xref: /petsc/src/mat/impls/baij/seq/baijfact7.c (revision 48a46eb9bd028bec07ec0f396b1a3abb43f14558)

/*
    Factorization code for BAIJ format.
*/
#include <../src/mat/impls/baij/seq/baij.h>
#include <petsc/private/kernels/blockinvert.h>

/* ------------------------------------------------------------*/
/*
      Version for when blocks are 6 by 6
*/
PetscErrorCode MatLUFactorNumeric_SeqBAIJ_6_inplace(Mat C, Mat A, const MatFactorInfo *info) {
  Mat_SeqBAIJ    *a = (Mat_SeqBAIJ *)A->data, *b = (Mat_SeqBAIJ *)C->data;
  IS              isrow = b->row, isicol = b->icol;
  const PetscInt *ajtmpold, *ajtmp, *diag_offset = b->diag, *r, *ic, *bi = b->i, *bj = b->j, *ai = a->i, *aj = a->j, *pj;
  PetscInt        nz, row, i, j, n = a->mbs, idx;
  MatScalar      *pv, *v, *rtmp, *pc, *w, *x;
  MatScalar       p1, p2, p3, p4, m1, m2, m3, m4, m5, m6, m7, m8, m9, x1, x2, x3, x4;
  MatScalar       p5, p6, p7, p8, p9, x5, x6, x7, x8, x9, x10, x11, x12, x13, x14, x15, x16;
  MatScalar       x17, x18, x19, x20, x21, x22, x23, x24, x25, p10, p11, p12, p13, p14;
  MatScalar       p15, p16, p17, p18, p19, p20, p21, p22, p23, p24, p25, m10, m11, m12;
  MatScalar       m13, m14, m15, m16, m17, m18, m19, m20, m21, m22, m23, m24, m25;
  MatScalar       p26, p27, p28, p29, p30, p31, p32, p33, p34, p35, p36;
  MatScalar       x26, x27, x28, x29, x30, x31, x32, x33, x34, x35, x36;
  MatScalar       m26, m27, m28, m29, m30, m31, m32, m33, m34, m35, m36;
  MatScalar      *ba = b->a, *aa = a->a;
  PetscReal       shift = info->shiftamount;
  PetscBool       allowzeropivot, zeropivotdetected;

  PetscFunctionBegin;
  allowzeropivot = PetscNot(A->erroriffailure);
  PetscCall(ISGetIndices(isrow, &r));
  PetscCall(ISGetIndices(isicol, &ic));
  PetscCall(PetscMalloc1(36 * (n + 1), &rtmp));

  for (i = 0; i < n; i++) {
    nz    = bi[i + 1] - bi[i];
    ajtmp = bj + bi[i];
    for (j = 0; j < nz; j++) {
      x    = rtmp + 36 * ajtmp[j];
      x[0] = x[1] = x[2] = x[3] = x[4] = x[5] = x[6] = x[7] = x[8] = x[9] = 0.0;
      x[10] = x[11] = x[12] = x[13] = x[14] = x[15] = x[16] = x[17] = 0.0;
      x[18] = x[19] = x[20] = x[21] = x[22] = x[23] = x[24] = x[25] = 0.0;
      x[26] = x[27] = x[28] = x[29] = x[30] = x[31] = x[32] = x[33] = 0.0;
      x[34] = x[35] = 0.0;
    }
    /* load in initial (unfactored row) */
    idx      = r[i];
    nz       = ai[idx + 1] - ai[idx];
    ajtmpold = aj + ai[idx];
    v        = aa + 36 * ai[idx];
    for (j = 0; j < nz; j++) {
      x     = rtmp + 36 * ic[ajtmpold[j]];
      x[0]  = v[0];
      x[1]  = v[1];
      x[2]  = v[2];
      x[3]  = v[3];
      x[4]  = v[4];
      x[5]  = v[5];
      x[6]  = v[6];
      x[7]  = v[7];
      x[8]  = v[8];
      x[9]  = v[9];
      x[10] = v[10];
      x[11] = v[11];
      x[12] = v[12];
      x[13] = v[13];
      x[14] = v[14];
      x[15] = v[15];
      x[16] = v[16];
      x[17] = v[17];
      x[18] = v[18];
      x[19] = v[19];
      x[20] = v[20];
      x[21] = v[21];
      x[22] = v[22];
      x[23] = v[23];
      x[24] = v[24];
      x[25] = v[25];
      x[26] = v[26];
      x[27] = v[27];
      x[28] = v[28];
      x[29] = v[29];
      x[30] = v[30];
      x[31] = v[31];
      x[32] = v[32];
      x[33] = v[33];
      x[34] = v[34];
      x[35] = v[35];
      v += 36;
    }
    row = *ajtmp++;
    while (row < i) {
      pc  = rtmp + 36 * row;
      p1  = pc[0];
      p2  = pc[1];
      p3  = pc[2];
      p4  = pc[3];
      p5  = pc[4];
      p6  = pc[5];
      p7  = pc[6];
      p8  = pc[7];
      p9  = pc[8];
      p10 = pc[9];
      p11 = pc[10];
      p12 = pc[11];
      p13 = pc[12];
      p14 = pc[13];
      p15 = pc[14];
      p16 = pc[15];
      p17 = pc[16];
      p18 = pc[17];
      p19 = pc[18];
      p20 = pc[19];
      p21 = pc[20];
      p22 = pc[21];
      p23 = pc[22];
      p24 = pc[23];
      p25 = pc[24];
      p26 = pc[25];
      p27 = pc[26];
      p28 = pc[27];
      p29 = pc[28];
      p30 = pc[29];
      p31 = pc[30];
      p32 = pc[31];
      p33 = pc[32];
      p34 = pc[33];
      p35 = pc[34];
      p36 = pc[35];
      if (p1 != 0.0 || p2 != 0.0 || p3 != 0.0 || p4 != 0.0 || p5 != 0.0 || p6 != 0.0 || p7 != 0.0 || p8 != 0.0 || p9 != 0.0 || p10 != 0.0 || p11 != 0.0 || p12 != 0.0 || p13 != 0.0 || p14 != 0.0 || p15 != 0.0 || p16 != 0.0 || p17 != 0.0 || p18 != 0.0 || p19 != 0.0 || p20 != 0.0 || p21 != 0.0 || p22 != 0.0 || p23 != 0.0 || p24 != 0.0 || p25 != 0.0 || p26 != 0.0 || p27 != 0.0 || p28 != 0.0 || p29 != 0.0 || p30 != 0.0 || p31 != 0.0 || p32 != 0.0 || p33 != 0.0 || p34 != 0.0 || p35 != 0.0 || p36 != 0.0) {
        pv    = ba + 36 * diag_offset[row];
        pj    = bj + diag_offset[row] + 1;
        x1    = pv[0];
        x2    = pv[1];
        x3    = pv[2];
        x4    = pv[3];
        x5    = pv[4];
        x6    = pv[5];
        x7    = pv[6];
        x8    = pv[7];
        x9    = pv[8];
        x10   = pv[9];
        x11   = pv[10];
        x12   = pv[11];
        x13   = pv[12];
        x14   = pv[13];
        x15   = pv[14];
        x16   = pv[15];
        x17   = pv[16];
        x18   = pv[17];
        x19   = pv[18];
        x20   = pv[19];
        x21   = pv[20];
        x22   = pv[21];
        x23   = pv[22];
        x24   = pv[23];
        x25   = pv[24];
        x26   = pv[25];
        x27   = pv[26];
        x28   = pv[27];
        x29   = pv[28];
        x30   = pv[29];
        x31   = pv[30];
        x32   = pv[31];
        x33   = pv[32];
        x34   = pv[33];
        x35   = pv[34];
        x36   = pv[35];
        pc[0] = m1 = p1 * x1 + p7 * x2 + p13 * x3 + p19 * x4 + p25 * x5 + p31 * x6;
        pc[1] = m2 = p2 * x1 + p8 * x2 + p14 * x3 + p20 * x4 + p26 * x5 + p32 * x6;
        pc[2] = m3 = p3 * x1 + p9 * x2 + p15 * x3 + p21 * x4 + p27 * x5 + p33 * x6;
        pc[3] = m4 = p4 * x1 + p10 * x2 + p16 * x3 + p22 * x4 + p28 * x5 + p34 * x6;
        pc[4] = m5 = p5 * x1 + p11 * x2 + p17 * x3 + p23 * x4 + p29 * x5 + p35 * x6;
        pc[5] = m6 = p6 * x1 + p12 * x2 + p18 * x3 + p24 * x4 + p30 * x5 + p36 * x6;

        pc[6] = m7 = p1 * x7 + p7 * x8 + p13 * x9 + p19 * x10 + p25 * x11 + p31 * x12;
        pc[7] = m8 = p2 * x7 + p8 * x8 + p14 * x9 + p20 * x10 + p26 * x11 + p32 * x12;
        pc[8] = m9 = p3 * x7 + p9 * x8 + p15 * x9 + p21 * x10 + p27 * x11 + p33 * x12;
        pc[9] = m10 = p4 * x7 + p10 * x8 + p16 * x9 + p22 * x10 + p28 * x11 + p34 * x12;
        pc[10] = m11 = p5 * x7 + p11 * x8 + p17 * x9 + p23 * x10 + p29 * x11 + p35 * x12;
        pc[11] = m12 = p6 * x7 + p12 * x8 + p18 * x9 + p24 * x10 + p30 * x11 + p36 * x12;

        pc[12] = m13 = p1 * x13 + p7 * x14 + p13 * x15 + p19 * x16 + p25 * x17 + p31 * x18;
        pc[13] = m14 = p2 * x13 + p8 * x14 + p14 * x15 + p20 * x16 + p26 * x17 + p32 * x18;
        pc[14] = m15 = p3 * x13 + p9 * x14 + p15 * x15 + p21 * x16 + p27 * x17 + p33 * x18;
        pc[15] = m16 = p4 * x13 + p10 * x14 + p16 * x15 + p22 * x16 + p28 * x17 + p34 * x18;
        pc[16] = m17 = p5 * x13 + p11 * x14 + p17 * x15 + p23 * x16 + p29 * x17 + p35 * x18;
        pc[17] = m18 = p6 * x13 + p12 * x14 + p18 * x15 + p24 * x16 + p30 * x17 + p36 * x18;

        pc[18] = m19 = p1 * x19 + p7 * x20 + p13 * x21 + p19 * x22 + p25 * x23 + p31 * x24;
        pc[19] = m20 = p2 * x19 + p8 * x20 + p14 * x21 + p20 * x22 + p26 * x23 + p32 * x24;
        pc[20] = m21 = p3 * x19 + p9 * x20 + p15 * x21 + p21 * x22 + p27 * x23 + p33 * x24;
        pc[21] = m22 = p4 * x19 + p10 * x20 + p16 * x21 + p22 * x22 + p28 * x23 + p34 * x24;
        pc[22] = m23 = p5 * x19 + p11 * x20 + p17 * x21 + p23 * x22 + p29 * x23 + p35 * x24;
        pc[23] = m24 = p6 * x19 + p12 * x20 + p18 * x21 + p24 * x22 + p30 * x23 + p36 * x24;

        pc[24] = m25 = p1 * x25 + p7 * x26 + p13 * x27 + p19 * x28 + p25 * x29 + p31 * x30;
        pc[25] = m26 = p2 * x25 + p8 * x26 + p14 * x27 + p20 * x28 + p26 * x29 + p32 * x30;
        pc[26] = m27 = p3 * x25 + p9 * x26 + p15 * x27 + p21 * x28 + p27 * x29 + p33 * x30;
        pc[27] = m28 = p4 * x25 + p10 * x26 + p16 * x27 + p22 * x28 + p28 * x29 + p34 * x30;
        pc[28] = m29 = p5 * x25 + p11 * x26 + p17 * x27 + p23 * x28 + p29 * x29 + p35 * x30;
        pc[29] = m30 = p6 * x25 + p12 * x26 + p18 * x27 + p24 * x28 + p30 * x29 + p36 * x30;

        pc[30] = m31 = p1 * x31 + p7 * x32 + p13 * x33 + p19 * x34 + p25 * x35 + p31 * x36;
        pc[31] = m32 = p2 * x31 + p8 * x32 + p14 * x33 + p20 * x34 + p26 * x35 + p32 * x36;
        pc[32] = m33 = p3 * x31 + p9 * x32 + p15 * x33 + p21 * x34 + p27 * x35 + p33 * x36;
        pc[33] = m34 = p4 * x31 + p10 * x32 + p16 * x33 + p22 * x34 + p28 * x35 + p34 * x36;
        pc[34] = m35 = p5 * x31 + p11 * x32 + p17 * x33 + p23 * x34 + p29 * x35 + p35 * x36;
        pc[35] = m36 = p6 * x31 + p12 * x32 + p18 * x33 + p24 * x34 + p30 * x35 + p36 * x36;

        nz = bi[row + 1] - diag_offset[row] - 1;
        pv += 36;
        for (j = 0; j < nz; j++) {
          x1  = pv[0];
          x2  = pv[1];
          x3  = pv[2];
          x4  = pv[3];
          x5  = pv[4];
          x6  = pv[5];
          x7  = pv[6];
          x8  = pv[7];
          x9  = pv[8];
          x10 = pv[9];
          x11 = pv[10];
          x12 = pv[11];
          x13 = pv[12];
          x14 = pv[13];
          x15 = pv[14];
          x16 = pv[15];
          x17 = pv[16];
          x18 = pv[17];
          x19 = pv[18];
          x20 = pv[19];
          x21 = pv[20];
          x22 = pv[21];
          x23 = pv[22];
          x24 = pv[23];
          x25 = pv[24];
          x26 = pv[25];
          x27 = pv[26];
          x28 = pv[27];
          x29 = pv[28];
          x30 = pv[29];
          x31 = pv[30];
          x32 = pv[31];
          x33 = pv[32];
          x34 = pv[33];
          x35 = pv[34];
          x36 = pv[35];
          x   = rtmp + 36 * pj[j];
          x[0] -= m1 * x1 + m7 * x2 + m13 * x3 + m19 * x4 + m25 * x5 + m31 * x6;
          x[1] -= m2 * x1 + m8 * x2 + m14 * x3 + m20 * x4 + m26 * x5 + m32 * x6;
          x[2] -= m3 * x1 + m9 * x2 + m15 * x3 + m21 * x4 + m27 * x5 + m33 * x6;
          x[3] -= m4 * x1 + m10 * x2 + m16 * x3 + m22 * x4 + m28 * x5 + m34 * x6;
          x[4] -= m5 * x1 + m11 * x2 + m17 * x3 + m23 * x4 + m29 * x5 + m35 * x6;
          x[5] -= m6 * x1 + m12 * x2 + m18 * x3 + m24 * x4 + m30 * x5 + m36 * x6;

          x[6] -= m1 * x7 + m7 * x8 + m13 * x9 + m19 * x10 + m25 * x11 + m31 * x12;
          x[7] -= m2 * x7 + m8 * x8 + m14 * x9 + m20 * x10 + m26 * x11 + m32 * x12;
          x[8] -= m3 * x7 + m9 * x8 + m15 * x9 + m21 * x10 + m27 * x11 + m33 * x12;
          x[9] -= m4 * x7 + m10 * x8 + m16 * x9 + m22 * x10 + m28 * x11 + m34 * x12;
          x[10] -= m5 * x7 + m11 * x8 + m17 * x9 + m23 * x10 + m29 * x11 + m35 * x12;
          x[11] -= m6 * x7 + m12 * x8 + m18 * x9 + m24 * x10 + m30 * x11 + m36 * x12;

          x[12] -= m1 * x13 + m7 * x14 + m13 * x15 + m19 * x16 + m25 * x17 + m31 * x18;
          x[13] -= m2 * x13 + m8 * x14 + m14 * x15 + m20 * x16 + m26 * x17 + m32 * x18;
          x[14] -= m3 * x13 + m9 * x14 + m15 * x15 + m21 * x16 + m27 * x17 + m33 * x18;
          x[15] -= m4 * x13 + m10 * x14 + m16 * x15 + m22 * x16 + m28 * x17 + m34 * x18;
          x[16] -= m5 * x13 + m11 * x14 + m17 * x15 + m23 * x16 + m29 * x17 + m35 * x18;
          x[17] -= m6 * x13 + m12 * x14 + m18 * x15 + m24 * x16 + m30 * x17 + m36 * x18;

          x[18] -= m1 * x19 + m7 * x20 + m13 * x21 + m19 * x22 + m25 * x23 + m31 * x24;
          x[19] -= m2 * x19 + m8 * x20 + m14 * x21 + m20 * x22 + m26 * x23 + m32 * x24;
          x[20] -= m3 * x19 + m9 * x20 + m15 * x21 + m21 * x22 + m27 * x23 + m33 * x24;
          x[21] -= m4 * x19 + m10 * x20 + m16 * x21 + m22 * x22 + m28 * x23 + m34 * x24;
          x[22] -= m5 * x19 + m11 * x20 + m17 * x21 + m23 * x22 + m29 * x23 + m35 * x24;
          x[23] -= m6 * x19 + m12 * x20 + m18 * x21 + m24 * x22 + m30 * x23 + m36 * x24;

          x[24] -= m1 * x25 + m7 * x26 + m13 * x27 + m19 * x28 + m25 * x29 + m31 * x30;
          x[25] -= m2 * x25 + m8 * x26 + m14 * x27 + m20 * x28 + m26 * x29 + m32 * x30;
          x[26] -= m3 * x25 + m9 * x26 + m15 * x27 + m21 * x28 + m27 * x29 + m33 * x30;
          x[27] -= m4 * x25 + m10 * x26 + m16 * x27 + m22 * x28 + m28 * x29 + m34 * x30;
          x[28] -= m5 * x25 + m11 * x26 + m17 * x27 + m23 * x28 + m29 * x29 + m35 * x30;
          x[29] -= m6 * x25 + m12 * x26 + m18 * x27 + m24 * x28 + m30 * x29 + m36 * x30;

          x[30] -= m1 * x31 + m7 * x32 + m13 * x33 + m19 * x34 + m25 * x35 + m31 * x36;
          x[31] -= m2 * x31 + m8 * x32 + m14 * x33 + m20 * x34 + m26 * x35 + m32 * x36;
          x[32] -= m3 * x31 + m9 * x32 + m15 * x33 + m21 * x34 + m27 * x35 + m33 * x36;
          x[33] -= m4 * x31 + m10 * x32 + m16 * x33 + m22 * x34 + m28 * x35 + m34 * x36;
          x[34] -= m5 * x31 + m11 * x32 + m17 * x33 + m23 * x34 + m29 * x35 + m35 * x36;
          x[35] -= m6 * x31 + m12 * x32 + m18 * x33 + m24 * x34 + m30 * x35 + m36 * x36;

          pv += 36;
        }
        PetscCall(PetscLogFlops(432.0 * nz + 396.0));
      }
      row = *ajtmp++;
    }
    /* finished row so stick it into b->a */
    pv = ba + 36 * bi[i];
    pj = bj + bi[i];
    nz = bi[i + 1] - bi[i];
    for (j = 0; j < nz; j++) {
      x      = rtmp + 36 * pj[j];
      pv[0]  = x[0];
      pv[1]  = x[1];
      pv[2]  = x[2];
      pv[3]  = x[3];
      pv[4]  = x[4];
      pv[5]  = x[5];
      pv[6]  = x[6];
      pv[7]  = x[7];
      pv[8]  = x[8];
      pv[9]  = x[9];
      pv[10] = x[10];
      pv[11] = x[11];
      pv[12] = x[12];
      pv[13] = x[13];
      pv[14] = x[14];
      pv[15] = x[15];
      pv[16] = x[16];
      pv[17] = x[17];
      pv[18] = x[18];
      pv[19] = x[19];
      pv[20] = x[20];
      pv[21] = x[21];
      pv[22] = x[22];
      pv[23] = x[23];
      pv[24] = x[24];
      pv[25] = x[25];
      pv[26] = x[26];
      pv[27] = x[27];
      pv[28] = x[28];
      pv[29] = x[29];
      pv[30] = x[30];
      pv[31] = x[31];
      pv[32] = x[32];
      pv[33] = x[33];
      pv[34] = x[34];
      pv[35] = x[35];
      pv += 36;
    }
    /* invert diagonal block */
    w = ba + 36 * diag_offset[i];
    PetscCall(PetscKernel_A_gets_inverse_A_6(w, shift, allowzeropivot, &zeropivotdetected));
    if (zeropivotdetected) C->factorerrortype = MAT_FACTOR_NUMERIC_ZEROPIVOT;
  }

  PetscCall(PetscFree(rtmp));
  PetscCall(ISRestoreIndices(isicol, &ic));
  PetscCall(ISRestoreIndices(isrow, &r));

  C->ops->solve          = MatSolve_SeqBAIJ_6_inplace;
  C->ops->solvetranspose = MatSolveTranspose_SeqBAIJ_6_inplace;
  C->assembled           = PETSC_TRUE;

  PetscCall(PetscLogFlops(1.333333333333 * 6 * 6 * 6 * b->mbs)); /* from inverting diagonal blocks */
  PetscFunctionReturn(0);
}

PetscErrorCode MatLUFactorNumeric_SeqBAIJ_6(Mat B, Mat A, const MatFactorInfo *info) {
  Mat             C = B;
  Mat_SeqBAIJ    *a = (Mat_SeqBAIJ *)A->data, *b = (Mat_SeqBAIJ *)C->data;
  IS              isrow = b->row, isicol = b->icol;
  const PetscInt *r, *ic;
  PetscInt        i, j, k, nz, nzL, row;
  const PetscInt  n = a->mbs, *ai = a->i, *aj = a->j, *bi = b->i, *bj = b->j;
  const PetscInt *ajtmp, *bjtmp, *bdiag = b->diag, *pj, bs2 = a->bs2;
  MatScalar      *rtmp, *pc, *mwork, *v, *pv, *aa = a->a;
  PetscInt        flg;
  PetscReal       shift = info->shiftamount;
  PetscBool       allowzeropivot, zeropivotdetected;

  PetscFunctionBegin;
  allowzeropivot = PetscNot(A->erroriffailure);
  PetscCall(ISGetIndices(isrow, &r));
  PetscCall(ISGetIndices(isicol, &ic));

  /* generate work space needed by the factorization */
  PetscCall(PetscMalloc2(bs2 * n, &rtmp, bs2, &mwork));
  PetscCall(PetscArrayzero(rtmp, bs2 * n));

  for (i = 0; i < n; i++) {
    /* zero rtmp */
    /* L part */
    nz    = bi[i + 1] - bi[i];
    bjtmp = bj + bi[i];
    for (j = 0; j < nz; j++) PetscCall(PetscArrayzero(rtmp + bs2 * bjtmp[j], bs2));

    /* U part */
    nz    = bdiag[i] - bdiag[i + 1];
    bjtmp = bj + bdiag[i + 1] + 1;
    for (j = 0; j < nz; j++) PetscCall(PetscArrayzero(rtmp + bs2 * bjtmp[j], bs2));

    /* load in initial (unfactored row) */
    nz    = ai[r[i] + 1] - ai[r[i]];
    ajtmp = aj + ai[r[i]];
    v     = aa + bs2 * ai[r[i]];
    for (j = 0; j < nz; j++) PetscCall(PetscArraycpy(rtmp + bs2 * ic[ajtmp[j]], v + bs2 * j, bs2));

    /* elimination */
    bjtmp = bj + bi[i];
    nzL   = bi[i + 1] - bi[i];
    for (k = 0; k < nzL; k++) {
      row = bjtmp[k];
      pc  = rtmp + bs2 * row;
      for (flg = 0, j = 0; j < bs2; j++) {
        if (pc[j] != 0.0) {
          flg = 1;
          break;
        }
      }
      if (flg) {
        pv = b->a + bs2 * bdiag[row];
        /* PetscKernel_A_gets_A_times_B(bs,pc,pv,mwork); *pc = *pc * (*pv); */
        PetscCall(PetscKernel_A_gets_A_times_B_6(pc, pv, mwork));

        pj = b->j + bdiag[row + 1] + 1; /* beginning of U(row,:) */
        pv = b->a + bs2 * (bdiag[row + 1] + 1);
        nz = bdiag[row] - bdiag[row + 1] - 1; /* num of entries inU(row,:), excluding diag */
        for (j = 0; j < nz; j++) {
          /* PetscKernel_A_gets_A_minus_B_times_C(bs,rtmp+bs2*pj[j],pc,pv+bs2*j); */
          /* rtmp+bs2*pj[j] = rtmp+bs2*pj[j] - (*pc)*(pv+bs2*j) */
          v = rtmp + bs2 * pj[j];
          PetscCall(PetscKernel_A_gets_A_minus_B_times_C_6(v, pc, pv));
          pv += bs2;
        }
        PetscCall(PetscLogFlops(432.0 * nz + 396)); /* flops = 2*bs^3*nz + 2*bs^3 - bs2) */
      }
    }

    /* finished row so stick it into b->a */
    /* L part */
    pv = b->a + bs2 * bi[i];
    pj = b->j + bi[i];
    nz = bi[i + 1] - bi[i];
    for (j = 0; j < nz; j++) PetscCall(PetscArraycpy(pv + bs2 * j, rtmp + bs2 * pj[j], bs2));

    /* Mark diagonal and invert diagonal for simpler triangular solves */
    pv = b->a + bs2 * bdiag[i];
    pj = b->j + bdiag[i];
    PetscCall(PetscArraycpy(pv, rtmp + bs2 * pj[0], bs2));
    PetscCall(PetscKernel_A_gets_inverse_A_6(pv, shift, allowzeropivot, &zeropivotdetected));
    if (zeropivotdetected) C->factorerrortype = MAT_FACTOR_NUMERIC_ZEROPIVOT;

    /* U part */
    pv = b->a + bs2 * (bdiag[i + 1] + 1);
    pj = b->j + bdiag[i + 1] + 1;
    nz = bdiag[i] - bdiag[i + 1] - 1;
    for (j = 0; j < nz; j++) PetscCall(PetscArraycpy(pv + bs2 * j, rtmp + bs2 * pj[j], bs2));
  }

  PetscCall(PetscFree2(rtmp, mwork));
  PetscCall(ISRestoreIndices(isicol, &ic));
  PetscCall(ISRestoreIndices(isrow, &r));

  C->ops->solve          = MatSolve_SeqBAIJ_6;
  C->ops->solvetranspose = MatSolveTranspose_SeqBAIJ_6;
  C->assembled           = PETSC_TRUE;

  PetscCall(PetscLogFlops(1.333333333333 * 6 * 6 * 6 * n)); /* from inverting diagonal blocks */
  PetscFunctionReturn(0);
}

PetscErrorCode MatLUFactorNumeric_SeqBAIJ_6_NaturalOrdering_inplace(Mat C, Mat A, const MatFactorInfo *info) {
  Mat_SeqBAIJ *a = (Mat_SeqBAIJ *)A->data, *b = (Mat_SeqBAIJ *)C->data;
  PetscInt     i, j, n = a->mbs, *bi = b->i, *bj = b->j;
  PetscInt    *ajtmpold, *ajtmp, nz, row;
  PetscInt    *diag_offset = b->diag, *ai = a->i, *aj = a->j, *pj;
  MatScalar   *pv, *v, *rtmp, *pc, *w, *x;
  MatScalar    x1, x2, x3, x4, x5, x6, x7, x8, x9, x10, x11, x12, x13, x14, x15;
  MatScalar    x16, x17, x18, x19, x20, x21, x22, x23, x24, x25;
  MatScalar    p1, p2, p3, p4, p5, p6, p7, p8, p9, p10, p11, p12, p13, p14, p15;
  MatScalar    p16, p17, p18, p19, p20, p21, p22, p23, p24, p25;
  MatScalar    m1, m2, m3, m4, m5, m6, m7, m8, m9, m10, m11, m12, m13, m14, m15;
  MatScalar    m16, m17, m18, m19, m20, m21, m22, m23, m24, m25;
  MatScalar    p26, p27, p28, p29, p30, p31, p32, p33, p34, p35, p36;
  MatScalar    x26, x27, x28, x29, x30, x31, x32, x33, x34, x35, x36;
  MatScalar    m26, m27, m28, m29, m30, m31, m32, m33, m34, m35, m36;
  MatScalar   *ba = b->a, *aa = a->a;
  PetscReal    shift = info->shiftamount;
  PetscBool    allowzeropivot, zeropivotdetected;

  PetscFunctionBegin;
  allowzeropivot = PetscNot(A->erroriffailure);
  PetscCall(PetscMalloc1(36 * (n + 1), &rtmp));
  for (i = 0; i < n; i++) {
    nz    = bi[i + 1] - bi[i];
    ajtmp = bj + bi[i];
    for (j = 0; j < nz; j++) {
      x    = rtmp + 36 * ajtmp[j];
      x[0] = x[1] = x[2] = x[3] = x[4] = x[5] = x[6] = x[7] = x[8] = x[9] = 0.0;
      x[10] = x[11] = x[12] = x[13] = x[14] = x[15] = x[16] = x[17] = 0.0;
      x[18] = x[19] = x[20] = x[21] = x[22] = x[23] = x[24] = x[25] = 0.0;
      x[26] = x[27] = x[28] = x[29] = x[30] = x[31] = x[32] = x[33] = 0.0;
      x[34] = x[35] = 0.0;
    }
    /* load in initial (unfactored row) */
    nz       = ai[i + 1] - ai[i];
    ajtmpold = aj + ai[i];
    v        = aa + 36 * ai[i];
    for (j = 0; j < nz; j++) {
      x     = rtmp + 36 * ajtmpold[j];
      x[0]  = v[0];
      x[1]  = v[1];
      x[2]  = v[2];
      x[3]  = v[3];
      x[4]  = v[4];
      x[5]  = v[5];
      x[6]  = v[6];
      x[7]  = v[7];
      x[8]  = v[8];
      x[9]  = v[9];
      x[10] = v[10];
      x[11] = v[11];
      x[12] = v[12];
      x[13] = v[13];
      x[14] = v[14];
      x[15] = v[15];
      x[16] = v[16];
      x[17] = v[17];
      x[18] = v[18];
      x[19] = v[19];
      x[20] = v[20];
      x[21] = v[21];
      x[22] = v[22];
      x[23] = v[23];
      x[24] = v[24];
      x[25] = v[25];
      x[26] = v[26];
      x[27] = v[27];
      x[28] = v[28];
      x[29] = v[29];
      x[30] = v[30];
      x[31] = v[31];
      x[32] = v[32];
      x[33] = v[33];
      x[34] = v[34];
      x[35] = v[35];
      v += 36;
    }
    row = *ajtmp++;
    while (row < i) {
      pc  = rtmp + 36 * row;
      p1  = pc[0];
      p2  = pc[1];
      p3  = pc[2];
      p4  = pc[3];
      p5  = pc[4];
      p6  = pc[5];
      p7  = pc[6];
      p8  = pc[7];
      p9  = pc[8];
      p10 = pc[9];
      p11 = pc[10];
      p12 = pc[11];
      p13 = pc[12];
      p14 = pc[13];
      p15 = pc[14];
      p16 = pc[15];
      p17 = pc[16];
      p18 = pc[17];
      p19 = pc[18];
      p20 = pc[19];
      p21 = pc[20];
      p22 = pc[21];
      p23 = pc[22];
      p24 = pc[23];
      p25 = pc[24];
      p26 = pc[25];
      p27 = pc[26];
      p28 = pc[27];
      p29 = pc[28];
      p30 = pc[29];
      p31 = pc[30];
      p32 = pc[31];
      p33 = pc[32];
      p34 = pc[33];
      p35 = pc[34];
      p36 = pc[35];
      if (p1 != 0.0 || p2 != 0.0 || p3 != 0.0 || p4 != 0.0 || p5 != 0.0 || p6 != 0.0 || p7 != 0.0 || p8 != 0.0 || p9 != 0.0 || p10 != 0.0 || p11 != 0.0 || p12 != 0.0 || p13 != 0.0 || p14 != 0.0 || p15 != 0.0 || p16 != 0.0 || p17 != 0.0 || p18 != 0.0 || p19 != 0.0 || p20 != 0.0 || p21 != 0.0 || p22 != 0.0 || p23 != 0.0 || p24 != 0.0 || p25 != 0.0 || p26 != 0.0 || p27 != 0.0 || p28 != 0.0 || p29 != 0.0 || p30 != 0.0 || p31 != 0.0 || p32 != 0.0 || p33 != 0.0 || p34 != 0.0 || p35 != 0.0 || p36 != 0.0) {
        pv    = ba + 36 * diag_offset[row];
        pj    = bj + diag_offset[row] + 1;
        x1    = pv[0];
        x2    = pv[1];
        x3    = pv[2];
        x4    = pv[3];
        x5    = pv[4];
        x6    = pv[5];
        x7    = pv[6];
        x8    = pv[7];
        x9    = pv[8];
        x10   = pv[9];
        x11   = pv[10];
        x12   = pv[11];
        x13   = pv[12];
        x14   = pv[13];
        x15   = pv[14];
        x16   = pv[15];
        x17   = pv[16];
        x18   = pv[17];
        x19   = pv[18];
        x20   = pv[19];
        x21   = pv[20];
        x22   = pv[21];
        x23   = pv[22];
        x24   = pv[23];
        x25   = pv[24];
        x26   = pv[25];
        x27   = pv[26];
        x28   = pv[27];
        x29   = pv[28];
        x30   = pv[29];
        x31   = pv[30];
        x32   = pv[31];
        x33   = pv[32];
        x34   = pv[33];
        x35   = pv[34];
        x36   = pv[35];
        pc[0] = m1 = p1 * x1 + p7 * x2 + p13 * x3 + p19 * x4 + p25 * x5 + p31 * x6;
        pc[1] = m2 = p2 * x1 + p8 * x2 + p14 * x3 + p20 * x4 + p26 * x5 + p32 * x6;
        pc[2] = m3 = p3 * x1 + p9 * x2 + p15 * x3 + p21 * x4 + p27 * x5 + p33 * x6;
        pc[3] = m4 = p4 * x1 + p10 * x2 + p16 * x3 + p22 * x4 + p28 * x5 + p34 * x6;
        pc[4] = m5 = p5 * x1 + p11 * x2 + p17 * x3 + p23 * x4 + p29 * x5 + p35 * x6;
        pc[5] = m6 = p6 * x1 + p12 * x2 + p18 * x3 + p24 * x4 + p30 * x5 + p36 * x6;

        pc[6] = m7 = p1 * x7 + p7 * x8 + p13 * x9 + p19 * x10 + p25 * x11 + p31 * x12;
        pc[7] = m8 = p2 * x7 + p8 * x8 + p14 * x9 + p20 * x10 + p26 * x11 + p32 * x12;
        pc[8] = m9 = p3 * x7 + p9 * x8 + p15 * x9 + p21 * x10 + p27 * x11 + p33 * x12;
        pc[9] = m10 = p4 * x7 + p10 * x8 + p16 * x9 + p22 * x10 + p28 * x11 + p34 * x12;
        pc[10] = m11 = p5 * x7 + p11 * x8 + p17 * x9 + p23 * x10 + p29 * x11 + p35 * x12;
        pc[11] = m12 = p6 * x7 + p12 * x8 + p18 * x9 + p24 * x10 + p30 * x11 + p36 * x12;

        pc[12] = m13 = p1 * x13 + p7 * x14 + p13 * x15 + p19 * x16 + p25 * x17 + p31 * x18;
        pc[13] = m14 = p2 * x13 + p8 * x14 + p14 * x15 + p20 * x16 + p26 * x17 + p32 * x18;
        pc[14] = m15 = p3 * x13 + p9 * x14 + p15 * x15 + p21 * x16 + p27 * x17 + p33 * x18;
        pc[15] = m16 = p4 * x13 + p10 * x14 + p16 * x15 + p22 * x16 + p28 * x17 + p34 * x18;
        pc[16] = m17 = p5 * x13 + p11 * x14 + p17 * x15 + p23 * x16 + p29 * x17 + p35 * x18;
        pc[17] = m18 = p6 * x13 + p12 * x14 + p18 * x15 + p24 * x16 + p30 * x17 + p36 * x18;

        pc[18] = m19 = p1 * x19 + p7 * x20 + p13 * x21 + p19 * x22 + p25 * x23 + p31 * x24;
        pc[19] = m20 = p2 * x19 + p8 * x20 + p14 * x21 + p20 * x22 + p26 * x23 + p32 * x24;
        pc[20] = m21 = p3 * x19 + p9 * x20 + p15 * x21 + p21 * x22 + p27 * x23 + p33 * x24;
        pc[21] = m22 = p4 * x19 + p10 * x20 + p16 * x21 + p22 * x22 + p28 * x23 + p34 * x24;
        pc[22] = m23 = p5 * x19 + p11 * x20 + p17 * x21 + p23 * x22 + p29 * x23 + p35 * x24;
        pc[23] = m24 = p6 * x19 + p12 * x20 + p18 * x21 + p24 * x22 + p30 * x23 + p36 * x24;

        pc[24] = m25 = p1 * x25 + p7 * x26 + p13 * x27 + p19 * x28 + p25 * x29 + p31 * x30;
        pc[25] = m26 = p2 * x25 + p8 * x26 + p14 * x27 + p20 * x28 + p26 * x29 + p32 * x30;
        pc[26] = m27 = p3 * x25 + p9 * x26 + p15 * x27 + p21 * x28 + p27 * x29 + p33 * x30;
        pc[27] = m28 = p4 * x25 + p10 * x26 + p16 * x27 + p22 * x28 + p28 * x29 + p34 * x30;
        pc[28] = m29 = p5 * x25 + p11 * x26 + p17 * x27 + p23 * x28 + p29 * x29 + p35 * x30;
        pc[29] = m30 = p6 * x25 + p12 * x26 + p18 * x27 + p24 * x28 + p30 * x29 + p36 * x30;

        pc[30] = m31 = p1 * x31 + p7 * x32 + p13 * x33 + p19 * x34 + p25 * x35 + p31 * x36;
        pc[31] = m32 = p2 * x31 + p8 * x32 + p14 * x33 + p20 * x34 + p26 * x35 + p32 * x36;
        pc[32] = m33 = p3 * x31 + p9 * x32 + p15 * x33 + p21 * x34 + p27 * x35 + p33 * x36;
        pc[33] = m34 = p4 * x31 + p10 * x32 + p16 * x33 + p22 * x34 + p28 * x35 + p34 * x36;
        pc[34] = m35 = p5 * x31 + p11 * x32 + p17 * x33 + p23 * x34 + p29 * x35 + p35 * x36;
        pc[35] = m36 = p6 * x31 + p12 * x32 + p18 * x33 + p24 * x34 + p30 * x35 + p36 * x36;

        nz = bi[row + 1] - diag_offset[row] - 1;
        pv += 36;
        for (j = 0; j < nz; j++) {
          x1  = pv[0];
          x2  = pv[1];
          x3  = pv[2];
          x4  = pv[3];
          x5  = pv[4];
          x6  = pv[5];
          x7  = pv[6];
          x8  = pv[7];
          x9  = pv[8];
          x10 = pv[9];
          x11 = pv[10];
          x12 = pv[11];
          x13 = pv[12];
          x14 = pv[13];
          x15 = pv[14];
          x16 = pv[15];
          x17 = pv[16];
          x18 = pv[17];
          x19 = pv[18];
          x20 = pv[19];
          x21 = pv[20];
          x22 = pv[21];
          x23 = pv[22];
          x24 = pv[23];
          x25 = pv[24];
          x26 = pv[25];
          x27 = pv[26];
          x28 = pv[27];
          x29 = pv[28];
          x30 = pv[29];
          x31 = pv[30];
          x32 = pv[31];
          x33 = pv[32];
          x34 = pv[33];
          x35 = pv[34];
          x36 = pv[35];
          x   = rtmp + 36 * pj[j];
          x[0] -= m1 * x1 + m7 * x2 + m13 * x3 + m19 * x4 + m25 * x5 + m31 * x6;
          x[1] -= m2 * x1 + m8 * x2 + m14 * x3 + m20 * x4 + m26 * x5 + m32 * x6;
          x[2] -= m3 * x1 + m9 * x2 + m15 * x3 + m21 * x4 + m27 * x5 + m33 * x6;
          x[3] -= m4 * x1 + m10 * x2 + m16 * x3 + m22 * x4 + m28 * x5 + m34 * x6;
          x[4] -= m5 * x1 + m11 * x2 + m17 * x3 + m23 * x4 + m29 * x5 + m35 * x6;
          x[5] -= m6 * x1 + m12 * x2 + m18 * x3 + m24 * x4 + m30 * x5 + m36 * x6;

          x[6] -= m1 * x7 + m7 * x8 + m13 * x9 + m19 * x10 + m25 * x11 + m31 * x12;
          x[7] -= m2 * x7 + m8 * x8 + m14 * x9 + m20 * x10 + m26 * x11 + m32 * x12;
          x[8] -= m3 * x7 + m9 * x8 + m15 * x9 + m21 * x10 + m27 * x11 + m33 * x12;
          x[9] -= m4 * x7 + m10 * x8 + m16 * x9 + m22 * x10 + m28 * x11 + m34 * x12;
          x[10] -= m5 * x7 + m11 * x8 + m17 * x9 + m23 * x10 + m29 * x11 + m35 * x12;
          x[11] -= m6 * x7 + m12 * x8 + m18 * x9 + m24 * x10 + m30 * x11 + m36 * x12;

          x[12] -= m1 * x13 + m7 * x14 + m13 * x15 + m19 * x16 + m25 * x17 + m31 * x18;
          x[13] -= m2 * x13 + m8 * x14 + m14 * x15 + m20 * x16 + m26 * x17 + m32 * x18;
          x[14] -= m3 * x13 + m9 * x14 + m15 * x15 + m21 * x16 + m27 * x17 + m33 * x18;
          x[15] -= m4 * x13 + m10 * x14 + m16 * x15 + m22 * x16 + m28 * x17 + m34 * x18;
          x[16] -= m5 * x13 + m11 * x14 + m17 * x15 + m23 * x16 + m29 * x17 + m35 * x18;
          x[17] -= m6 * x13 + m12 * x14 + m18 * x15 + m24 * x16 + m30 * x17 + m36 * x18;

          x[18] -= m1 * x19 + m7 * x20 + m13 * x21 + m19 * x22 + m25 * x23 + m31 * x24;
          x[19] -= m2 * x19 + m8 * x20 + m14 * x21 + m20 * x22 + m26 * x23 + m32 * x24;
          x[20] -= m3 * x19 + m9 * x20 + m15 * x21 + m21 * x22 + m27 * x23 + m33 * x24;
          x[21] -= m4 * x19 + m10 * x20 + m16 * x21 + m22 * x22 + m28 * x23 + m34 * x24;
          x[22] -= m5 * x19 + m11 * x20 + m17 * x21 + m23 * x22 + m29 * x23 + m35 * x24;
          x[23] -= m6 * x19 + m12 * x20 + m18 * x21 + m24 * x22 + m30 * x23 + m36 * x24;

          x[24] -= m1 * x25 + m7 * x26 + m13 * x27 + m19 * x28 + m25 * x29 + m31 * x30;
          x[25] -= m2 * x25 + m8 * x26 + m14 * x27 + m20 * x28 + m26 * x29 + m32 * x30;
          x[26] -= m3 * x25 + m9 * x26 + m15 * x27 + m21 * x28 + m27 * x29 + m33 * x30;
          x[27] -= m4 * x25 + m10 * x26 + m16 * x27 + m22 * x28 + m28 * x29 + m34 * x30;
          x[28] -= m5 * x25 + m11 * x26 + m17 * x27 + m23 * x28 + m29 * x29 + m35 * x30;
          x[29] -= m6 * x25 + m12 * x26 + m18 * x27 + m24 * x28 + m30 * x29 + m36 * x30;

          x[30] -= m1 * x31 + m7 * x32 + m13 * x33 + m19 * x34 + m25 * x35 + m31 * x36;
          x[31] -= m2 * x31 + m8 * x32 + m14 * x33 + m20 * x34 + m26 * x35 + m32 * x36;
          x[32] -= m3 * x31 + m9 * x32 + m15 * x33 + m21 * x34 + m27 * x35 + m33 * x36;
          x[33] -= m4 * x31 + m10 * x32 + m16 * x33 + m22 * x34 + m28 * x35 + m34 * x36;
          x[34] -= m5 * x31 + m11 * x32 + m17 * x33 + m23 * x34 + m29 * x35 + m35 * x36;
          x[35] -= m6 * x31 + m12 * x32 + m18 * x33 + m24 * x34 + m30 * x35 + m36 * x36;

          pv += 36;
        }
        PetscCall(PetscLogFlops(432.0 * nz + 396.0));
      }
      row = *ajtmp++;
    }
    /* finished row so stick it into b->a */
    pv = ba + 36 * bi[i];
    pj = bj + bi[i];
    nz = bi[i + 1] - bi[i];
    for (j = 0; j < nz; j++) {
      x      = rtmp + 36 * pj[j];
      pv[0]  = x[0];
      pv[1]  = x[1];
      pv[2]  = x[2];
      pv[3]  = x[3];
      pv[4]  = x[4];
      pv[5]  = x[5];
      pv[6]  = x[6];
      pv[7]  = x[7];
      pv[8]  = x[8];
      pv[9]  = x[9];
      pv[10] = x[10];
      pv[11] = x[11];
      pv[12] = x[12];
      pv[13] = x[13];
      pv[14] = x[14];
      pv[15] = x[15];
      pv[16] = x[16];
      pv[17] = x[17];
      pv[18] = x[18];
      pv[19] = x[19];
      pv[20] = x[20];
      pv[21] = x[21];
      pv[22] = x[22];
      pv[23] = x[23];
      pv[24] = x[24];
      pv[25] = x[25];
      pv[26] = x[26];
      pv[27] = x[27];
      pv[28] = x[28];
      pv[29] = x[29];
      pv[30] = x[30];
      pv[31] = x[31];
      pv[32] = x[32];
      pv[33] = x[33];
      pv[34] = x[34];
      pv[35] = x[35];
      pv += 36;
    }
    /* invert diagonal block */
    w = ba + 36 * diag_offset[i];
    PetscCall(PetscKernel_A_gets_inverse_A_6(w, shift, allowzeropivot, &zeropivotdetected));
    if (zeropivotdetected) C->factorerrortype = MAT_FACTOR_NUMERIC_ZEROPIVOT;
  }

  PetscCall(PetscFree(rtmp));

  C->ops->solve          = MatSolve_SeqBAIJ_6_NaturalOrdering_inplace;
  C->ops->solvetranspose = MatSolveTranspose_SeqBAIJ_6_NaturalOrdering_inplace;
  C->assembled           = PETSC_TRUE;

  PetscCall(PetscLogFlops(1.333333333333 * 6 * 6 * 6 * b->mbs)); /* from inverting diagonal blocks */
  PetscFunctionReturn(0);
}

PetscErrorCode MatLUFactorNumeric_SeqBAIJ_6_NaturalOrdering(Mat B, Mat A, const MatFactorInfo *info) {
  Mat             C = B;
  Mat_SeqBAIJ    *a = (Mat_SeqBAIJ *)A->data, *b = (Mat_SeqBAIJ *)C->data;
  PetscInt        i, j, k, nz, nzL, row;
  const PetscInt  n = a->mbs, *ai = a->i, *aj = a->j, *bi = b->i, *bj = b->j;
  const PetscInt *ajtmp, *bjtmp, *bdiag = b->diag, *pj, bs2 = a->bs2;
  MatScalar      *rtmp, *pc, *mwork, *v, *pv, *aa = a->a;
  PetscInt        flg;
  PetscReal       shift = info->shiftamount;
  PetscBool       allowzeropivot, zeropivotdetected;

  PetscFunctionBegin;
  allowzeropivot = PetscNot(A->erroriffailure);

  /* generate work space needed by the factorization */
  PetscCall(PetscMalloc2(bs2 * n, &rtmp, bs2, &mwork));
  PetscCall(PetscArrayzero(rtmp, bs2 * n));

  for (i = 0; i < n; i++) {
    /* zero rtmp */
    /* L part */
    nz    = bi[i + 1] - bi[i];
    bjtmp = bj + bi[i];
    for (j = 0; j < nz; j++) PetscCall(PetscArrayzero(rtmp + bs2 * bjtmp[j], bs2));

    /* U part */
    nz    = bdiag[i] - bdiag[i + 1];
    bjtmp = bj + bdiag[i + 1] + 1;
    for (j = 0; j < nz; j++) PetscCall(PetscArrayzero(rtmp + bs2 * bjtmp[j], bs2));

    /* load in initial (unfactored row) */
    nz    = ai[i + 1] - ai[i];
    ajtmp = aj + ai[i];
    v     = aa + bs2 * ai[i];
    for (j = 0; j < nz; j++) PetscCall(PetscArraycpy(rtmp + bs2 * ajtmp[j], v + bs2 * j, bs2));

    /* elimination */
    bjtmp = bj + bi[i];
    nzL   = bi[i + 1] - bi[i];
    for (k = 0; k < nzL; k++) {
      row = bjtmp[k];
      pc  = rtmp + bs2 * row;
      for (flg = 0, j = 0; j < bs2; j++) {
        if (pc[j] != 0.0) {
          flg = 1;
          break;
        }
      }
      if (flg) {
        pv = b->a + bs2 * bdiag[row];
        /* PetscKernel_A_gets_A_times_B(bs,pc,pv,mwork); *pc = *pc * (*pv); */
        PetscCall(PetscKernel_A_gets_A_times_B_6(pc, pv, mwork));

        pj = b->j + bdiag[row + 1] + 1; /* beginning of U(row,:) */
        pv = b->a + bs2 * (bdiag[row + 1] + 1);
        nz = bdiag[row] - bdiag[row + 1] - 1; /* num of entries inU(row,:), excluding diag */
        for (j = 0; j < nz; j++) {
          /* PetscKernel_A_gets_A_minus_B_times_C(bs,rtmp+bs2*pj[j],pc,pv+bs2*j); */
          /* rtmp+bs2*pj[j] = rtmp+bs2*pj[j] - (*pc)*(pv+bs2*j) */
          v = rtmp + bs2 * pj[j];
          PetscCall(PetscKernel_A_gets_A_minus_B_times_C_6(v, pc, pv));
          pv += bs2;
        }
        PetscCall(PetscLogFlops(432.0 * nz + 396)); /* flops = 2*bs^3*nz + 2*bs^3 - bs2) */
      }
    }

    /* finished row so stick it into b->a */
    /* L part */
    pv = b->a + bs2 * bi[i];
    pj = b->j + bi[i];
    nz = bi[i + 1] - bi[i];
    for (j = 0; j < nz; j++) PetscCall(PetscArraycpy(pv + bs2 * j, rtmp + bs2 * pj[j], bs2));

    /* Mark diagonal and invert diagonal for simpler triangular solves */
    pv = b->a + bs2 * bdiag[i];
    pj = b->j + bdiag[i];
    PetscCall(PetscArraycpy(pv, rtmp + bs2 * pj[0], bs2));
    PetscCall(PetscKernel_A_gets_inverse_A_6(pv, shift, allowzeropivot, &zeropivotdetected));
    if (zeropivotdetected) C->factorerrortype = MAT_FACTOR_NUMERIC_ZEROPIVOT;

    /* U part */
    pv = b->a + bs2 * (bdiag[i + 1] + 1);
    pj = b->j + bdiag[i + 1] + 1;
    nz = bdiag[i] - bdiag[i + 1] - 1;
    for (j = 0; j < nz; j++) PetscCall(PetscArraycpy(pv + bs2 * j, rtmp + bs2 * pj[j], bs2));
  }
  PetscCall(PetscFree2(rtmp, mwork));

  C->ops->solve          = MatSolve_SeqBAIJ_6_NaturalOrdering;
  C->ops->solvetranspose = MatSolveTranspose_SeqBAIJ_6_NaturalOrdering;
  C->assembled           = PETSC_TRUE;

  PetscCall(PetscLogFlops(1.333333333333 * 6 * 6 * 6 * n)); /* from inverting diagonal blocks */
  PetscFunctionReturn(0);
}