SuperWord::unrolling_analysis() question

[Berg, Michael C]

John, it is pretty much that issue(unrolling for the available supported vector), I am testing some changes now for Roland.

-----Original Message-----
From: John Rose [mailto:john.r.rose at oracle.com] 
Sent: Wednesday, April 27, 2016 2:55 PM
To: rwestrel at redhat.com
Cc: Berg, Michael C <michael.c.berg at intel.com>; hotspot-compiler-dev at openjdk.java.net
Subject: Re: SuperWord::unrolling_analysis() question

It is reasonable to look ahead into the loop to find the largest applicable vector size, before choosing an unroll factor.
A loop which works on bytes and doubles at the same time will want to unroll only up to vector-of-double.
But a loop which works only on bytes will want to unroll more.
Is that what we are talking about here?
- John

> On Apr 27, 2016, at 8:53 AM, Roland Westrelin <rwestrel at redhat.com> wrote:
> 
> 
> Hi Michael,
> 
> Thanks for the answer.
> 
>> The answer could be conditional if we had a machines with enough byte 
>> or short components to make vectors with, I chose INT as it is the 
>> current consistent minimum configuration for complete vector mapping.
>> The best answer would be to create some code which mines the common 
>> type used in the current loops expressions, but I think we would be 
>> stuck with two passes over the code, the first to bind the common 
>> type, the second for finding the optimal sub vector mapping.  Or 
>> possibly moving the question to the machine layer as a query, where 
>> compiler writers choose the minimum consistent configuration based on 
>> current info on the machine we compile on.
> 
> Would two passes like sketched here:
> 
> http://cr.openjdk.java.net/~roland/vect-unroll-analysis/webrev/
> 
> would do the job?
> 
> Roland.