[Vm-dev] Re: [squeak-dev] identityHash bits not incremented for
new objects?
Igor Stasenko
siguctua at gmail.com
Mon Aug 1 08:03:04 UTC 2011
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.
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