Improve alias analysis for HConstants (and fix small snafu in MustAlias).

src/hydrogen-alias-analysis.h

 // 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 47 matching lines @@
       // An allocation can never alias a parameter or a constant.
       if (b->IsParameter()) return kNoAlias;
       if (b->IsConstant()) return kNoAlias;
     }
     if (b->IsAllocate() || b->IsInnerAllocatedObject()) {
       // An allocation can never alias a parameter or a constant.
       if (a->IsParameter()) return kNoAlias;
       if (a->IsConstant()) return kNoAlias;
     }
 
-    // TODO(titzer): return MustAlias for two equivalent constants.
+    // Constant objects can be distinguished statically.
+    if (a->IsConstant()) {
+      // TODO(titzer): DataEquals() is more efficient, but that's protected.
+      return a->Equals(b) ? kMustAlias : kNoAlias;
+    }
     return kMayAlias;
   }
 
   // Checks whether the objects referred to by the given instructions may
   // ever be aliases. Note that this is more conservative than checking
   // {Query(a, b) == kMayAlias}, since this method considers kMustAlias
   // objects to also be may-aliasing.
   inline bool MayAlias(HValue* a, HValue* b) {
     return Query(a, b) != kNoAlias;
   }
 
   inline bool MustAlias(HValue* a, HValue* b) {
-    return Query(a, b) == kMayAlias;
+    return Query(a, b) == kMustAlias;
   }
 
   inline bool NoAlias(HValue* a, HValue* b) {
     return Query(a, b) == kNoAlias;
   }
 
   // Returns the actual value of an instruction. In the case of a chain
   // of informative definitions, return the root of the chain.
   HValue* ActualValue(HValue* obj) {
     while (obj->IsInformativeDefinition()) {  // Walk a chain of idefs.
       obj = obj->RedefinedOperand();
     }
     return obj;
   }
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_HYDROGEN_ALIAS_ANALYSIS_H_