Memory alignment in derived types

[Wee_Beng_T_]

Hi,

If I have the derived type below :

type test

integer :: k,ij(2)

real :: xy(2)

end type test

type(test) :: t1(10,10)

Are the variables aligned in this way :

t1(1,1)%k,t1(1,1)%ij(1),t1(1,1)%ij(2),t1(1,1)%xy(1),t1(1,1)%xy(2)

t1(1,2)%k,t1(1,2)%ij(1),t1(1,2)%ij(2),t1(1,2)%xy(1),t1(1,2)%xy(2) ... ?

or

t1(1,1)%k,t1(1,2)%k,t1(1,3)%k, ...

t1(1,1)%ij(1),t1(1,2)%ij(1),t1(1,3)%ij(1) ... ?

or even

t1(1,1)%k,t1(1,2)%k,t1(1,3)%k, ...

t1(1,1)%ij(1),t1(1,1)%ij(2),t1(1,2)%ij(1),t1(1,2)%ij(2) ... ?

Can I choose how the variables are stored in memory?

Also, do I need to use -align records during compilation whenever I have derived types?

thank you!

[jimdempseyatthecove]

Look in the documentation for SEQUENCE

Jim Dempsey

[Steven_L_Intel1]

Without SEQUENCE or BIND(C), you cannot depend on the relative order of components within the derived type, but the answer to your general question is that the first layout you show is what you get. Within each element of t1, each of its components are together.

SEQUENCE prevents the compiler from rearranging the components, but does not necessarily prevent padding between components for alignment. Intel Fortran does not add padding, by default, for SEQUENCE types, though you can request that with -align sequence.

BIND(C) specifies that the order of the components, and alignment padding, is exactly the same as what the "companion C compiler" would do for an equivalent struct (though array components are still laid out in Fortran's column-major order.)

You do not "need" to use -align records - but you can if you wish. For the type you show, no alignment would be added even if you added that option, since all the components are naturally aligned.

[Wee_Beng_T_]

Hi,

I googled for the use of SEQUENCE and it gives me the idea that it will ensure that the memory will be aligned according to the first layout, is that so?

Is there anyway I can get layout 2 or even 3 thru some means?

This is because in my code, I have something like:

do j = 1,10

do i = 1,10

ii = t1(i,j)%ij(1); jj = t1(i,j)%ij(2)

expression involving ii,jj ....

end do

end do

Is it better/faster if I just declare the array ij(2,10,10)?

thank you!

[Steven_L_Intel1 · Accepted solution]

SEQUENCE will give you the first layout. I would not use the word "aligned" here, as it means something different than I think you are using.

I don't see a problem, offhand, with the current layout and your proposed code. I suggest that you write your program for clarity and correctness first. Run the program through Intel VTune Amplifier to see if you are getting memory contention or cache miss issues. Ideally you want to keep your memory references near each other if they are small.