Re: upvalues and polymorphism

[Roberto Ierusalimschy <roberto@...>]

> In fact, for this specific reason, I'd like to see the "lookup" VM codes
> dereference a complete table lookup, instead of just one slot. It would be
> much more efficient to come up with a cache method for:
> 
> ...
> 
> Am I wrong in assuming that table lookup is somewhat costly in lua?

As lhf aswered, table lookup takes constant time in Lua. The constant can be
a little heavy, but not too much... Just in case, it is important to remember
that, in critical cases, a local variable can be explicitly used to "cache"
table lookups. As a tipical example, the following loop

      local i = 1
      while i<=MAX do
        obj:f(a.b.c.d[i])
        i = i+1
      end

can be rewritten as

      local i = 1
      local t = a.b.c.d
      local f = obj.f
      while i<=MAX do
        f(obj, t[i])
        i = i+1
      end

with good impact on performance.

-- Roberto