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|ID||Project||Category||View Status||Date Submitted||Last Update|
|0004747||OCaml||OCaml standard library||public||2009-03-16 10:35||2012-09-17 13:51|
|Target Version||4.00.2+dev||Fixed in Version|
|Summary||0004747: Hashtbl.resize is not tail recursive|
|Description||The function in Hashtbl.resize that copies the linked list attached to one bucket isn't tail recursive, which means that if you have the (admittedly undesirable) situation of a very long chain of buckets it can cause a stack overflow.|
let ht = Hashtbl.create 100
let () = for x = 1 to 2500000 do Hashtbl.add ht 0 () done
|Tags||No tags attached.|
|Attached Files||hashtbl.patch [^] (1,130 bytes) 2012-06-20 14:31 [Show Content]|
I encountered this bug while using Coq extraction feature, because of a misuse of Hashtbl.add instead of Hashtbl.replace that has been reported upstream.
I attached a proposed patch.
Beware that other functions build results in a non-tail-rec fashion over buckets:
Therefore, this patch may only delay stack overflows some more, as long as it can shrink buckets enough. A more thorough patch would probably incur both performance and efficiency loss.
This patch will slow down the resizing process too, since it builds the reversed bucket first.
Valentin Robert has given me a full patch. However, due to the non-critical status¹ and potential for performance regressions, there is no need to do that before the release. I'm changing the "target version" to reflect this.
¹: If the bucket becomes so long that the overflows becomes a problem, it means that there is a problem with the way hashtables are used. If it comes from hashing conflicts, you will get terrible performances due to the linked-list bucket implementation (linear cost for finding a given element), and a difference implementation must be preferred. If it comes from an over-use of the multiple binding semantics ("adding" a lot of different values on the same key), you should probably switch to a different data structure. One might argue that having a Stack Overflow is a good signal for a design defect that could otherwise go unnoticed. I'm still in favor of fixing this bug, but it can wait after the release.
|2009-03-16 10:35||shinwell||New Issue|
|2009-04-29 15:41||doligez||Status||new => acknowledged|
|2012-06-20 14:31||varobert||File Added: hashtbl.patch|
|2012-06-20 14:34||varobert||Note Added: 0007590|
|2012-07-04 16:55||doligez||Priority||normal => high|
|2012-07-04 16:55||doligez||Status||acknowledged => confirmed|
|2012-07-04 17:18||doligez||Target Version||=> 4.00.0+dev|
|2012-07-04 17:33||gasche||Assigned To||=> gasche|
|2012-07-04 17:33||gasche||Status||confirmed => assigned|
|2012-07-12 14:28||gasche||Note Added: 0007730|
|2012-07-12 14:28||gasche||Target Version||4.00.0+dev => 4.01.0+dev|
|2012-07-27 12:49||frisch||Category||OCaml general => OCaml standard library|
|2012-07-31 13:36||doligez||Target Version||4.01.0+dev => 4.00.1+dev|
|2012-09-17 13:51||doligez||Target Version||4.00.1+dev => 4.00.2+dev|
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