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RE: More of a Loop distribution.



-----Original Message-----
From: Richard Biener [mailto:richard.guenther@gmail.com] 
Sent: Thursday, August 13, 2015 3:23 PM
To: Ajit Kumar Agarwal
Cc: Jeff Law; gcc@gcc.gnu.org; Vinod Kathail; Shail Aditya Gupta; Vidhumouli Hunsigida; Nagaraju Mekala
Subject: Re: More of a Loop distribution.

On Thu, Aug 13, 2015 at 9:37 AM, Ajit Kumar Agarwal <ajit.kumar.agarwal@xilinx.com> wrote:
> All:
>
> Loop distribution considers DDG to decide on distributing the Loops. 
> The Loops with control statements like IF-THEN-ELSE can also be 
> Distributed. Instead of Data Dependency Graph, the Control Dependence Graph should be considered in order to distribute the loops In presence of control Statements.
>
> Also the presence of multiple exits in the Loop can also be considered for Loop distribution transformation.
>
> Thus the above transformation helps in the Libquantum benchmarks for SPEC 2006.
>
> There are following articles that looks interesting to me.
>
> "Loop Distribution in presence of arbitrarily control flow Ken Kennedy et.al."
> "Loop Distribution in presence of Multiple Exits Bor-Ming Hsieh etal."
>
>
> I don't think the loop distribution in presence of control flow is implemented in GCC/LLVM.
>
> I think it is feasible to consider the above for the implementation in GCC.

>>It's true that loop distribution does not try to distribute based on any control structure heuristics but it only considers data locality.  It does however already >>compute the control dependence graph (and uses it to add control edges to the DDG to properly add data dependence edges to uses of control statements >>necessary in partitions).

>>So it should be a matter of specifying the proper set of starting statements it tries separating.

Thanks.

>>Not sure which kind of distribution you are after, can you give an example?

I would like to have a distribution of the loop having control flow. For example

For (I = 2 ; I < N; i++)
{
    If  (condition)
       {
  S1:         A[i] = ...
  S2:        D[i] = A[i-1]...
       }
}

The above loop can be distributed with two loops having one loop with S1  inside IF and another loop with S2 with the IF.
The two scenario can be true.

1. The condition inside IF have a check on A[i] and is dependent on S1. In this case the distribution is difficult. And the above article
From Ken Kennedy et.al does store the partial results of comparison in an temporary array and that array is compared inside the IF
Condition. This makes the loop distributed. This is what I was looking for which I found in the above article.

2. The condition inside the IF in the above loop is not dependent on the S1 and S2 , and this case the loop can be distributed.

In the above two scenario the GCC can't distribute the loop, as the control dependency graph ( control structure ) is not used. The advantage of
The above loop distribution makes the loop vectorizable which otherwise not possible due to presence of multiple statements inside the IF and
Also may not be IF-converted due to presence of multiple statements. If we distribute the loop for the above two scenarios the individual loops 
in the distributed loop can be vectorized which is otherwise not possible.

Thanks & Regards
Ajit


Richard.

> Thanks & Regards
> Ajit

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