Here is my problem: I have a fortran code with a certain amount of nested loops and first I wanted to know if it's possible to optimize (rearranging) them in order to get a time gain? Second I wonder if I could use OpenMP to optimize them?
I have seen a lot of posts about nested do loops in fortran and how to optimize them but I didn't find one example that is suited to mine. I have also searched about OpenMP for nested do loops in fortran but I'm level 0 in OpenMP and it's difficult for me to know how to use it in my case.
Here are two very similar examples of loops that I have, first:
do p=1,N
do q=1,N
do ab=1,nVV
cd = 0
do c=nO+1,N
do d=c+1,N
cd = cd + 1
A(p,q,ab) = A(p,q,ab) + (B(p,q,c,d) - B(p,q,d,c))*C(cd,ab)
end do
end do
kl = 0
do k=1,nO
do l=k+1,nO
kl = kl + 1
A(p,q,ab) = A(p,q,ab) + (B(p,q,k,l) - B(p,q,l,k))*D(kl,ab)
end do
end do
end do
do ij=1,nOO
cd = 0
do c=nO+1,N
do d=c+1,N
cd = cd + 1
E(p,q,ij) = E(p,q,ij) + (B(p,q,c,d) - B(p,q,d,c))*F(cd,ij)
end do
end do
kl = 0
do k=1,nO
do l=k+1,nO
kl = kl + 1
E(p,q,ij) = E(p,q,ij) + (B(p,q,k,l) - B(p,q,l,k))*G(kl,ij)
end do
end do
end do
end do
end do
and the other one is:
do p=1,N
do q=1,N
do ab=1,nVV
cd = 0
do c=nO+1,N
do d=nO+1,N
cd = cd + 1
A(p,q,ab) = A(p,q,ab) + B(p,q,c,d)*C(cd,ab)
end do
end do
kl = 0
do k=1,nO
do l=1,nO
kl = kl + 1
A(p,q,ab) = A(p,q,ab) + B(p,q,k,l)*D(kl,ab)
end do
end do
end do
do ij=1,nOO
cd = 0
do c=nO+1,N
do d=nO+1,N
cd = cd + 1
E(p,q,ij) = E(p,q,ij) + B(p,q,c,d)*F(cd,ij)
end do
end do
kl = 0
do k=1,nO
do l=1,nO
kl = kl + 1
E(p,q,ij) = E(p,q,ij) + B(p,q,k,l)*G(kl,ij)
end do
end do
end do
end do
end do
The very small difference between the two examples is mainly in the indices of the loops. I don't know if you need more info about the different integers in the loops but you have in general: nO < nOO < N < nVV. So I don't know if it's possible to optimize these loops and/or possibly put them in a way that will facilitate the use of OpenMP (I don't know yet if I will use OpenMP, it will depend on how much I can gain by optimizing the loops without it).
I already tried to rearrange the loops in different ways without any success (no time gain) and I also tried a little bit of OpenMP but I don't know much about it, so again no success.
