FFT/generic : Replace explicit openacc directives with do concurrent

[mathrack]

Following commit ef6dfb9, explicit openacc directive allow the generic fft to run on GPU. Do concurrent should be used instead of explicit openacc directives.

[mathrack]

However, tests performed on the cluster Bede during the last GPU hackathon suggest that the nvidia compiler is not currently able to deal with this kind of do concurrent. Attached is a small example showing the issue.
example.tar.gz
The output on Bede is :

 Compiled with nvfortran 22.1-0
 Compiler options : 
 main.f90 -Mfree -Kieee -Minfo=accel,ftn,inline,loop,vect,opt,stdpar -stdpar=gpu -gpu=cc70,managed,lineinfo,deepcopy -acc -target=gpu -traceback -O3 -Mvect=simd -Mflushz -Mcache_align -Mrecip-div -Mfactorize -Minstrument -g -c -I/opt/software/builder/developers/compilers/nvhpc/22.1/1/default/Linux_ppc64le/22.1/comm_libs/openmpi/openmpi-3.1.5/include -I/opt/software/builder/developers/compilers/nvhpc/22.1/1/default/Linux_ppc64le/22.1/comm_libs/openmpi/openmpi-3.1.5/lib
 Absolute error for derxacc :    6.9501907648138697E-002
 Absolute error for derxDC :     985420388520.4409

[fspiga]

!$acc routine(...) seq cannot be simply removed.

The example attached refers to der* routines, FFT/2decomp, the very generic adhoc FFT implementation ... what part exactly?

[mathrack]

The objective would be to keep the !$acc routine() seq and replace

!$acc parallel loop gang vector collapse(2) private(buffer)
    do k = 1, nz
    do j = 1, ny
    ...
    enddo
    enddo

with

    do concurrent (k=1:nz, j=1:ny) local(buffer)
    ...
    enddo

or alternatively a 3D shared buffer and

    do concurrent (k=1:nz, j=1:ny)
    ...
    enddo

The example refers to der* subroutines but fixing the example should also fix the current fft/generic