t>(*handle());
+    intptr_t hash = reinterpret_cast<intptr_t>(*handle());
+    // Prevent smis from having fewer hash values when truncated to
+    // the least significant bits.
+    const int kShiftSize = kSmiShiftSize + kSmiTagSize;
+    return hash ^ (hash >> kShiftSize);
   }
 
 #ifdef DEBUG
   virtual void Verify() { }
 #endif
 
   DECLARE_CONCRETE_INSTRUCTION(Constant)
 
  protected:
   virtual Range* InferRange();
@@ skipping 2295 matching lines @@
   DECLARE_CONCRETE_INSTRUCTION(LoadFieldByIndex);
 };
 
 
 #undef DECLARE_INSTRUCTION
 #undef DECLARE_CONCRETE_INSTRUCTION
 
 } }  // namespace v8::internal
 
 #endif  // V8_HYDROGEN_INSTRUCTIONS_H_