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

[David T. Lewis]

On Mon, Aug 01, 2011 at 01:55:16PM +0200, Nicolas Cellier wrote:
> 
> Well distributed ?
> In examples like Igor's one you'll likely get consecutive hashes...
> 
> Nicolas

I don't know, but I would say that if you have a suitable hash test, please
try comparing the results of that test on Cog versus a interpreter VM (and
check it again after Igor's patch becomes available) to see if it makes
a significant difference.

Dave


> 
> 2011/8/1 David T. Lewis <lewis at mail.msen.com>:
> >
> > On Mon, Aug 01, 2011 at 10:03:04AM +0200, Igor Stasenko wrote:
> >>
> >> 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)
> >>
> >
> > Nice catch Igor!
> >
> > I think the intented implementation was probably this:
> >
> > ? ? ? ?^freeStart >> ShiftForWord
> >
> > Nasty sort of bug, it's the sort of typo that looks fine until you
> > stop to think about it ;)
> >
> > Dave
> >
> >