[LispM] bug in hash-table-rehash?

[Greg Gilley]

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 <mailto: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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