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RE: vectorization cost macro TARGET_VECTORIZE_ADD_STMT_COST
- From: Ajit Kumar Agarwal <ajit dot kumar dot agarwal at xilinx dot com>
- To: Richard Biener <richard dot guenther at gmail dot com>
- Cc: Jeff Law <law at redhat dot com>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>, "Vinod Kathail" <vinodk at xilinx dot com>, Shail Aditya Gupta <shailadi at xilinx dot com>, Vidhumouli Hunsigida <vidhum at xilinx dot com>, Nagaraju Mekala <nmekala at xilinx dot com>
- Date: Tue, 4 Aug 2015 13:39:11 +0000
- Subject: RE: vectorization cost macro TARGET_VECTORIZE_ADD_STMT_COST
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-----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.
Thanks for the valuable information and suggestions.
Thanks & Regards
Ajit
Richard.
> Thanks & Regards
> Ajit