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      1 // Copyright 2013 the V8 project authors. All rights reserved.
      2 // Use of this source code is governed by a BSD-style license that can be
      3 // found in the LICENSE file.
      4 
      5 #ifndef V8_HYDROGEN_ALIAS_ANALYSIS_H_
      6 #define V8_HYDROGEN_ALIAS_ANALYSIS_H_
      7 
      8 #include "src/hydrogen.h"
      9 
     10 namespace v8 {
     11 namespace internal {
     12 
     13 enum HAliasing {
     14   kMustAlias,
     15   kMayAlias,
     16   kNoAlias
     17 };
     18 
     19 
     20 // Defines the interface to alias analysis for the rest of the compiler.
     21 // A simple implementation can use only local reasoning, but a more powerful
     22 // analysis might employ points-to analysis.
     23 class HAliasAnalyzer : public ZoneObject {
     24  public:
     25   // Simple alias analysis distinguishes allocations, parameters,
     26   // and constants using only local reasoning.
     27   HAliasing Query(HValue* a, HValue* b) {
     28     // The same SSA value always references the same object.
     29     if (a == b) return kMustAlias;
     30 
     31     if (a->IsAllocate() || a->IsInnerAllocatedObject()) {
     32       // Two non-identical allocations can never be aliases.
     33       if (b->IsAllocate()) return kNoAlias;
     34       if (b->IsInnerAllocatedObject()) return kNoAlias;
     35       // An allocation can never alias a parameter or a constant.
     36       if (b->IsParameter()) return kNoAlias;
     37       if (b->IsConstant()) return kNoAlias;
     38     }
     39     if (b->IsAllocate() || b->IsInnerAllocatedObject()) {
     40       // An allocation can never alias a parameter or a constant.
     41       if (a->IsParameter()) return kNoAlias;
     42       if (a->IsConstant()) return kNoAlias;
     43     }
     44 
     45     // Constant objects can be distinguished statically.
     46     if (a->IsConstant()) {
     47       // TODO(titzer): DataEquals() is more efficient, but that's protected.
     48       return a->Equals(b) ? kMustAlias : kNoAlias;
     49     }
     50     return kMayAlias;
     51   }
     52 
     53   // Checks whether the objects referred to by the given instructions may
     54   // ever be aliases. Note that this is more conservative than checking
     55   // {Query(a, b) == kMayAlias}, since this method considers kMustAlias
     56   // objects to also be may-aliasing.
     57   inline bool MayAlias(HValue* a, HValue* b) {
     58     return Query(a, b) != kNoAlias;
     59   }
     60 
     61   inline bool MustAlias(HValue* a, HValue* b) {
     62     return Query(a, b) == kMustAlias;
     63   }
     64 
     65   inline bool NoAlias(HValue* a, HValue* b) {
     66     return Query(a, b) == kNoAlias;
     67   }
     68 };
     69 
     70 
     71 } }  // namespace v8::internal
     72 
     73 #endif  // V8_HYDROGEN_ALIAS_ANALYSIS_H_
     74