"I'm sure I've seen references to re-ordering tests & loops in some Keil literature somewhere.."

One of the optimisation levels is called 'loop rotation'. However, I don't know what this actually means and have drawn a blank trying to find out.

"How on earth the compiler can be sure that it's safe to do this worries me, though.."

In the simple case where the loop variable isn't used in the body of the loop it shouldn't be a problem. In other cases.. well, I don't know. One thing I do know, though, is that compiler writers are very clever chaps.