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<h4 class="subsection">4.3.2 Planner Flags</h4>

<p>All of the planner routines in FFTW accept an integer <code>flags</code>
argument, which is a bitwise OR (&lsquo;<samp><span class="samp">|</span></samp>&rsquo;) 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.

   <p><em>Important:</em> the planner overwrites the input array during
planning unless a saved plan (see <a href="Wisdom.html#Wisdom">Wisdom</a>) is available for that
problem, so you should initialize your input data after creating the
plan.  The only exceptions to this are the <code>FFTW_ESTIMATE</code> and
<code>FFTW_WISDOM_ONLY</code> flags, as mentioned below.

   <p>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 <code>FFTW_ESTIMATE</code> mode any available
wisdom is used, whereas  in <code>FFTW_PATIENT</code> mode only wisdom created
in patient or exhaustive mode can be used.  See <a href="Words-of-Wisdom_002dSaving-Plans.html#Words-of-Wisdom_002dSaving-Plans">Words of Wisdom-Saving Plans</a>.

<h5 class="subsubheading">Planning-rigor flags</h5>

     <ul>
<li><a name="index-FFTW_005fESTIMATE-171"></a><code>FFTW_ESTIMATE</code> specifies 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.

     <li><a name="index-FFTW_005fMEASURE-172"></a><code>FFTW_MEASURE</code> tells FFTW to find an optimized plan by actually
<em>computing</em> several FFTs and measuring their execution time.
Depending on your machine, this can take some time (often a few
seconds).  <code>FFTW_MEASURE</code> is the default planning option.

     <li><a name="index-FFTW_005fPATIENT-173"></a><code>FFTW_PATIENT</code> is like <code>FFTW_MEASURE</code>, but considers a wider
range of algorithms and often produces a &ldquo;more optimal&rdquo; plan
(especially for large transforms), but at the expense of several times
longer planning time (especially for large transforms).

     <li><a name="index-FFTW_005fEXHAUSTIVE-174"></a><code>FFTW_EXHAUSTIVE</code> is like <code>FFTW_PATIENT</code>, 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.

     <li><a name="index-FFTW_005fWISDOM_005fONLY-175"></a><code>FFTW_WISDOM_ONLY</code> is a special planning mode in which the plan
is only created if wisdom is available for the given problem, and
otherwise a <code>NULL</code> plan is returned.  This can be combined with
other flags, e.g. &lsquo;<samp><span class="samp">FFTW_WISDOM_ONLY | FFTW_PATIENT</span></samp>&rsquo; creates a
plan only if wisdom is available that was created in
<code>FFTW_PATIENT</code> or <code>FFTW_EXHAUSTIVE</code> mode.  The
<code>FFTW_WISDOM_ONLY</code> flag 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.

</ul>

<h5 class="subsubheading">Algorithm-restriction flags</h5>

     <ul>
<li><a name="index-FFTW_005fDESTROY_005fINPUT-176"></a><code>FFTW_DESTROY_INPUT</code> specifies that an out-of-place transform is
allowed to <em>overwrite its input</em> array with arbitrary data; this
can sometimes allow more efficient algorithms to be employed.
<a name="index-out_002dof_002dplace-177"></a>
<li><a name="index-FFTW_005fPRESERVE_005fINPUT-178"></a><code>FFTW_PRESERVE_INPUT</code> specifies that an out-of-place transform must
<em>not change its input</em> array.  This is ordinarily the
<em>default</em>, except for c2r and hc2r (i.e. complex-to-real)
transforms for which <code>FFTW_DESTROY_INPUT</code> is the default.  In the
latter cases, passing <code>FFTW_PRESERVE_INPUT</code> will 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
<code>NULL</code> if one is requested.
<a name="index-c2r-179"></a><a name="index-hc2r-180"></a>
<li><a name="index-FFTW_005fUNALIGNED-181"></a><a name="index-alignment-182"></a><code>FFTW_UNALIGNED</code> specifies 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 <em>not necessary</em>, 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 <code>fftw_malloc</code> makes this flag unnecessary
even then.)

</ul>

<h5 class="subsubheading">Limiting planning time</h5>

<pre class="example">     extern void fftw_set_timelimit(double seconds);
</pre>
   <p><a name="index-fftw_005fset_005ftimelimit-183"></a>
This function instructs FFTW to spend at most <code>seconds</code> seconds
(approximately) in the planner.  If <code>seconds ==
FFTW_NO_TIMELIMIT</code> (the default value, which is negative), then
planning time is unbounded.  Otherwise, FFTW plans with a
progressively wider range of algorithms until the the given time limit
is reached or the given range of algorithms is explored, returning the
best available plan.
<a name="index-FFTW_005fNO_005fTIMELIMIT-184"></a>

   <p>For example, specifying <code>FFTW_PATIENT</code> first plans in
<code>FFTW_ESTIMATE</code> mode, then in <code>FFTW_MEASURE</code> mode, then
finally (time permitting) in <code>FFTW_PATIENT</code>.  If
<code>FFTW_EXHAUSTIVE</code> is specified instead, the planner will further
progress to <code>FFTW_EXHAUSTIVE</code> mode.

   <p>Note that the <code>seconds</code> 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
in <code>FFTW_ESTIMATE</code> mode (which is thus equivalent to a time limit
of 0).

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author	Chris Cannam <cannam@all-day-breakfast.com>
date	Mon, 02 Mar 2020 14:03:47 +0000
parents	89f5e221ed7b
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