Re: Lua 5.1 has a serious issue

[Mike Pall <mikelu-0601@...>]

Hi,

Daniel Silverstone wrote:
> [...] Lua 5.1's new # operator.
> Specifically it is that it does not invoke the __len metamethod on
> tables, only on userdata.

Ah. News. Well, as of May 19th 2005:
  http://lua-users.org/lists/lua-l/2005-05/msg00281.html

> It's fairly easy to replace the chunk in lvm.c for handling OP_LEN for
> tables to be something like:

It's also fairly slow. Measuring #t on a plain empty table in a
tight loop minus the loop overhead is 2x slower (!). Not good.

A proper fix is to make __len a fast tag method. Then the
overhead is down to 2% (much less when the table has elements).
Much better.

I've had a 'nice to have, but probably too expensive' opinion
before. But in the light of this benchmark, I'd say go for it.

> And such would save a lot of hassle and prevent us from having another
> second-class metamethod to go alongside __gc.

This has a very different rationale and is an inappropriate
comparison. Please google for (e.g.) Java finalizers or
reachability states to understand the problem. I severly doubt
there is an elegant generic solution.

> Otherwise I can see
> many distributions applying such a patch as a "bugfix" which
> will make varying and effectively incompatible versions of Lua
> occupy our distributions.

Yes, they can do that. Just not call it Lua anymore.

PS: The choice of the subject line and the general tone of your
    message is not helpful for your agenda. Developers are
    usually most impressed by proper research, good code and
    convincing benchmarks.

Bye,
     Mike