dom1 prevents vectorization via partial loop peeling?

[Alan Lawrence]

Richard Biener wrote:
> 
> Well.  In this case we hit
> 
>   /* If one of the loop header's edge is an exit edge then do not
>      apply if-conversion.  */
>   FOR_EACH_EDGE (e, ei, loop->header->succs)
>     if (loop_exit_edge_p (loop, e))
>       return false;
> 
> which is simply because even after if-conversion we'll at least end up
> with a non-empty latch block which is what the vectorizer doesn't support.
> DOM rotated the loop into this non-canonical form.  Running loop header
> copying again would probably undo this.

Indeed - header copying causes the loop to be if-converted, and then actually 
vectorized, too. (Well, I had to make do_while_loop_p a bit more aggressive in 
saying things weren't do-while loops.)

I merely moved pass_ch to immediately after pass_dominator, a few stages later; 
I have also tried cloning it, but do you foresee any problems with just moving 
it? (It passes bootstrap, I'm running the full testsuite atm).

Cheers,
Alan