[llvm-dev] Jacobi 5 Point Stencil Code not Vectorizing
hameeza ahmed via llvm-dev
llvm-dev at lists.llvm.org
Tue Oct 24 11:37:34 PDT 2017
Thank you.
But how to resolve it, my work depends on it.
On Tue, Oct 24, 2017 at 6:30 PM, Michael Kruse <llvmdev at meinersbur.de>
wrote:
> Your problem is due to GVN partial reduction elimination (PRE) which
> introduces a PHI node the current loop vectorizer cannot handle:
>
> opt -O3 stencil.ll -pass-remarks=loop-vectorize
> -pass-remarks-missed=loop-vectorize
> -pass-remarks-analysis=loop-vectorize
> remark: <unknown>:0:0: loop not vectorized: value that could not be
> identified as reduction is used outside the loop
> remark: <unknown>:0:0: loop not vectorized
>
> The message is not entirely accurate. The PHI is not used outside of
> the loop, but is in the loop header, therefore cannot be if-converted
> and is not a reduction. Polly also had difficulties with this
> construction as well.
>
> As presented at the dev meeting, VPlan will be able to insert a
> shuffle instruction when encountering this situation.
>
>
> Michael
>
>
>
>
>
> 2017-10-23 8:58 GMT+02:00 hameeza ahmed via llvm-dev <
> llvm-dev at lists.llvm.org>:
> > stencil.ll is attached here.
> >
> > On Mon, Oct 23, 2017 at 11:37 AM, Serge Preis <spreis at yandex-team.ru>
> wrote:
> >>
> >>
> >>
> >> Hello,
> >>
> >> To me this is an issue in llvm loop vectorizer (if N is large enough to
> >> prevent complete unrolling of j-loop).
> >>
> >> Woud you mind to share stencil.ll than I would say more definitely what
> >> the issue is.
> >>
> >> Regards,
> >> Serge.
> >>
> >>
> >> 22.10.2017, 03:21, "hameeza ahmed" <hahmed2305 at gmail.com>:
> >>
> >> Hello,
> >>
> >> The following stencil function that you wrote few months ago (for
> >> computing 2D-5 point Jacobi stencil) is vectorizable.
> >>
> >> 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[j] = 0.25 * (a[i][j] + a[i-1][j] + a[i+1][j] +
> >> a[i][j-1] + a[i][j+1]);
> >> for (j = 1; j <= N-2; j++)
> >> a[i][j] = b[j];
> >> }
> >> }
> >> }
> >>
> >> but when i write the above code in main i am getting error
> >>
> >> opt -S -O3 stencil.ll -o stencil_o3.ll
> >> remark: <unknown>:0:0: loop not vectorized: value that could not be
> >> identified as reduction is used outside the loop
> >>
> >> Why is that so?
> >>
> >> my code is follows:
> >> int a[N][N], b[N];
> >> int main(void)
> >> {
> >> 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[j] = 0.25 * (a[i][j] + a[i-1][j] + a[i+1][j] +
> >> a[i][j-1] + a[i][j+1]);
> >> for (j = 1; j <= N-2; j++)
> >> a[i][j] = b[j];
> >> }
> >> }
> >> }
> >>
> >>
> >> Please help.
> >>
> >> Thank You.
> >>
> >>
> >> On Mon, Jul 3, 2017 at 10:19 AM, Serge Preis <spreis at yandex-team.ru>
> >> wrote:
> >>
> >> Hello,
> >>
> >> 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.
> >>
> >> 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.
> >>
> >> 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)
> >>
> >> // 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[j] = 0.25 * (a[i][j] + a[i-1][j] + a[i+1][j] +
> >> a[i][j-1] + a[i][j+1]);
> >> for (j = 1; j <= N-2; j++)
> >> a[i][j] = b[j];
> >> }
> >> }
> >> }
> >>
> >> 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.
> >>
> >> void stencil(int a[][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+=16) {
> >> int b[16]; // This should become a register on KNL
> >> for (v = 0; v < 16; v++) {
> >> 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]);
> >> }
> >> for (v = 0; v < 16; v++) { // This will be a single
> store
> >> operation
> >> a[i][j+v] = b[v];
> >> }
> >> }
> >> // You should explicitly take care about the tail of j-loop
> >> #if !MASKED_SHORT_LOOP_VECTORIZATION_SUPPORTED // This is not an
> actual
> >> name, just a designator
> >> for (;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]);
> >> }
> >> #else
> >> for (v = 0; v < 16-(N-2-j); v++) { // This would become
> masked
> >> non-loop
> >> 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]);
> >> }
> >> for (v = 0; v < 16-(N-2-j); v++) { // This will be a single
> >> masked store operation
> >> a[i][j+v] = b[v];
> >> }
> >> #endif
> >> }
> >> }
> >>
> >> 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).
> >>
> >> 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).
> >>
> >> And by the way why do you divide by 4, not by 5 as number of points
> >> suggest?
> >>
> >> Serge Preis
> >>
> >>
> >> 02.07.2017, 05:11, "hameeza ahmed via llvm-dev" <
> llvm-dev at lists.llvm.org>:
> >> > 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?
> >> >>
> >> >> please reply. i m waiting.
> >> >>
> >> >> 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;
> >> >>>>
> >> >>>> Please correct me.
> >> >>>>
> >> >>>> Thank You
> >> > ,
> >> >
> >> > _______________________________________________
> >> > LLVM Developers mailing list
> >> > llvm-dev at lists.llvm.org
> >> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
>
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