<div dir="ltr">Hello,<div><br></div><div>The following stencil function that you wrote few months ago (for computing 2D-5 point Jacobi stencil) is vectorizable.</div><div><br></div><div><div>void stencil(int a[][N], int b[N])</div><div>{</div><div> int i, j, k;</div><div> for (k = 0; k < N; k++) {</div><div> for (i = 1; i <= N-2; i++) {</div><div> for (j = 1; j <= N-2; j++)</div><div> b[j] = 0.25 * (a[i][j] + a[i-1][j] + a[i+1][j] + a[i][j-1] + a[i][j+1]);</div><div> for (j = 1; j <= N-2; j++)</div><div> a[i][j] = b[j];</div><div> }</div><div> }</div><div>}</div></div><div><br></div><div>but when i write the above code in main i am getting error </div><div><br></div><div><div>opt -S -O3 stencil.ll -o stencil_o3.ll</div><div>remark: <unknown>:0:0: loop not vectorized: value that could not be identified as reduction is used outside the loop</div></div><div><br></div><div>Why is that so?</div><div><br></div><div>my code is follows:</div><div>int a[N][N], b[N];<br></div><div><div>int main(void)</div><div>{</div><div> int i, j, k;</div><div> for (k = 0; k < N; k++) {</div><div> for (i = 1; i <= N-2; i++) {</div><div> for (j = 1; j <= N-2; j++)</div><div> b[j] = 0.25 * (a[i][j] + a[i-1][j] + a[i+1][j] + a[i][j-1] + a[i][j+1]);</div><div> for (j = 1; j <= N-2; j++)</div><div> a[i][j] = b[j];</div><div> }</div><div> }</div><div>}</div></div><div><br></div><div><br></div><div>Please help.</div><div><br></div><div>Thank You.</div><div><br></div></div><div class="gmail_extra"><br><div class="gmail_quote">On Mon, Jul 3, 2017 at 10:19 AM, Serge Preis <span dir="ltr"><<a href="mailto:spreis@yandex-team.ru" target="_blank">spreis@yandex-team.ru</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hello,<br>
<br>
This is not equivalent rewrite: your original code definitely shouldn't vectorize because it has backward cross-iteration (loop-carried) dependency: your value on iteration j+1 depend on value from iteration j you've just written. In case of vectorization you need to do load-operation-store on multiple consecutive values at once and it is impossible in this case.<br>
<br>
In your new code all values of a[] in right-hand side of the main loop are from previous k iteration (because on current k iteration you're writing to 'b'). So there is no way to vectorize loop in its original form, but new form is definitely vectorizable.<br>
<br>
I am second to recommend you filing a bug over 'restrict' behavior. And you may in fact save some memory by making 'b' 1D array (this is not equivalent rewrite once again though)<br>
<span class=""><br>
// This function computes 2D-5 point Jacobi stencil<br>
</span>void stencil(int a[][N], int b[N])<br>
<span class="">{<br>
int i, j, k;<br>
for (k = 0; k < N; k++) {<br>
for (i = 1; i <= N-2; i++) {<br>
for (j = 1; j <= N-2; j++)<br>
</span> b[j] = 0.25 * (a[i][j] + a[i-1][j] + a[i+1][j] + a[i][j-1] + a[i][j+1]);<br>
<span class=""> for (j = 1; j <= N-2; j++)<br>
</span> a[i][j] = b[j];<br>
}<br>
}<br>
}<br>
<br>
There is a way to vectorize your code even more efficient. This will look like more low-level, but probably closest to what you would expect from vectorization.<br>
<br>
void stencil(int a[][N])<br>
<span class="">{<br>
int i, j, k;<br>
for (k = 0; k < 100; k++) { <br>
for (i = 1; i <= N-2; i++) {<br>
</span> for (j = 1; j <= N-2; j+=16) {<br>
int b[16]; // This should become a register on KNL<br>
for (v = 0; v < 16; v++) {<br>
b[v] = 0.25 * (a[i][j+v] + a[i-1][j+v] + a[i+1][j+v] + a[i][j-1+v] + a[i][j+1+v]);<br>
}<br>
for (v = 0; v < 16; v++) { // This will be a single store operation<br>
a[i][j+v] = b[v];<br>
}<br>
}<br>
// You should explicitly take care about the tail of j-loop<br>
#if !MASKED_SHORT_LOOP_<wbr>VECTORIZATION_SUPPORTED // This is not an actual name, just a designator<br>
for (;j <= N-2; j++) {<br>
<span class=""> a[i][j] = 0.25 * (a[i][j] + a[i-1][j] + a[i+1][j] + a[i][j-1] + a[i][j+1]);<br>
}<br>
</span>#else<br>
for (v = 0; v < 16-(N-2-j); v++) { // This would become masked non-loop<br>
b[v] = 0.25 * (a[i][j+v] + a[i-1][j+v] + a[i+1][j+v] + a[i][j-1+v] + a[i][j+1+v]);<br>
}<br>
for (v = 0; v < 16-(N-2-j); v++) { // This will be a single masked store operation<br>
a[i][j+v] = b[v];<br>
}<br>
#endif<br>
}<br>
}<br>
<br>
Unfortunalely compiler cannot do this for you: this is not equivalent transformation of original code. I am also not aware of any way to express this desired behavior less explicitly (e.g. OpenMP SIMD pragma won't work in this case).<br>
<br>
Minor note: You're using 'int' for data, than multiply by 0.25 (divide by 4) and than write it back to 'int'. This will cost you 2 conversion to/from double while you may just place (...) / 4 which should be optimized to simple sequecnce with shifts (not to single shift due to signedness, but still better than conversions with changes of element size 4->8->4 and data size INT->FP->INT).<br>
<br>
And by the way why do you divide by 4, not by 5 as number of points suggest?<br>
<br>
Serge Preis<br>
<br>
<br>
02.07.2017, 05:11, "hameeza ahmed via llvm-dev" <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>>:<br>
<div><div class="h5">> further i modified the code to the following;<br>
><br>
> #include <stdio.h><br>
> #define N 100351<br>
><br>
> // This function computes 2D-5 point Jacobi stencil<br>
> void stencil(int a[restrict][N], int b[restrict][N])<br>
> {<br>
> int i, j, k;<br>
> for (k = 0; k < N; k++) {<br>
> for (i = 1; i <= N-2; i++)<br>
> for (j = 1; j <= N-2; j++)<br>
> b[i][j] = 0.25 * (a[i][j] + a[i-1][j] + a[i+1][j] + a[i][j-1] + a[i][j+1]);<br>
><br>
> for (i = 1; i <= N-2; i++)<br>
> for (j = 1; j <= N-2; j++)<br>
> a[i][j] = b[i][j];<br>
><br>
> }<br>
> }<br>
><br>
> but still no vectorization in IR. Also, when I set vector width explicitly to 64, it gives the following error:<br>
><br>
> remark: <unknown>:0:0: loop not vectorized: call instruction cannot be vectorized<br>
> remark: <unknown>:0:0: loop not vectorized: value that could not be identified as reduction is used outside the loop<br>
><br>
> I need serious help on this. Please guide me.<br>
><br>
> On Sun, Jul 2, 2017 at 1:54 AM, hameeza ahmed <<a href="mailto:hahmed2305@gmail.com">hahmed2305@gmail.com</a>> wrote:<br>
>> Does it happen due to loop carried dependence? if yes what is the solution to vectorize such codes?<br>
>><br>
>> please reply. i m waiting.<br>
>><br>
>> On Jul 1, 2017 12:30 PM, "hameeza ahmed" <<a href="mailto:hahmed2305@gmail.com">hahmed2305@gmail.com</a>> wrote:<br>
>>> I even tried polly but still my llvm IR does not contain vector instructions. i used the following command;<br>
>>><br>
>>> clang -S -emit-llvm stencil.c -march=knl -O3 -mllvm -polly -mllvm -polly-vectorizer=stripmine -o stencil_poly.ll<br>
>>><br>
>>> Please specify what is wrong with my code?<br>
>>><br>
>>> On Sat, Jul 1, 2017 at 4:08 PM, hameeza ahmed <<a href="mailto:hahmed2305@gmail.com">hahmed2305@gmail.com</a>> wrote:<br>
>>>> Hello,<br>
>>>><br>
>>>> I am trying to vectorize following stencil code;<br>
>>>><br>
>>>> #include <stdio.h><br>
>>>> #define N 100351<br>
>>>><br>
>>>> // This function computes 2D-5 point Jacobi stencil<br>
>>>> void stencil(int a[restrict][N])<br>
>>>> {<br>
>>>> int i, j, k;<br>
>>>> for (k = 0; k < 100; k++)<br>
>>>> { for (i = 1; i <= N-2; i++)<br>
>>>> { for (j = 1; j <= N-2; j++)<br>
>>>> { a[i][j] = 0.25 * (a[i][j] + a[i-1][j] + a[i+1][j] + a[i][j-1] + a[i][j+1]);<br>
>>>> }<br>
>>>> }<br>
>>>> }}<br>
>>>><br>
>>>> I have used the following commands<br>
>>>><br>
>>>> clang -S -emit-llvm stencil.c -march=knl -O3 -mllvm -disable-llvm-optzns -o stencil.ll<br>
>>>><br>
>>>> opt -S -O3 stencil.ll -o stencil_o3.ll<br>
>>>><br>
>>>> llc -x86-asm-syntax=intel stencil_o3.ll -o stencil.s<br>
>>>><br>
>>>> But the code is not vectorized. It still uses the scalar instructions;<br>
>>>><br>
>>>> Please correct me.<br>
>>>><br>
>>>> Thank You<br>
</div></div>> ,<br>
><br>
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</blockquote></div><br></div>