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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.

Richard.

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



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