Re: [PATCH, vec-tails 07/10] Support loop epilogue combining

[Jeff Law <law at redhat dot com>]

On 07/25/2016 12:32 PM, Richard Biener wrote:
> On July 25, 2016 8:01:17 PM GMT+02:00, Jeff Law <law@redhat.com> wrote:
> > On 07/22/2016 05:36 AM, Richard Biener wrote:
> > > The thing that needs work I think is re-running of if-conversion.
> > I wonder if we could revamp if-conversion to work on a subset of the
> > CFG?   I can see that potentially being useful in other contexts.
> > Would
> > that work for you Richi?
>
> Well, you need to make it not need post-dominators or preserve them (or compute "post-dominators" on SESE regions).
Oh, but it'd be so nice to have DOMs and/or PDOMs on regions.  But 
that's probably out of scope for gcc-7.


> What doesn't work with the idea to clone the epilogue using __built-in_vectorized()
> For the if- vs. Not if-converted loop?
I must be missing something.   I don't see how 
builtin_vectorized_function helps, but maybe I've got the wrong built-in 
or don't understand what you're suggesting.

It sounds like this is the biggest impediment to moving forward.  So 
let's reset and make sure we're all on the same page here.

Ilya, what's the fundamental reason why we need to run if-conversion 
again?    Yes, I know you want to if-convert the epilogue, but why?

What are the consequences of not doing if-conversion on the epilogue? 
Presumably we miss a vectorization opportunity on the tail.  But that 
may be a reasonable limitation to allow the existing work to move 
forward while you go back and revamp things a little.

Jeff