On August 14, 2015 5:03:58 PM GMT+02:00, Ajit Kumar Agarwal <ajit.kumar.agar...@xilinx.com> wrote: > > >-----Original Message----- >From: Richard Biener [mailto:richard.guent...@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.agar...@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
