1:       program main              !   Solves the linear system  J x = f
  2:  #include <petsc/finclude/petsc.h>
  3:       use petscksp
  4:       implicit none
  5:       Vec x,f
  6:       Mat J
  7:       DM da
  8:       KSP ksp
  9:       PetscErrorCode ierr
 10:       PetscInt eight,one

 12:       eight = 8
 13:       one = 1
 14:       call PetscInitialize(PETSC_NULL_CHARACTER,ierr)
 15:       if (ierr .ne. 0) then
 16:         print*,'Unable to initialize PETSc'
 17:         stop
 18:       endif
 19:       call DMDACreate1d(MPI_COMM_WORLD,DM_BOUNDARY_NONE,eight,one,one,PETSC_NULL_INTEGER,da,ierr);CHKERRA(ierr)
 20:       call DMSetFromOptions(da,ierr)
 21:       call DMSetUp(da,ierr)
 22:       call DMCreateGlobalVector(da,x,ierr);CHKERRA(ierr)
 23:       call VecDuplicate(x,f,ierr);CHKERRA(ierr)
 24:       call DMSetMatType(da,MATAIJ,ierr);CHKERRA(ierr)
 25:       call DMCreateMatrix(da,J,ierr);CHKERRA(ierr)

 27:       call ComputeRHS(da,f,ierr);CHKERRA(ierr)
 28:       call ComputeMatrix(da,J,ierr);CHKERRA(ierr)

 30:       call KSPCreate(MPI_COMM_WORLD,ksp,ierr);CHKERRA(ierr)
 31:       call KSPSetOperators(ksp,J,J,ierr);CHKERRA(ierr)
 32:       call KSPSetFromOptions(ksp,ierr);CHKERRA(ierr)
 33:       call KSPSolve(ksp,f,x,ierr);CHKERRA(ierr)

 35:       call MatDestroy(J,ierr);CHKERRA(ierr)
 36:       call VecDestroy(x,ierr);CHKERRA(ierr)
 37:       call VecDestroy(f,ierr);CHKERRA(ierr)
 38:       call KSPDestroy(ksp,ierr);CHKERRA(ierr)
 39:       call DMDestroy(da,ierr);CHKERRA(ierr)
 40:       call PetscFinalize(ierr)
 41:       end

 43: ! AVX512 crashes without this..
 44:       block data init
 45:       implicit none
 46:       PetscScalar sd
 47:       common /cb/ sd
 48:       data sd /0/
 49:       end
 50:       subroutine knl_workarround(xx)
 51:       implicit none
 52:       PetscScalar xx
 53:       PetscScalar sd
 54:       common /cb/ sd
 55:       sd = sd+xx
 56:       end

 58:       subroutine  ComputeRHS(da,x,ierr)
 59:       use petscdmda
 60:       implicit none
 61:       DM da
 62:       Vec x
 63:       PetscErrorCode ierr
 64:       PetscInt xs,xm,i,mx
 65:       PetscScalar hx
 66:       PetscScalar, pointer :: xx(:)
 67:       call DMDAGetInfo(da,PETSC_NULL_INTEGER,mx,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER, &
 68:      &                 PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER, &
 69:      &                 PETSC_NULL_INTEGER,ierr);CHKERRQ(ierr)
 70:       call DMDAGetCorners(da,xs,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,xm,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,ierr);CHKERRQ(ierr)
 71:       hx     = 1.0_PETSC_REAL_KIND/(mx-1)
 72:       call DMDAVecGetArrayF90(da,x,xx,ierr);CHKERRQ(ierr)
 73:       do i=xs,xs+xm-1
 74:         call knl_workarround(xx(i))
 75:         xx(i) = i*hx
 76:       enddo
 77:       call DMDAVecRestoreArrayF90(da,x,xx,ierr);CHKERRQ(ierr)
 78:       return
 79:       end

 81:       subroutine ComputeMatrix(da,J,ierr)
 82:       use petscdm
 83:       use petscmat
 84:       implicit none
 85:       Mat J
 86:       DM da
 87:       PetscErrorCode ierr
 88:       PetscInt xs,xm,i,mx
 89:       PetscScalar hx,one

 91:       one = 1.0
 92:       call DMDAGetInfo(da,PETSC_NULL_INTEGER,mx,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER, &
 93:      &                 PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,   &
 94:      &                 PETSC_NULL_INTEGER,ierr);CHKERRQ(ierr)
 95:       call DMDAGetCorners(da,xs,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,xm,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,ierr);CHKERRQ(ierr)
 96:       hx     = 1.0_PETSC_REAL_KIND/(mx-1)
 97:       do i=xs,xs+xm-1
 98:         if ((i .eq. 0) .or. (i .eq. mx-1)) then
 99:           call MatSetValue(J,i,i,one,INSERT_VALUES,ierr);CHKERRQ(ierr)
100:         else
101:           call MatSetValue(J,i,i-1,-hx,INSERT_VALUES,ierr);CHKERRQ(ierr)
102:           call MatSetValue(J,i,i+1,-hx,INSERT_VALUES,ierr);CHKERRQ(ierr)
103:           call MatSetValue(J,i,i,2*hx,INSERT_VALUES,ierr);CHKERRQ(ierr)
104:         endif
105:       enddo
106:       call MatAssemblyBegin(J,MAT_FINAL_ASSEMBLY,ierr);CHKERRQ(ierr)
107:       call MatAssemblyEnd(J,MAT_FINAL_ASSEMBLY,ierr);CHKERRQ(ierr)
108:       return
109:       end

111: !/*TEST
112: !
113: !   test:
114: !      args: -ksp_converged_reason
115: !
116: !TEST*/