Optimization

[Oskar Enoksson <osken393@student.liu.se>]

Hi! I'm working with computational mathematics using c and c++.

Does g++/gcc have any optimization that brings out constant subexpressions
outside loops? Consider the following:

void sqr(float *x, int *step) {
  int i,j,k;

  for (i=0; i<100; i++)
    for (j=0; j<100; j++)
      for (k=0; k<100; k++)
        *(x+i*step[2]+j*step[1]+k*step[0]) *= 
          *(x+i*step[2]+j*step[1]+k*step[0]);
}

The resulting assembler with egcs 1.0.2 (i686-pc-linux-gnulibc1) is not
very efficient, because the integer expressions are calculated again
and again inside the innermost loop. Can I make the compiler translate the
above code into something like the following:

void sqr(float *x, int *step) {
  int i,j,k; 
  float *xi,*xj;

  for (i=0; i<100; i++) {
    xi=x+i*step[2];
    for (j=0; j<100; j++)  {
      xj=xi+j*step[1];
      for (k=0; k<100; k++)  {
        *(xj+k*step[0]) *=
          *(xj+k*step[0]);
      }
    }
  }
}

If this is impossible with the current version, would it be hard to
add this optimization to the compiler? The resulting assembler is much
more efficient.

/Oskar