BlockVector whose blocks refer to the same Vector

[FH-9]

Hi,

I am solving the auxiliary gradient equations for the Navier-Stokes equations in the context of DG methods. I have a NonlinearForm object with which I am calling its Mult method. For the same vector of global DG DOFs, I need to call Mult 3 times for the components of the gradient in 3D, which seems inefficient. I would prefer to call Mult once and get all three gradient component vectors at the same time. I don't think this is possible with the regular NonlinearForm since its Mult method accepts only two arguments of the same size. So I have been looking into BlockNonlinearForm and its MultBlocked method. But now both arguments need to be BlockVectors with the same number of blocks (Block L-Vector to Block L-Vector), whereas in my case, it is (Block 1-vector to Block 3-Vector). Would it be reasonable to make my first BlockVector (the DG DOF vector) have repeated blocks that refer to the same Vector? This is what I have come up with so far:

Array<int> bOffsets(4);
bOffsets[0] = 0; bOffsets[1] = vfes->GetVSize();
bOffsets[2] = 2 * vfes->GetVSize(); bOffsets[3] = 3 * vfes->GetVSize();

BlockVector x_blocks(bOffsets);
x_blocks.GetBlock(0).SetDataAndSize(x.GetData(), vfes->GetVSize());
x_blocks.GetBlock(1).SetDataAndSize(x.GetData(), vfes->GetVSize());
x_blocks.GetBlock(2).SetDataAndSize(x.GetData(), vfes->GetVSize());

BlockVector grad_blocks(bOffsets);

blockNonlinearForm->MultBlocked(x_blocks, grad_blocks);

Would this be correct? Are there any pitfalls with this implementation? Or maybe a better of achieving the same? Note also that I do not intend to modify x_blocks in any way.

I would greatly appreciate your input.

Thanks!