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Plans for all transform types in FFTW are stored as type
Chris@19: fftw_plan (an opaque pointer type), and are created by one of the
Chris@19: various planning routines described in the following sections.
Chris@19: An fftw_plan contains all information necessary to compute the
Chris@19: transform, including the pointers to the input and output arrays.
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void fftw_execute(const fftw_plan plan); Chris@19:Chris@19:
Chris@19: This executes the plan, to compute the corresponding transform on
Chris@19: the arrays for which it was planned (which must still exist). The plan
Chris@19: is not modified, and fftw_execute can be called as many times as
Chris@19: desired.
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To apply a given plan to a different array, you can use the new-array execute Chris@19: interface. See New-array Execute Functions. Chris@19: Chris@19:
fftw_execute (and equivalents) is the only function in FFTW
Chris@19: guaranteed to be thread-safe; see Thread safety.
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This function: Chris@19:
void fftw_destroy_plan(fftw_plan plan); Chris@19:Chris@19:
deallocates the plan and all its associated data.
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FFTW's planner saves some other persistent data, such as the Chris@19: accumulated wisdom and a list of algorithms available in the current Chris@19: configuration. If you want to deallocate all of that and reset FFTW Chris@19: to the pristine state it was in when you started your program, you can Chris@19: call: Chris@19: Chris@19:
void fftw_cleanup(void); Chris@19:Chris@19:
Chris@19: After calling fftw_cleanup, all existing plans become undefined,
Chris@19: and you should not attempt to execute them nor to destroy them. You can
Chris@19: however create and execute/destroy new plans, in which case FFTW starts
Chris@19: accumulating wisdom information again.
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fftw_cleanup does not deallocate your plans, however. To prevent
Chris@19: memory leaks, you must still call fftw_destroy_plan before
Chris@19: executing fftw_cleanup.
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Occasionally, it may useful to know FFTW's internal “cost” metric
Chris@19: that it uses to compare plans to one another; this cost is
Chris@19: proportional to an execution time of the plan, in undocumented units,
Chris@19: if the plan was created with the FFTW_MEASURE or other
Chris@19: timing-based options, or alternatively is a heuristic cost function
Chris@19: for FFTW_ESTIMATE plans. (The cost values of measured and
Chris@19: estimated plans are not comparable, being in different units. Also,
Chris@19: costs from different FFTW versions or the same version compiled
Chris@19: differently may not be in the same units. Plans created from wisdom
Chris@19: have a cost of 0 since no timing measurement is performed for them.
Chris@19: Finally, certain problems for which only one top-level algorithm was
Chris@19: possible may have required no measurements of the cost of the whole
Chris@19: plan, in which case fftw_cost will also return 0.) The cost
Chris@19: metric for a given plan is returned by:
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double fftw_cost(const fftw_plan plan); Chris@19:Chris@19:
Chris@19: The following two routines are provided purely for academic purposes Chris@19: (that is, for entertainment). Chris@19: Chris@19:
void fftw_flops(const fftw_plan plan, Chris@19: double *add, double *mul, double *fma); Chris@19:Chris@19:
Chris@19: Given a plan, set add, mul, and fma to an
Chris@19: exact count of the number of floating-point additions, multiplications,
Chris@19: and fused multiply-add operations involved in the plan's execution. The
Chris@19: total number of floating-point operations (flops) is add + mul +
Chris@19: 2*fma, or add + mul + fma if the hardware supports fused
Chris@19: multiply-add instructions (although the number of FMA operations is only
Chris@19: approximate because of compiler voodoo). (The number of operations
Chris@19: should be an integer, but we use double to avoid overflowing
Chris@19: int for large transforms; the arguments are of type double
Chris@19: even for single and long-double precision versions of FFTW.)
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void fftw_fprint_plan(const fftw_plan plan, FILE *output_file); Chris@19: void fftw_print_plan(const fftw_plan plan); Chris@19: char *fftw_sprint_plan(const fftw_plan plan); Chris@19:Chris@19:
Chris@19: This outputs a “nerd-readable” representation of the plan to
Chris@19: the given file, to stdout, or two a newly allocated
Chris@19: NUL-terminated string (which the caller is responsible for deallocating
Chris@19: with free), respectively.
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