Re: compiler optimization

From: Silvio Bierman (sbierman_at_idfix.nl)
Date: 02/23/04 

Date: Mon, 23 Feb 2004 13:19:30 +0100

"Murat Tasan" <tasan@eecs.cwru.edu> wrote in message
news:Pine.SOL.4.53.0402222133280.27212@homer...
> i've asked a similar question to this in the past, but the example i gave
> was not good enough i think to warrant any good answers... so i've come up
> with another example that should be clear.
>
> consider a loop that adds some values from an array of doubles x... such
> as (where "a" is a constant value):
>
> for(int i = 0; i < x.length; i++)
> result += (x[i] - a) / x.length;
>
> now, one can also obviously do:
>
> for(int i = 0; i < x.length; i++)
> result += (x[i] - a);
> result = result / x.length;
>
> if the code is executed directly as it is written, the second version can
> be far more efficient (only one division, vs x.length divisions)... and
> let us assume that precision is not a direct concern at the moment.
>
> does anyone know if the java compiler will attempt to see this
> optimization and make the appropriate change if the code is given as it is
> in the first example?
>
> thanks much,
>
> murat
>
>
> --
> Murat Tasan
> mxt6@po.cwru.edu
> tasan@eecs.cwru.edu
> murat.tasan@cwru.edu
> http://genomics.cwru.edu
>

No, Java compilers will only optimize code in ways that will not influence
the result. Doing multiple divisions will result in a different result than
one division considering rounding etc. You will have to do that yourself.

Silvio Bierman