It just occurred to me ...

[Niceno]

... if we were storing boundary condition information in faces, rather than in boundary cells, we would avoid all those

if(c2 < 0) then
  ! Do what needs to be done
end if

That would shorten the code, and make it faster to an extent

[Niceno]

Dear Egor, I see you assigned this issue to your own self. I guess it means you like the idea or at least agree with it?

[palkinev]

Definitely, Bojan,
For some models like Rsm we solve numerous similar of equations with tons of checks like this.
I feel that it should improve performance for those models.

I am working on this issue today.