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: Friday, August 14, 2015 9:59 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 August 14, 2015 5:03:58 PM GMT+02:00, Ajit Kumar Agarwal <ajit.kumar.agarwal@xilinx.com> wrote:
>
>
>-----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.

>>Sure.  But the number of iterations may only be available symbolically, thus the cost be only useful for the dynamic check at runtime.  A better static >>estimate would also be useful.

Thanks. For the cases the loop bound can be known at the compile time, through Value Range Analysis. Already GCC uses the value range
 Information/Analysis To calculate the Loop bound. We can use the same loop bound info to get the static estimate on the number of iterations.

  Based on the above estimates, the above cost calculation as I have mentioned can be used for Vectorization cost Analysis.

Thanks & Regards
Ajit

Richard.

>Thanks & Regards
>Ajit
>
>Richard.
>
>
>> Thanks & Regards
>> Ajit