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- Subject: Re: lists with nil play nice for Lua 5.2
- From: Lavergne Thomas <thomas.lavergne@...>
- Date: Mon, 6 Aug 2007 10:39:38 +0200
On Fri, Aug 03, 2007 at 09:47:12PM +0100, David Given wrote:
> Roberto Ierusalimschy wrote:
> > This 'n' would increase the memory needed to store each table.
> > That would demand a new test when writing an element to the table (to see
> > whether the key is in the array part).
> > Again this demand a test at each writing, to know whether the element
> > being written is nil.
> This is all correct; but since all this happens in C code inside the VM, the
> overhead is trivial. Storing 'n' is just another machine word in the table
> structure (and may replace the existing #length field, if there is one).
> Comparing for nil is trivial. I would be very surprised if any of this turned
> into more than a dozen or so machine instructions. Any extra overhead should
> be completely absorbed in general housekeeping.
This is what I have in mind when writting my previous post. This cost is
extremly low when I compare it to the code (both in lua and c) I need to
ensure a predictable execution of some of the programs I work on.
I'm prfectly ok to loose a little of speed if, in exchange, I can be
sure of the way my code run. I think unpredictable result of some core
operations in a language is very big flaw.
> One possible complication I can see, however, is what happens if the user
> creates a table with sparse indices and then fills in the gaps later. Does the
> VM move stuff from the hash table portion of the table into the array portion
> (your definition!) of the table when this happens, or does the data all end up
> in the hash table?
This is implementation dependant, but, in my point of view, what I call
the array part (integer index from 1 to #t) can be stored in hash or
array. The array trick is just an implementation optimisation : in some
case you will have code working quicker, but in all case you will have
exactly the same result for your computation.
Thomas Lavergne "Le vrai rêveur est celui qui rêve
de l'impossible." (Elsa Triolet)