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

[Jeff Law <law@redhat.com>]

On 07/26/2016 03:57 AM, Ilya Enkovich wrote:
>>
>> 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.
>
> If we have some control-flow in a loop then we have to if-convert it
> for vectorizer. We need to preserve both versions: if-converted one
> for vectorizer and the original one to be used if vectorization
> fails.  For epilogues we have similar situation and need two
> versions.  I do it by running if-conversion on a copy of original
> loop. Note that it doesn't run full if-conversion pass. If-conversion
> is called for epilogue loop only.
Right.  So what I think Richi wants you to try is to use the 
if-converted loop to construct the if-converted epilogue.  It seems 
conceptually simple and low cost -- the question is on the 
implementation side.  I have no clue how painful that would be.

jeff