Code Optimization Exercises:
============================

First something really simple. What optimizations can we perform on
the following pieces of code?

1. 

    x = y / w + z / w

Answer:
-------

    x = (y + z) / w



2. 

    DO i = 1,10
      print*,i*10
    END DO


Answer:
-------

Use constant increments instead of multiplies if this is possible.
Some compilers are clever enough to figure this out for you in some
simple cases.

    DO i = 1,100,10
      print*,i
    END DO



3.

    INTEGER :: i,j
    REAL :: d,f
    REAL, DIMENSION(width;height) :: array
    DO i=1,width
      DO j=1,height
        array(i,j)=(2.0*i+j)/d
	f = 1.5*d
      END DO
    END DO


Answer:
-------

In fortran arrays the first index should change the most, declare the
variables in order of type size (largest first), replace division with
multiplication and multiplication with addition. Take the loop
invariant code out of the loop. You may also do some loop unrolling.
Note that, compiler may do these kind of optimizations automatically
in simple cases.


    REAL, DIMENSION(width,height) :: array
    REAL :: d,dd,f
    INTEGER :: i,j

    dd = 1.0/d
    f = 1.5*d
    DO j=1,height
      DO i=1,width
        array(i,j)=(i+i+j)*dd
      END DO
    END DO



Then we'll get down to business...

4. Profile your application by using gprof (if you do not have your
application with you, try LDGB in the exercises folder).


5. Try to enhance the performance of your application by tweaking
compiler flags. Note, check that the results are not affected!


6. Examine, if the performance of your code can be optimized and
attempt to do it.