Re: Optimization

[Joern Rennecke <amylaar@cygnus.co.uk>]

> 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.

There is loop invariant code motion in loop.c, but it doesn't work in this
case, because the alias analysis can't prove that that step[0] ... step[2]
are not changed by the stores.