All of the planner routines in FFTW accept an integer
argument, which is a bitwise OR (‘|’) of zero or more of the flag
constants defined below. These flags control the rigor (and time) of
the planning process, and can also impose (or lift) restrictions on the
type of transform algorithm that is employed.
Important: the planner overwrites the input array during
planning unless a saved plan (see Wisdom) is available for that
problem, so you should initialize your input data after creating the
plan. The only exceptions to this are the
FFTW_WISDOM_ONLY flags, as mentioned below.
In all cases, if wisdom is available for the given problem that was
created with equal-or-greater planning rigor, then the more rigorous
wisdom is used. For example, in
FFTW_ESTIMATE mode any available
wisdom is used, whereas in
FFTW_PATIENT mode only wisdom created
in patient or exhaustive mode can be used. See Words of Wisdom-Saving Plans.
FFTW_ESTIMATEspecifies that, instead of actual measurements of different algorithms, a simple heuristic is used to pick a (probably sub-optimal) plan quickly. With this flag, the input/output arrays are not overwritten during planning.
FFTW_MEASUREtells FFTW to find an optimized plan by actually computing several FFTs and measuring their execution time. Depending on your machine, this can take some time (often a few seconds).
FFTW_MEASUREis the default planning option.
FFTW_MEASURE, but considers a wider range of algorithms and often produces a “more optimal” plan (especially for large transforms), but at the expense of several times longer planning time (especially for large transforms).
FFTW_PATIENT, but considers an even wider range of algorithms, including many that we think are unlikely to be fast, to produce the most optimal plan but with a substantially increased planning time.
FFTW_WISDOM_ONLYis a special planning mode in which the plan is only created if wisdom is available for the given problem, and otherwise a
NULLplan is returned. This can be combined with other flags, e.g. ‘FFTW_WISDOM_ONLY | FFTW_PATIENT’ creates a plan only if wisdom is available that was created in
FFTW_WISDOM_ONLYflag is intended for users who need to detect whether wisdom is available; for example, if wisdom is not available one may wish to allocate new arrays for planning so that user data is not overwritten.
FFTW_DESTROY_INPUTspecifies that an out-of-place transform is allowed to overwrite its input array with arbitrary data; this can sometimes allow more efficient algorithms to be employed.
FFTW_PRESERVE_INPUTspecifies that an out-of-place transform must not change its input array. This is ordinarily the default, except for c2r and hc2r (i.e. complex-to-real) transforms for which
FFTW_DESTROY_INPUTis the default. In the latter cases, passing
FFTW_PRESERVE_INPUTwill attempt to use algorithms that do not destroy the input, at the expense of worse performance; for multi-dimensional c2r transforms, however, no input-preserving algorithms are implemented and the planner will return
NULLif one is requested.
FFTW_UNALIGNEDspecifies that the algorithm may not impose any unusual alignment requirements on the input/output arrays (i.e. no SIMD may be used). This flag is normally not necessary, since the planner automatically detects misaligned arrays. The only use for this flag is if you want to use the new-array execute interface to execute a given plan on a different array that may not be aligned like the original. (Using
fftw_mallocmakes this flag unnecessary even then. You can also use
fftw_alignment_ofto detect whether two arrays are equivalently aligned.)
extern void fftw_set_timelimit(double seconds);
This function instructs FFTW to spend at most
(approximately) in the planner. If
FFTW_NO_TIMELIMIT (the default value, which is negative), then
planning time is unbounded. Otherwise, FFTW plans with a
progressively wider range of algorithms until the given time limit
is reached or the given range of algorithms is explored, returning the
best available plan.
For example, specifying
FFTW_PATIENT first plans in
FFTW_ESTIMATE mode, then in
FFTW_MEASURE mode, then
finally (time permitting) in
FFTW_EXHAUSTIVE is specified instead, the planner will further
Note that the
seconds argument specifies only a rough limit; in
practice, the planner may use somewhat more time if the time limit is
reached when the planner is in the middle of an operation that cannot
be interrupted. At the very least, the planner will complete planning
FFTW_ESTIMATE mode (which is thus equivalent to a time limit