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

[Richard Biener <richard.guenther@gmail.com>]

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).

What doesn't work with the idea to clone the epilogue using __built-in_vectorized()
For the if- vs. Not if-converted loop?

Richard.

>We've already got Bin doing that for DOM...
>
>
>> Also I don't like at
>> all that we have many variants of vectorizing but somehow the
>decision which one
>> to choose is rather unclear.  The way the epilogue vectorization code
>> is hooked in
>> is rather awkward and bound to be a maintainance burden (well, maybe
>a
>> small one).
>I think it's going to be a small one.  I suspect that we really need 
>another architecture with masking capabilities to really be able to see
>
>how the costing models ought to work and bring sanity to that decision.
>
>>
>> And last, I double there is a case for a masked vectorized loop - I
>can bet that
>> doing a non-masked vectorized loop plus a masked epilogue (with no
>iteration
>> then!) will be always faster unless you hit the window of very few
>iterations
>> (or optimizing for size - in which case vectorizing is questionable
>on
>> its own and
>> disabled IIRC).
>Ilya, does this case make a noticeable difference with the ICC 
>implementation?
>
>Jeff