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fftw3: New-array Execute Functions

 
 4.6 New-array Execute Functions
 ===============================
 
 Normally, one executes a plan for the arrays with which the plan was
 created, by calling 'fftw_execute(plan)' as described in Using
 Plans.  However, it is possible for sophisticated users to apply a
 given plan to a _different_ array using the "new-array execute"
 functions detailed below, provided that the following conditions are
 met:
 
    * The array size, strides, etcetera are the same (since those are set
      by the plan).
 
    * The input and output arrays are the same (in-place) or different
      (out-of-place) if the plan was originally created to be in-place or
      out-of-place, respectively.
 
    * For split arrays, the separations between the real and imaginary
      parts, 'ii-ri' and 'io-ro', are the same as they were for the input
      and output arrays when the plan was created.  (This condition is
      automatically satisfied for interleaved arrays.)
 
    * The "alignment" of the new input/output arrays is the same as that
      of the input/output arrays when the plan was created, unless the
      plan was created with the 'FFTW_UNALIGNED' flag.  Here, the
      alignment is a platform-dependent quantity (for example, it is the
      address modulo 16 if SSE SIMD instructions are used, but the
      address modulo 4 for non-SIMD single-precision FFTW on the same
      machine).  In general, only arrays allocated with 'fftw_malloc' are
      guaranteed to be equally aligned (SIMD alignment and
      fftw_malloc).
 
    The alignment issue is especially critical, because if you don't use
 'fftw_malloc' then you may have little control over the alignment of
 arrays in memory.  For example, neither the C++ 'new' function nor the
 Fortran 'allocate' statement provide strong enough guarantees about data
 alignment.  If you don't use 'fftw_malloc', therefore, you probably have
 to use 'FFTW_UNALIGNED' (which disables most SIMD support).  If
 possible, it is probably better for you to simply create multiple plans
 (creating a new plan is quick once one exists for a given size), or
 better yet re-use the same array for your transforms.
 
    For rare circumstances in which you cannot control the alignment of
 allocated memory, but wish to determine where a given array is aligned
 like the original array for which a plan was created, you can use the
 'fftw_alignment_of' function:
      int fftw_alignment_of(double *p);
    Two arrays have equivalent alignment (for the purposes of applying a
 plan) if and only if 'fftw_alignment_of' returns the same value for the
 corresponding pointers to their data (typecast to 'double*' if
 necessary).
 
    If you are tempted to use the new-array execute interface because you
 want to transform a known bunch of arrays of the same size, you should
 probably go use the advanced interface instead (Advanced
 Interface)).
 
    The new-array execute functions are:
 
      void fftw_execute_dft(
           const fftw_plan p,
           fftw_complex *in, fftw_complex *out);
 
      void fftw_execute_split_dft(
           const fftw_plan p,
           double *ri, double *ii, double *ro, double *io);
 
      void fftw_execute_dft_r2c(
           const fftw_plan p,
           double *in, fftw_complex *out);
 
      void fftw_execute_split_dft_r2c(
           const fftw_plan p,
           double *in, double *ro, double *io);
 
      void fftw_execute_dft_c2r(
           const fftw_plan p,
           fftw_complex *in, double *out);
 
      void fftw_execute_split_dft_c2r(
           const fftw_plan p,
           double *ri, double *ii, double *out);
 
      void fftw_execute_r2r(
           const fftw_plan p,
           double *in, double *out);
 
    These execute the 'plan' to compute the corresponding transform on
 the input/output arrays specified by the subsequent arguments.  The
 input/output array arguments have the same meanings as the ones passed
 to the guru planner routines in the preceding sections.  The 'plan' is
 not modified, and these routines can be called as many times as desired,
 or intermixed with calls to the ordinary 'fftw_execute'.
 
    The 'plan' _must_ have been created for the transform type
 corresponding to the execute function, e.g.  it must be a complex-DFT
 plan for 'fftw_execute_dft'.  Any of the planner routines for that
 transform type, from the basic to the guru interface, could have been
 used to create the plan, however.
 

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