Good luck beating the performance of a Fortran optimizing compiler.

Thus a Fortran compiler doesn't have to do anything special to make sure it preserves order when storing and accessing from two different arrays. Therefore the compiler can be much more aggressive about unrolling loops and changing the order of operations.

By contrast, given a loop like

  for( i=0; i<N; ++i ) {
    y[i] = 2*x[i];
    }

This is one reason you see a lot of C code that does numerical work hand-unrolled, with explicit stores, like this:

  for( i=0; i<N; i+=4 ) {
    xi = x[i];
    xip1 = x[i+1]; xip2 = x[i+2]; xip3 = x[i+3];
    y[i] = 2*xi; ... y[i+3] = 2*xip3;
    }

There's a HUGE number of languages that don't allow aliasing in this way. I wasn't thinking about "how to improve C", I had APL, J, K, Q etc. in mind, as well as languages that implement mass array processing with similar semantics in terms of a more conventional syntax.

Personally, I think that idea is hard to justify these days, especially when, in my experience, finding 20-30% speed differences between optimizing Fortran compilers isn't all that hard.