# [llvm-dev] Jacobi 5 Point Stencil Code not Vectorizing

hameeza ahmed via llvm-dev llvm-dev at lists.llvm.org
Sat Jul 1 15:33:09 PDT 2017

```I am able to vectorize it with the following code;

#include <stdio.h>
#define N 100351

// This function computes 2D-5 point Jacobi stencil
void stencil(int a[][N], int b[][N])
{
int i, j, k;
for (k = 0; k < N; k++) {
for (i = 1; i <= N-2; i++)
for (j = 1; j <= N-2; j++)
b[i][j] = 0.25 * (a[i][j] + a[i-1][j] + a[i+1][j] + a[i][j-1] +
a[i][j+1]);

for (i = 1; i <= N-2; i++)
for (j = 1; j <= N-2; j++)
a[i][j] = b[i][j];

}
}

I removed restrict over here.

On Sun, Jul 2, 2017 at 3:11 AM, hameeza ahmed <hahmed2305 at gmail.com> wrote:

> further i modified the code to the following;
>
> #include <stdio.h>
> #define N 100351
>
> // This function computes 2D-5 point Jacobi stencil
> void stencil(int a[restrict][N], int b[restrict][N])
> {
>    int i, j, k;
>    for (k = 0; k < N; k++) {
>        for (i = 1; i <= N-2; i++)
>            for (j = 1; j <= N-2; j++)
>         b[i][j] = 0.25 * (a[i][j] + a[i-1][j] + a[i+1][j] + a[i][j-1] +
> a[i][j+1]);
>
> for (i = 1; i <= N-2; i++)
> for (j = 1; j <= N-2; j++)
>  a[i][j] = b[i][j];
>
> }
> }
>
>    but still no vectorization in IR. Also, when I set vector width
> explicitly to 64, it gives the following error:
>
> remark: <unknown>:0:0: loop not vectorized: call instruction cannot be
> vectorized
> remark: <unknown>:0:0: loop not vectorized: value that could not be
> identified as reduction is used outside the loop
>
> I need serious help on this. Please guide me.
>
> On Sun, Jul 2, 2017 at 1:54 AM, hameeza ahmed <hahmed2305 at gmail.com>
> wrote:
>
>> Does it happen due to loop carried dependence? if yes what is the
>> solution to vectorize such codes?
>>
>>
>>
>> On Jul 1, 2017 12:30 PM, "hameeza ahmed" <hahmed2305 at gmail.com> wrote:
>>
>>> I even tried polly but still my llvm IR does not contain vector
>>> instructions. i used the following command;
>>>
>>> clang  -S -emit-llvm stencil.c -march=knl -O3 -mllvm -polly -mllvm
>>> -polly-vectorizer=stripmine -o stencil_poly.ll
>>>
>>> Please specify what is wrong with my code?
>>>
>>>
>>> On Sat, Jul 1, 2017 at 4:08 PM, hameeza ahmed <hahmed2305 at gmail.com>
>>> wrote:
>>>
>>>> Hello,
>>>>
>>>> I am trying to vectorize following stencil code;
>>>>
>>>> #include <stdio.h>
>>>> #define N 100351
>>>>
>>>> // This function computes 2D-5 point Jacobi stencil
>>>> void stencil(int a[restrict][N])
>>>> {
>>>>    int i, j, k;
>>>>    for (k = 0; k < 100; k++)
>>>>      {  for (i = 1; i <= N-2; i++)
>>>>          {  for (j = 1; j <= N-2; j++)
>>>>         { a[i][j] = 0.25 * (a[i][j] + a[i-1][j] + a[i+1][j] +
>>>> a[i][j-1] + a[i][j+1]);
>>>>               }
>>>> }
>>>> }}
>>>>
>>>> I have used the following commands
>>>>
>>>> clang  -S -emit-llvm stencil.c -march=knl -O3 -mllvm
>>>> -disable-llvm-optzns -o stencil.ll
>>>>
>>>> opt  -S -O3 stencil.ll -o stencil_o3.ll
>>>>
>>>> llc -x86-asm-syntax=intel stencil_o3.ll -o stencil.s
>>>>
>>>> But the code is not vectorized. It still uses the scalar instructions;
>>>>
>>>>
>>>> Thank You
>>>>
>>>>
>>>>
>>>>
>>>>
>>>
>
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