[LispM] bug in hash-table-rehash?

[Scott L. Burson]

Right.  I'm not certain, but I think an empty leader probably took zero
space, so the pointers would have the same value.

-- Scott


On Sun, Mar 11, 2018 at 5:28 PM, Greg Gilley <ggilley at gerg.org> wrote:

> But %structure-total-size doesn’t include the leader if it’s passed the
> array header pointer (in this case hash-table). So the blt-typed is being
> handed the pointers to the leader to start the copy, but the size doesn’t
> include the leader if I believe the microcode comment.
>
> Greg
>
>
> On Mar 11, 2018, at 5:18 PM, Scott L. Burson <Scott at ergy.com> wrote:
>
> You may have found a bug.  Since rehashing hash tables is presumably a
> frequent operation, and thus this code is likely to be well exercised, I
> strongly suspect that in practice, hash tables never had nonempty leaders.
>
> -- Scott
>
>
> On Sun, Mar 11, 2018 at 4:24 PM, Greg Gilley <ggilley at gerg.org> wrote:
>
>>
>> This is in the MIT sys99 code and the Explorer code:
>>
>> ((null grow) (setq hash-table (follow-structure-forwarding hash-table))
>>     (setf (hash-table-lock new-hash-table) (hash-table-lock hash-table))
>>     (%blt-typed (%find-structure-leader new-hash-table)
>> (%find-structure-leader hash-table)
>> (%structure-total-size hash-table) 1)
>>     (return-array (prog1 new-hash-table (setf new-hash-table nil)))
>>     hash-table)
>>
>> In the function hash-table-rehash, if it didn’t grow the table (after a
>> gc perhaps), then it copies the re-hashed table back to the original hash
>> table.
>>
>> However, it doesn’t seem to copy the whole table. if I read the comments
>> from the microcode (this is from sys99, but seems similar to the lmi
>> microcode):
>>
>>
>> ;;; Given the address of the base of a structure, return information on
>> its size.
>> ;;; Note that if given the address of an array header, the leader (if
>> any) is
>> ;;; not counted, but if given the address of the leader, the leader is
>> ;;; counted.  I.e. nothing before the given address is counted.
>> ;;; In the case of an RPLACD-forwarded list, the 2 words pointed to by the
>> ;;; forwarding-pointer are counted, and the forwarding-pointer itself
>> isn't.
>>
>> It looks as though it’s not copying the complete hash table over because
>> it’s passing in the pointer to the hash-table and the not the hash-table
>> leader.
>>
>> Anyone familiar enough with the code to confirm my diagnosis?
>>
>> Thanks,
>>
>> Greg
>>
>>
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>>
>
>
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