0005290: Hashing of custom blocks

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0005290

OCaml

OCaml general

public

2011-06-17 01:00

2011-07-23 12:25

Reporter

Boris Yakobowski

Assigned To

xleroy

Priority

normal

Severity

major

Reproducibility

always

Status

resolved

Resolution

open

Platform

OS Version

Product Version

3.12.0

Target Version

Fixed in Version

Summary

0005290: Hashing of custom blocks

Description

Hashtbl.hash returns 0 on custom blocks, at least those of the standard library (verified on bigints, channels, and by a cursory grep). This is consistent with its documentation "associates a positive integer to any value of any type". However, it is IMHO a very poor choice, as this will always silently lead to abysmal performances when this function is used on such values. It would be more logical to have this function fail, just as polymorphic equality and comparison. Since Hashtbl are about to undergo a massive rewrite in 3.13, please consider deprecating the old behavior and have the function fail.

(For the record, this really bit us. After some reorganization, we started using the polymorphic hashing function, and Frama-C performances dropped terribly. It took us one afternoon to track the culprit.)

(Also, providing a good hash function for bigints/nats, one bound in the custom block, would be great. For example, keep only the 2^(n-1) lowest or highest bits. This cannot be done efficiently in pure Caml.)

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Notes
(0006015) Boris Yakobowski (reporter) 2011-06-17 11:55	A precision: the custom blocks are on Nats, not on bigints.

(0006018) xleroy (administrator) 2011-06-20 19:03	Some standard library types have proper hash functions, e.g. Int32.t, Nativeint.t and Int64.t, but you are correct that channels and nats don't while they should. I put that on my to do list.

(0006058) xleroy (administrator) 2011-07-23 12:25	Tentative fix in SVN trunk, will go in 3.13.

Issue History
Date Modified	Username	Field	Change
2011-06-17 01:00	Boris Yakobowski	New Issue
2011-06-17 10:55	Pascal Cuoq	Note Added: 0006014
2011-06-17 10:56	Pascal Cuoq	Note Deleted: 0006014
2011-06-17 11:55	Boris Yakobowski	Note Added: 0006015
2011-06-20 19:03	xleroy	Note Added: 0006018
2011-06-20 19:03	xleroy	Status	new => assigned
2011-06-20 19:04	xleroy	Assigned To	=> xleroy
2011-07-23 12:25	xleroy	Note Added: 0006058
2011-07-23 12:25	xleroy	Status	assigned => resolved