[Vm-dev] Re: [squeak-dev] identityHash bits not incremented for new objects?

[Igor Stasenko]

On 1 August 2011 09:47, Mariano Martinez Peck <marianopeck at gmail.com> wrote:
>
> Hi Igor. Does the same test work correctlly in a interpreter VM?  because I can see:
>
> ObjectMemory >> newObjectHash
>     "Answer a new 16-bit pseudo-random number for use as an identity hash."
>
>     lastHash := 13849 + (27181 * lastHash) bitAnd: 65535.
>     ^ lastHash
>
>
> In which case, it seems that every new hash will be different than the previous one.
>
> But NewObjectMemory >> newObjectHash
>     "Derive the new object hash from the allocation pointer.  This is less costly than
>      using lastHash because it avoids the read-modify-write cycle to update lastHash.
>      Since the size of eden is a power of two and larger than the hash range this provides
>      a well-distributed and fairly random set of values."
>     <inline: true>
>     ^freeStart >> BytesPerWord

aha.. so this is the reason why hash is not incrementing!
Since BytesPerWord == 4
it means that hash will change only if old freeStart - new freeStart value >= 16

and since two short strings 'ab' fit in 16 bytes, they receive same hash value.

So, instead it should be:

    ^freeStart // BytesPerWord

since freeStart is aligned on BytesPerWord, but not (2^BytesPerWord)

>
> so...I have no idea, but maybe for two consequently allocated objects, the instVar freeStart can remain with the same value ?
>
>
>
> On Mon, Aug 1, 2011 at 8:43 AM, Igor Stasenko <siguctua at gmail.com> wrote:
>>
>> I found that #testBecomeIdentityHash sometimes failing, sometimes not.
>>
>> It seems like VM 'forgets' to produce different identityHash bits for
>> two consequently allocated objects,
>> while test assumes that they are always different.
>>
>> If i insert a statement (see the code), test no longer fails.
>>
>> testBecomeIdentityHash
>>        "Note. The identity hash of both objects seems to change after the become:"
>>
>>        | a b c d |
>>
>>        a := 'ab' copy.
>>        b := 'cd' copy.
>>
>> >>>     [ b identityHash = a identityHash ] whileTrue: [ b := b copy ].
>>
>>        c := a.
>>        d := b.
>>
>>        a become: b.
>>
>>        self
>>                assert: a identityHash = c identityHash;
>>                assert: b identityHash = d identityHash;
>>                deny: a identityHash = b identityHash.
>>
>> A simple piece of code reveals the problem:
>>
>> (1 to: 20) collect: [:i |
>>        'ab' copy basicIdentityHash ] #(954 954 955 955 956 956 957 957 958
>> 958 959 959 960 960 961 961 962 962 963 963)
>>
>>
>> --
>> Best regards,
>> Igor Stasenko AKA sig.
>>
>
>
>
> --
> Mariano
> http://marianopeck.wordpress.com
>
>
>



-- 
Best regards,
Igor Stasenko AKA sig.