[llvm-dev] DenseMapInfo::getHashValue() of two sometimes identical pointers

[Jeroen Dobbelaere via llvm-dev]

Hi all,

is there a recommended way to implement a 'DenseMapInfo<..>::getHashValue()'
for a class that contains multiple pointers of which some are potentially identical ?

The standard used xor (^) is canceling out the hash contribution of those identical pointers.

More concrete: I am adding a 'PtrProvenance' member to MemoryLocation. When I add
the member to the hash computation in the way it seems intended,

https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/Analysis/MemoryLocation.h#L340-L344

I would do:

  static unsigned getHashValue(const MemoryLocation &Val) {
      return DenseMapInfo<const Value *>::getHashValue(Val.Ptr) ^
           DenseMapInfo<const Value *>::getHashValue(Val.PtrProvenance) ^ // Handle new member
           DenseMapInfo<LocationSize>::getHashValue(Val.Size) ^
           DenseMapInfo<AAMDNodes>::getHashValue(Val.AATags);
  }


But, in the initial case, it is very likely that Val.Ptr == Val.PtrProvenance.

Is there a recommended (standard) way to fix this ?

Thanks,

Jeroen Dobbelaere