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

[Mariano Martinez Peck]

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

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
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