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digitalmars.D.bugs - [Issue 19072] New: Object.toHash and typeid(void*).getHash(&ptr)


          Issue ID: 19072
           Summary: Object.toHash and typeid(void*).getHash(&ptr) should
                    be more varied in their low bits
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: druntime
          Assignee: nobody puremagic.com
          Reporter: n8sh.secondary hotmail.com

The low bits of `Object.toHash` are insufficiently varied. Depending on the
platform the bottom 4, 3, or 2 bits of the result will always be zero. This is
bad because the low bits of a hash code are typically the most significant for
hashtable implementations. D's builtin AA deals with this and other potential
defects by rehashing the hash codes it receives. Some 3rd party hashtable
implementations do the same, but others assume that keys with a defined
`toHash` have reasonable hashcodes that can be used without need for further
mixing. Fixing this would not be hard. The same can be done for raw pointers;
although it will not always be necessary for them the cost is low and it will
frequently be a benefit.

Jul 09 2018