From: Sean Middleditch <elanthis@...>
Date: 2002-04-05T00:41:59+09:00
Subject: Re: About efficiency

On Thu, 2002-04-04 at 01:27, nobu.nokada@softhome.net wrote:
> Hi,
> 
> At Wed, 3 Apr 2002 10:46:24 +0900,
> Sean Middleditch wrote:
> > Hmm, how would that work with loadable modules?  Either you'd have to
> > store a lot of meta information, or modules that try to access a
> > variable in the wrong way from another module would cause a crash or
> > some other bad side-effect.
> 
> I guess it's so hard.

^,^  That's how I look at it.

> 
> > 
> > You know, one cache I found useful in Scriptix (dunno if Ruby uses it)
> > is a name cache.  I.e., if you have a small loop that references the
> > same small number of variables/members/etc., the code has a string to
> > find the name against.  Well, comparing pointers is faster than
> > comparing a whole string.  So, the compiler caches the last 20 (or
> > whatever number) names is encountered, so the byte-code (or node tree)
> > can just have multiple copies of the same string in memory.  This
> > reduces memory usage for variable name lookups, plus makes the lookups
> > faster by reducing the complexity of the name comparisons.  This
> > optimization had a nice effect in Scriptix, so if Ruby doesn't use it
> > now, it might make a big difference if it did.
> 
> In ruby, identifiers are handled as numeric ID commonly.  Don't
> you mean this?

Sort of - are the names converted to ID's at compile time or run-time?

> > Hmm.  if you stored them all in a dynamically allocated array, that
> > might make it work (only appending entries).  The problem I see is
> > getting the interpreter to know what to cache when.  Doing it at run
> > time would just add over-head to the current member search, wouldn't
> > it?  (Maybe I'm just not seeing how this optimization works.)
> 
> Adding instance variables dynamically means that it's
> impossible to predict the order of them.  I guess it needs to
> map ID to index finally.

Well, with a cache, you wouldn't need to know the order until first
encountered.  If you don't allow the removal or deletion of members,
then once cached, the index will always be valid.

> > That's something I've been toying with.  A powerful optimizer can make
> > the runtime code a *lot* faster, but significantly slow down the
> > compiler.  If you load a script once and let it run for a very long time
> > (i.e., a full-scale application, or large mathematical computations), or
> > if you run a small script repeatedly (using scripts as callbacks in
> > triggers in an embedding application), then the greatly increased
> > compile time could well be worth it.
> 
> No doubt that inlining will make code faster much, if no method
> redefinition.  But it may be difficult to reduce drawback on
> the redefinition.

Perhaps if we had a way of marking a method as "constant" or "static" ?
Then the compiler would know that the value cannot change, and it can
inline to its heart's content.

> 
> > I've not looked much at the Ruby compiler, but perhaps two separate
> > compilers would be possible?  A simpler, lighter one that compiles
> > quickly and runs at moderate speed, and a complex one that may take many
> > seconds to compile a smaller app/script, but run at lightning speeds?
> > 
> > Also, a more complex compiler may be useful if Ruby let you save to some
> > form of bytecode (hell, even an binary or XML representation of the
> > current node tree scheme), then load that back in.  You could do like
> > you would with C - spend a lot of time compiling, but get applications
> > much much faster than a fully interpreted language.
> 
> IMHO, bytecode serialization may be a prerequisite to optimize
> more, a more complex compiler.  But I'm afraid about the
> drawback.

Which drawback?

> 
> -- 
> Nobu Nakada