7.5 Allocating aligned memory in Fortran

In order to obtain maximum performance in FFTW, you should store your data in arrays that have been specially aligned in memory (see SIMD alignment and fftw_malloc). Enforcing alignment also permits you to safely use the new-array execute functions (see New-array Execute Functions) to apply a given plan to more than one pair of in/out arrays. Unfortunately, standard Fortran arrays do not provide any alignment guarantees. The only way to allocate aligned memory in standard Fortran is to allocate it with an external C function, like the fftw_alloc_real and fftw_alloc_complex functions. Fortunately, Fortran 2003 provides a simple way to associate such allocated memory with a standard Fortran array pointer that you can then use normally.

We therefore recommend allocating all your input/output arrays using the following technique:

1. Declare a pointer, arr, to your array of the desired type and dimensions. For example, real(C_DOUBLE), pointer :: a(:,:) for a 2d real array, or complex(C_DOUBLE_COMPLEX), pointer :: a(:,:,:) for a 3d complex array.
2. The number of elements to allocate must be an integer(C_SIZE_T). You can either declare a variable of this type, e.g. integer(C_SIZE_T) :: sz, to store the number of elements to allocate, or you can use the int(..., C_SIZE_T) intrinsic function. e.g. set sz = L * M * N or use int(L * M * N, C_SIZE_T) for an L × M × N array.
3. Declare a type(C_PTR) :: p to hold the return value from FFTW’s allocation routine. Set p = fftw_alloc_real(sz) for a real array, or p = fftw_alloc_complex(sz) for a complex array.
4. Associate your pointer arr with the allocated memory p using the standard c_f_pointer subroutine: call c_f_pointer(p, arr, [...dimensions...]), where [...dimensions...]) are an array of the dimensions of the array (in the usual Fortran order). e.g. call c_f_pointer(p, arr, [L,M,N]) for an L × M × N array. (Alternatively, you can omit the dimensions argument if you specified the shape explicitly when declaring arr.) You can now use arr as a usual multidimensional array.
5. When you are done using the array, deallocate the memory by call fftw_free(p) on p.

For example, here is how we would allocate an L × M 2d real array:

  real(C_DOUBLE), pointer :: arr(:,:)
  type(C_PTR) :: p
  p = fftw_alloc_real(int(L * M, C_SIZE_T))
  call c_f_pointer(p, arr, [L,M])
  ...use arr and arr(i,j) as usual...
  call fftw_free(p)

and here is an L × M × N 3d complex array:

  complex(C_DOUBLE_COMPLEX), pointer :: arr(:,:,:)
  type(C_PTR) :: p
  p = fftw_alloc_complex(int(L * M * N, C_SIZE_T))
  call c_f_pointer(p, arr, [L,M,N])
  ...use arr and arr(i,j,k) as usual...
  call fftw_free(p)

See Reversing array dimensions for an example allocating a single array and associating both real and complex array pointers with it, for in-place real-to-complex transforms.