Re: Constant folding

[Kein-Hong Man <mkh@...>]

Mike Pall wrote:
> Kein-Hong Man wrote:
> > It is not heavy-duty constant folding, though. According to my 
> > observations, the terms of an expression are not reordered, so the 
> > following is not optimized:
> >
> >   local a = a+1+2
> >
> > since it is parsed as:
> >
> >   local a = (a+1)+2
>
> It would violate the language semantics to optimize this at
> compile time. '+' is left-associative in Lua. If 'a' is a table
> or userdata with an __add metamethod, it could completely change
> the execution path when the terms are reordered or folded.
>
> It even makes a difference with numbers (lua_Number = double):
>
> $ lua -e 'local x=2^53; print((x+1+1)-x, (x+2)-x)'
> 0       2
>
> Addition is only associative in specific (mathematical) groups.
> E.g. for numbers with arbitrary precision or fixed precision
> integers with wrap-around semantics. Floating-point numbers with
> a fixed precision do not follow this rule (and many others).
>
> Compiler writers sometimes forget about this (hint: check your
> favourite language). But Lua gets it right.

Good call. Thanks for the explanation, I was thinking in terms of 
integers. There is of course always a danger with shortcuts when 
dealing with floating-point formats. I've been applying C thinking 
to this thing... :-) Semantically, of course it is wrong. But in 
the real world, the user usually wants 1+2 optimized.

I guess the best way is to make it explicit, i.e. write:

  local a = a+1+2

as:

  local a = a+(1+2)

--
Cheers,
Kein-Hong Man (esq.)
Kuala Lumpur, Malaysia