OpenEuphoria

Re: Constructive criticism

• Posted by DerekParnell (admin) Mar 09, 2009
• 1343 views

Thank you.

So what? It is owned by the module its in and is private. What exactly is it a problem if this sequence exists for the entire life of the run-time?

Yes. This is an issue. We are working on a generic release management solution that will cover that aspect.

On the other hand, using the 'delete' is no more a problem than closing files. We do that without bitching about it. A deleted stack takes up 4-bytes. I can live with that.

public procedure rot(integer stack)  
    object temp  
  
    validate(stack)  
    if length(stacks[stack]) > 2 then  
        temp = stacks[stack][$]  
        stacks[stack][$] = stacks[stack][$-1]  
        stacks[stack][$-1] = stacks[stack][$-2]  
        stacks[stack][$-2] = temp  
    end if  
end procedure  

public procedure rot(integer stack)  
    object temp 
 
    validate(stack) 
    object s = stacks[stack] -- we do not copy here: COW 
    if length(s) > 2 then  
        temp = s[$] 
        s[$] = s[$-1]        -- Ooops: here s is copied completely! 
        s[$-1] = s[$-2]  
        s[$-2] = temp 
         
        stacks[stack] = s    -- COW 
    end if 
end procedure  

The first thing to note is that you have reminded me of that optimisation technique. Your version of the routine takes half the time than my original one. Even though you seem to be saying that it should be slower, it is fact much faster.

Secondly, the "common subexpression elimination" problem is of two types. The first is on a purely textual basis. I will be advocating a method of allowing some controlled textual substistutions in future versions of Euphoria. Something along the lines of ...

public procedure rot(integer stack)  
    object temp  
  
    validate(stack)  
    alias stacks[stack] as s 
    if length(s) > 2 then  
        temp = s[$]  
        s[$] = s[$-1]  
        s[$-1] = s[$-2]  
        s[$-2] = temp  
    end if  
end procedure  

This would "generate" the same code as my original version.

Further more, there is still many more optimisations that we can put into both the interpreter and translator that should allow caching of subexpressions. This would cause such code as above to automatically run as if it was coded more like your version.

And finally, it is anticipated that a 'swap' operator will be available in 4.1 that will further optimise this type of routine.

public procedure rot(integer stack)  
    validate(stack)  
    alias stacks[stack] as s 
    if length(s) > 2 then  
        s[$]  <=> s[$-1]  
        s[$-1] <=> s[$-2]  
    end if  
end procedure  

Try benchmarking it yourself and you will see that this is not so. The suggested optimisation works well.