[crankshaft] DCE must not eliminate (observable) math operations.

src/hydrogen-instructions.h

 // Copyright 2012 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.
 
 #ifndef V8_HYDROGEN_INSTRUCTIONS_H_
 #define V8_HYDROGEN_INSTRUCTIONS_H_
 
 #include <cstring>
 #include <iosfwd>
 
@@ skipping 2617 matching lines @@
       case kMathPowHalf:
         set_representation(Representation::Double());
         break;
       default:
         UNREACHABLE();
     }
     SetFlag(kUseGVN);
     SetFlag(kAllowUndefinedAsNaN);
   }
 
-  bool IsDeletable() const override { return true; }
+  bool IsDeletable() const override {
+    // TODO(crankshaft): This should be true, however the semantics of this
+    // instruction also include the ToNumber conversion that is mentioned in the
+    // spec, which is of course observable.
+    return false;
+  }
 
   HValue* SimplifiedDividendForMathFloorOfDiv(HDiv* hdiv);
   HValue* SimplifiedDivisorForMathFloorOfDiv(HDiv* hdiv);
 
   BuiltinFunctionId op_;
 };
 
 
 class HLoadRoot final : public HTemplateInstruction<0> {
  public:
@@ skipping 5412 matching lines @@
 };
 
 
 
 #undef DECLARE_INSTRUCTION
 #undef DECLARE_CONCRETE_INSTRUCTION
 
 } }  // namespace v8::internal
 
 #endif  // V8_HYDROGEN_INSTRUCTIONS_H_