Re: Optimization

[Jim Wilson <wilson@cygnus.com>]

	  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]);

This is a hard problem.  We can't move the loads from step[] outside
the loop unless we can know for sure that the stores into *(x+...) won't
modify any of the values pointed to by step.  Since we have no info about
where x or step point to, we make the worst case assumption that they
overlap, and hence the loads from step[] are not loop invariant.

It is actually a bit more complicated than that (we have type info to help
with aliasing), but it is still a complicated problem.

The ISO C9X restrict keyword would make this optimization easier, because
this would allow the programmer to tell the compiler that there is no aliasing.

Jim