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Chris@42:In order to obtain maximum performance in FFTW, you should store your
Chris@42: data in arrays that have been specially aligned in memory (see SIMD alignment and fftw_malloc). Enforcing alignment also permits you to
Chris@42: 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
Chris@42: arrays. Unfortunately, standard Fortran arrays do not provide
Chris@42: any alignment guarantees. The only way to allocate aligned
Chris@42: memory in standard Fortran is to allocate it with an external C
Chris@42: function, like the fftw_alloc_real and
Chris@42: fftw_alloc_complex functions. Fortunately, Fortran 2003 provides
Chris@42: a simple way to associate such allocated memory with a standard Fortran
Chris@42: array pointer that you can then use normally.
Chris@42:
We therefore recommend allocating all your input/output arrays using Chris@42: the following technique: Chris@42:
Chris@42:pointer, arr, to your array of the desired type
Chris@42: and dimensions. For example, real(C_DOUBLE), pointer :: a(:,:)
Chris@42: for a 2d real array, or complex(C_DOUBLE_COMPLEX), pointer ::
Chris@42: a(:,:,:) for a 3d complex array.
Chris@42:
Chris@42: integer(C_SIZE_T). You can either declare a variable of this
Chris@42: type, e.g. integer(C_SIZE_T) :: sz, to store the number of
Chris@42: elements to allocate, or you can use the int(..., C_SIZE_T)
Chris@42: intrinsic function. e.g. set sz = L * M * N or use
Chris@42: int(L * M * N, C_SIZE_T) for an L × M × N array.
Chris@42:
Chris@42: type(C_PTR) :: p to hold the return value from
Chris@42: FFTW’s allocation routine. Set p = fftw_alloc_real(sz) for a real array, or p = fftw_alloc_complex(sz) for a complex array.
Chris@42:
Chris@42: arr with the allocated memory p
Chris@42: using the standard c_f_pointer subroutine: call
Chris@42: c_f_pointer(p, arr, [...dimensions...]), where
Chris@42: [...dimensions...]) are an array of the dimensions of the array
Chris@42: (in the usual Fortran order). e.g. call c_f_pointer(p, arr,
Chris@42: [L,M,N]) for an L × M × N array. (Alternatively, you can
Chris@42: omit the dimensions argument if you specified the shape explicitly
Chris@42: when declaring arr.) You can now use arr as a usual
Chris@42: multidimensional array.
Chris@42:
Chris@42: call
Chris@42: fftw_free(p) on p.
Chris@42:
Chris@42: For example, here is how we would allocate an L × M 2d real array: Chris@42:
Chris@42:real(C_DOUBLE), pointer :: arr(:,:) Chris@42: type(C_PTR) :: p Chris@42: p = fftw_alloc_real(int(L * M, C_SIZE_T)) Chris@42: call c_f_pointer(p, arr, [L,M]) Chris@42: ...use arr and arr(i,j) as usual... Chris@42: call fftw_free(p) Chris@42:
and here is an L × M × N 3d complex array: Chris@42:
Chris@42:complex(C_DOUBLE_COMPLEX), pointer :: arr(:,:,:) Chris@42: type(C_PTR) :: p Chris@42: p = fftw_alloc_complex(int(L * M * N, C_SIZE_T)) Chris@42: call c_f_pointer(p, arr, [L,M,N]) Chris@42: ...use arr and arr(i,j,k) as usual... Chris@42: call fftw_free(p) Chris@42:
See Reversing array dimensions for an example allocating a Chris@42: single array and associating both real and complex array pointers with Chris@42: it, for in-place real-to-complex transforms. Chris@42:
Chris@42:Chris@42: Next: Accessing the wisdom API from Fortran, Previous: Plan execution in Fortran, Up: Calling FFTW from Modern Fortran [Contents][Index]
Chris@42: