Re: optimization improves code readability

[Leo Razoumov <slonik.az@...>]

On Thu, 22 Jan 2009, David Manura wrote:

> > > Date: Thu, 22 Jan 2009 07:37:20 -0500 (EST)
> > > From: Leo Razoumov
> > In Lua, on the other hand, some
> > optimizations make code *MORE* readable.
>
> Here's one optimization that is less readable:
>
>  local y = myvar^4 / (1 + myvar^2)
>
>  local x2 = myvar*myvar
>  local y = x2*x2 / (1 + x2)
>  -- Lua itself couldn't do this factorization due to
>  -- the possibility not only of overflow but also
>  -- metamethods, except in special cases if detected
>  -- by code analysis

The optimization above is, in essence, an *algorithmic* optimization
rather than a programming optimization. 
You are defining your own way of raising number to some power 
relying upon associativity of multiplication. 
In a finite precision math, such optimization could lead to 
non-bit-exact results when compared with an unoptimized case. 
I really do not want my compiler to do such things for me behind the scenes.

By the same token the following function

function mysum1(x)
	assert(math.abs(x)<1)
	local sum=1
	for i=1,9999 do
		sum= sum + x^i
    end
	return sum
end

can be "optimized" into

function mysum2(x)
	assert(math.abs(x)<1)
	return 1/(1-x)
end

x= 0.5
print(mysum1(x), mysum2(x), mysum1(x) - mysum2(x))
assert(mysum1(x) == mysum2(x))

--Leo--