[Mitgcm-support] TAF bug for tangent linear

[mitgcm-support at dev.mitgcm.org]

Hi Ralf,

there is a problem with the forward mode of TAF
when exploiting self-adjointness.

What happens is that in S/R g_solve_for_pressure
cg2d is called first to compute the g_... quantities
(g_cg2d_b,g_cg2d_x),
then called again to calculate the actual state
(cg2d_b,cg2d_x).
Before the second call the indices should be reset, thus
       firstresidual = 0.
       lastresidual = 0.
       numiters = cg2dmaxiters
but the reset is missing.

The flow directive should have the information,
since the correct resetting is done in the adjoint code:

C----------------------------------------
C subroutine  cg2d
C----------------------------------------
CADJ SUBROUTINE cg2d ADNAME  = cg2d
CADJ SUBROUTINE cg2d FTLNAME = cg2d
CADJ SUBROUTINE cg2d INPUT   = 1,2,3,4,5,6
CADJ SUBROUTINE cg2d OUTPUT  =   2,3,4,5
CADJ SUBROUTINE cg2d ACTIVE  = 1,2
CADJ SUBROUTINE cg2d DEPEND  =           6 


Another issue (or, why I do all this) is that
I try to do forward mode vs. finite difference gradient checks.
Assuming that
    g_fc = (grad J)_(ith-component) * (delta xx)_(ith-component)
I should be able to recover (grad J)_(ith-component)
by setting
    (delta xx)_(ith-component) = 1.
(all others = 0.).
Well, I get the correct order of magnitude,
but not really correct gradients.
Do you have an idea what else could go wrong
(I've tried it with my above hand-fix).

Cheers
Patrick

-- 
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Patrick Heimbach     Massachusetts Institute of Technology
FON: +1/617/253-5259                    EAPS, Room 54-1518
FAX: +1/617/253-4464               77 Massachusetts Avenue
mailto:heimbach at mit.edu                 Cambridge MA 02139
http://www.mit.edu/~heimbach/                       U.S.A.