[Vm-dev] Questions about hash

[Mariano Martinez Peck]

Hi folks. I am trying to understand a little how hash is assigned and how it
works in the SqueakVM. I have a couple of questions:

1) There are 12 bits in the ObjectHeader for the hash. I saw that the value
of this hash comes from #newObjectHash. But....this method answers a hash of
16 bit:
newObjectHash
    "Answer a new 16-bit pseudo-random number for use as an identity hash."

    lastHash := 13849 + (27181 * lastHash) bitAnd: 65535.
    ^ lastHash

Since in the OH we have 12 bits, each place where this method is used, then
we have to do something like this:

header1 := (classFormat bitAnd: 16r1FF00) bitOr: (hash << HashBitsOffset
bitAnd: HashBits).

or

    hash := self newObjectHash.
    header := (self longAt: newOop) bitAnd: 16r1FFFF.
    "use old ccIndex, format, size, and header-type fields"
    header := header bitOr: ((hash << 17) bitAnd: 16r1FFE0000).

....
so, we always need to trim it to 12 bits using (hash << HashBitsOffset
bitAnd: HashBits).

Now, I wonder, why #newObjectHash  does not answer a 12bits long hash
directly ?  is it because of the hash function performance?


2)  It looks to me that not, but can it change the hash of an object? if
true, why and where?

3) If #basicIdentityHash is ONLY used for Sets and friends, why not letting
the hash assignment as late/lazy as possible? i mean, instead of creating
and assigning a hash at object creation, what if we do this in #hashBitsOf:
oop
where we can check in the hash is empty and if true assign one.
Would this make sense or it would be worst?

Thanks

Mariano
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