[petsc-users] Is this a bug? MatMultAdd_SeqBAIJ_11

[Lawrence Mitchell]

Hi Daniel,

> On 21 Sep 2021, at 11:19, Daniel Stone <daniel.stone at opengosim.com> wrote:
> 
> Hello,
> 
> If we look at lines 2330-2331 in file baij2.c, it looks like there are some 
> mistakes in assigning the `sum..` variables to the z array, causing
> the function MatMultAdd_SeqBAIJ_11() to not produce the correct
> answer.
> 
> I don't have a good example program to demonstrate this yet - it's
> currently causing problems in a dev branch of pflotan_ogs that
> can produce blocksize 11 matrices. When in parallel, a standard
> matrix-vector multiplication calls MatMultAdd for the off-proc
> contributions, and the result is wrong when this is redirected
> to MatMultAdd_SeqBAIJ_11. Seems to be the root cause of
> several solvers failing such as fgmres.
> 
> Can anyone confirm that these two lines seem incorrect?

Looks wrong to me, I guess this patch is correct?

diff --git a/src/mat/impls/baij/seq/baij2.c b/src/mat/impls/baij/seq/baij2.c
index 2849ef9051..65513c8989 100644
--- a/src/mat/impls/baij/seq/baij2.c
+++ b/src/mat/impls/baij/seq/baij2.c
@@ -2328,7 +2328,7 @@ PetscErrorCode MatMultAdd_SeqBAIJ_11(Mat A,Vec xx,Vec yy,Vec zz)
       v    += 121;
     }
     z[0] = sum1; z[1] = sum2; z[2] = sum3; z[3] = sum4; z[4] = sum5; z[5] = sum6; z[6] = sum7;
-    z[7] = sum6; z[8] = sum7; z[9] = sum8; z[10] = sum9; z[11] = sum10;
+    z[7] = sum8; z[8] = sum9; z[9] = sum10; z[10] = sum11;
     if (!usecprow) {
       z += 11; y += 11;
     }


Lawrence