Tips and Tricks for Deeper Recursive Routines

[mohanmuthu]

I am developing a fortran code with multi-level recursions (at times, it is expected to go 8000+ levels deep) using compiler 12.1. From the forum topics I learnt that, using heap arrays is a good practice. But is there any other tip/trick which I should be taking care not to run into problem.

[jimdempseyatthecove]

What arguments are passed into and returned from each recursion? (show the call and the dummy and data declarations)

Does the recursion make use of ~large temporary storage? (such as a scratch array)

Is preservation of this temporary storage necessary as you recurs deeper? (prior values of scratch array required on return)

Is, or at what point, parallelization involved?

There isn't a single tip/trick. What you need to do is highly dependent upon the answers to above questions (and others yet to be asked).

Is recursion absolutely necessary, or is it merely convenient?

Jim Dempsey

[mohanmuthu]

Thank you Jim. Here's what routine looks like.

    RECURSIVE SUBROUTINE recur_3d(sp,ep,t1,t2,t5,t6,t7,t10,t11)
        USE mod_globalvars, ONLY: x,y,z,a,b,c !real*8(1 to 20000) - constant
        USE mod_globalvars, ONLY: tpi !integer(1 to 2000) - constant
        USE mod_globalvars, ONLY: tp,vtp,ind1,ind2,path1,path2,mean1,mean2  !integer or real*8(1 to 20000) - updates with every call
        IMPLICIT NONE
        INTEGER, INTENT(IN)         :: sp,ep
        INTEGER, INTENT(INOUT)      :: t1,t2,t5,t6,t7,t10,t11

        CALL recur_3d(tp(t5-1),vtp(t5-1),t1,t2,t5,t6,t7,t10,t11)
        ...
        CALL recur_3d(tp(m),ind1(b2),t1,t2,t5,t6,t7,t10,t11)
        ...
        CALL recur_3d(ind2(b2),vtp(m),t1,t2,t5,t6,t7,t10,t11)

All the local variables used in last row were updated in every recursion. Due to some proprietary content, I am unable to share the entire code.

[mecej4]

See if you can use the ideas in https://en.wikipedia.org/wiki/Tail_call .

[jimdempseyatthecove]

Something like this may be of use:

module YourModule
    ! Generic interface recur_3d
    INTERFACE recur_3d
        ! Explicit interface with all arguments
        RECURSIVE SUBROUTINE recur_3d_all(sp,ep,t1x,t2x,t5x,t6x,t7x,t10x,t11x)
         IMPLICIT NONE
         INTEGER, INTENT(IN)         :: sp,ep
         INTEGER, INTENT(INOUT)      :: t1x,t2x,t5x,t6x,t7x,t10x,t11x
        END SUBROUTINE recur_3d_all
        ! 
        ! Explicit interface with few arguments
        RECURSIVE SUBROUTINE recur_3d_few(sp,ep)
         USE mod_globalvars, ONLY: x,y,z,a,b,c !real*8(1 to 20000) - constant
         USE mod_globalvars, ONLY: tpi !integer(1 to 2000) - constant
         USE mod_globalvars, ONLY: tp,vtp,ind1,ind2,path1,path2,mean1,mean2  !integer or real*8(1 to 20000) - updates with every call
         IMPLICIT NONE
         INTEGER, INTENT(IN)         :: sp,ep
        END SUBROUTINE recur_3d_few
    END INTERFACE
    
    ! module scoped context variables
    INTEGER, PRIVATE      :: t1,t2,t5,t6,t7,t10,t11
    
    CONTAINS
    RECURSIVE SUBROUTINE recur_3d_all(sp,ep,t1x,t2x,t5x,t6x,t7x,t10x,t11x)
         IMPLICIT NONE
         INTEGER, INTENT(IN)         :: sp,ep
         INTEGER, INTENT(INOUT)      :: t1,t2,t5,t6,t7,t10,t11
         t1 = t1x; t2=t2x; t5=t5x; t6=t6x; t7=t7x; t10=t10x; t11=t11x
         recur_3d(sp,ep)
         t1x = t1; t2x=t2; t5x=t5; t6x=t6; t7x=t7; t10x=t10; t11x=t11
    END SUBROUTINE recur_3d_all
         
    RECURSIVE SUBROUTINE recur_3d_few(sp,ep)
        USE mod_globalvars, ONLY: x,y,z,a,b,c !real*8(1 to 20000) - constant
        USE mod_globalvars, ONLY: tpi !integer(1 to 2000) - constant
        USE mod_globalvars, ONLY: tp,vtp,ind1,ind2,path1,path2,mean1,mean2  !integer or real*8(1 to 20000) - updates with every call
        IMPLICIT NONE
        INTEGER, INTENT(IN)         :: sp,ep
        CALL recur_3d(tp(t5-1),vtp(t5-1))
        ...
        CALL recur_3d(tp(m),ind1(b2))
        ...
        CALL recur_3d(ind2(b2),vtp(m))
    END SUBROUTINE recur_3d_few
    end module YourModule
    
    PROGRAM YourProbram
        USE YourModule
        ...
        ! use initial call arguments here
        CALL recur_3d(ind2(b2),vtp(m),t1,t2,t5,t6,t7,t10,t11)

Jim Dempsey