This addresses a vectorisation regression for tail-folded loops that are counting down, e.g. loops as simple as this:
void foo(char *A, char *B, char *C, uint32_t N) {
while (N > 0) {
*C++ = *A++ + *B++;
N--;
}
}These are loops that can be vectorised, but when tail-folding is requested, it can't find a primary induction variable which we do need for predicating the loop. As a result, the loop isn't vectorised at all, which it is able to do when tail-folding is not attempted. So, this adds a check for the primary induction variable where we decide how to lower the scalar epilogue. I.e., when there isn't a primary induction variable, a scalar epilogue loop is allowed (i.e. don't request tail-folding) so that vectorisation could still be triggered.
Having this check for the primary induction variable make sense anyway, and in addition, in a follow-up of this I will look into discovering earlier the primary induction variable for counting down loops, so that this can also be tail-folded.
The ordering of the predicates tests seems to be a bit off from a readability point-of-view. If we're only thinking about predication, I would expect an early return if PredicateOptDisabled, which would also include Hints.getPredicate() == LoopVectorizeHints::FK_Disable. The last piece of logic would then only contain all the values that we require to enable the folding.