RE: vectorization cost macro TARGET_VECTORIZE_ADD_STMT_COST

[Ajit Kumar Agarwal <ajit dot kumar dot agarwal at xilinx dot com>]

-----Original Message-----
From: Richard Biener [mailto:richard.guenther@gmail.com] 
Sent: Monday, August 03, 2015 2:59 PM
To: Ajit Kumar Agarwal
Cc: Jeff Law; gcc@gcc.gnu.org; Vinod Kathail; Shail Aditya Gupta; Vidhumouli Hunsigida; Nagaraju Mekala
Subject: Re: vectorization cost macro TARGET_VECTORIZE_ADD_STMT_COST

On Sun, Aug 2, 2015 at 4:13 PM, Ajit Kumar Agarwal <ajit.kumar.agarwal@xilinx.com> wrote:
> All:
>
> The definition of the following macro that determine the statement cost that adds to vectorization cost.
>
> #define TARGET_VECTORIZE_ADD_STMT_COST.
>
> In the implementation of the above macro the following is done for many vectorization supported architectures like i386, ARM.
>
> if (where == vect_body && stmt_info && stmt_in_inner_loop_p (stmt_info))
>         count *= 50;  /* FIXME.  */
>
> I have the  following questions.
>
> 1. Why the multiplication factor of 50 is choosen?

>>It's a wild guess.  See tree-vect-loop.c:vect_get_single_scalar_iteration_cost.

> 2. The comment mentions that the inner loop relative to the loop being 
> vectorized is added more weight. If more weight is added to the inner 
> loop for the loop being vectorized, the chances of vectorizing the inner loop decreases. Why the inner loop cost is increased with relative to the loop being vectorized?

>>In fact adding more weight to the inner loop increases the chance of vectorizing it (if vectorizing the inner loop is profitable).
>>Both scalar and vector cost get biased by a factor of 50 (we assume 50 iterations of the inner loop for one iteration of the outer loop), so a non-profitable >>vectorization in the outer loop can be offsetted by profitable inner loop vectorization.

>>Yes, '50' can be improved if we actually know the iteration count of the inner loop or if we have profile-feedback.

Instead of vector and scalar cost get biased by a factor of 50, Can the benefit of vectorization calculated as follows

Benefit = scalar cost - vector cost/VF;
Cost = 0;
For ( I = 1; I < N; i++)
{
    Cost = cost + (final_value - Initial value)/steps;
}

Benefit = Benefit * cost;

Where 
N = No. of levels of the loop;
Final_value = Final iteration count of the loop.
Initial_value = Initial Iteration count of the loop.
Steps = steps of the iteration for the loop.
VF = vectorization factor.

Thus increase in the Levels of the loops increases  the benefit of vectorization. Also if the scalar cost is more than the vectorization cost then the 
Scalar cost - vector cost /VF increases with the same vectorization Factor thus increasing the benefit of vectorization.

Thanks & Regards
Ajit

Richard.


> Thanks & Regards
> Ajit