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Add FFTW3
author Chris Cannam
date Wed, 20 Mar 2013 15:35:50 +0000
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Chris@10 3 <title>Plan execution in Fortran - FFTW 3.3.3</title>
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Chris@10 13 This manual is for FFTW
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Chris@10 48 <div class="node">
Chris@10 49 <a name="Plan-execution-in-Fortran"></a>
Chris@10 50 <p>
Chris@10 51 Next:&nbsp;<a rel="next" accesskey="n" href="Allocating-aligned-memory-in-Fortran.html#Allocating-aligned-memory-in-Fortran">Allocating aligned memory in Fortran</a>,
Chris@10 52 Previous:&nbsp;<a rel="previous" accesskey="p" href="FFTW-Fortran-type-reference.html#FFTW-Fortran-type-reference">FFTW Fortran type reference</a>,
Chris@10 53 Up:&nbsp;<a rel="up" accesskey="u" href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran">Calling FFTW from Modern Fortran</a>
Chris@10 54 <hr>
Chris@10 55 </div>
Chris@10 56
Chris@10 57 <h3 class="section">7.4 Plan execution in Fortran</h3>
Chris@10 58
Chris@10 59 <p>In C, in order to use a plan, one normally calls <code>fftw_execute</code>,
Chris@10 60 which executes the plan to perform the transform on the input/output
Chris@10 61 arrays passed when the plan was created (see <a href="Using-Plans.html#Using-Plans">Using Plans</a>). The
Chris@10 62 corresponding subroutine call in modern Fortran is:
Chris@10 63 <pre class="example"> call fftw_execute(plan)
Chris@10 64 </pre>
Chris@10 65 <p><a name="index-fftw_005fexecute-554"></a>
Chris@10 66 However, we have had reports that this causes problems with some
Chris@10 67 recent optimizing Fortran compilers. The problem is, because the
Chris@10 68 input/output arrays are not passed as explicit arguments to
Chris@10 69 <code>fftw_execute</code>, the semantics of Fortran (unlike C) allow the
Chris@10 70 compiler to assume that the input/output arrays are not changed by
Chris@10 71 <code>fftw_execute</code>. As a consequence, certain compilers end up
Chris@10 72 repositioning the call to <code>fftw_execute</code>, assuming incorrectly
Chris@10 73 that it does nothing to the arrays.
Chris@10 74
Chris@10 75 <p>There are various workarounds to this, but the safest and simplest
Chris@10 76 thing is to not use <code>fftw_execute</code> in Fortran. Instead, use the
Chris@10 77 functions described in <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>, which take
Chris@10 78 the input/output arrays as explicit arguments. For example, if the
Chris@10 79 plan is for a complex-data DFT and was created for the arrays
Chris@10 80 <code>in</code> and <code>out</code>, you would do:
Chris@10 81 <pre class="example"> call fftw_execute_dft(plan, in, out)
Chris@10 82 </pre>
Chris@10 83 <p><a name="index-fftw_005fexecute_005fdft-555"></a>
Chris@10 84 There are a few things to be careful of, however:
Chris@10 85
Chris@10 86 <ul>
Chris@10 87 <li><a name="index-fftw_005fexecute_005fdft_005fr2c-556"></a><a name="index-fftw_005fexecute_005fdft_005fc2r-557"></a><a name="index-fftw_005fexecute_005fr2r-558"></a>You must use the correct type of execute function, matching the way
Chris@10 88 the plan was created. Complex DFT plans should use
Chris@10 89 <code>fftw_execute_dft</code>, Real-input (r2c) DFT plans should use use
Chris@10 90 <code>fftw_execute_dft_r2c</code>, and real-output (c2r) DFT plans should
Chris@10 91 use <code>fftw_execute_dft_c2r</code>. The various r2r plans should use
Chris@10 92 <code>fftw_execute_r2r</code>. Fortunately, if you use the wrong one you
Chris@10 93 will get a compile-time type-mismatch error (unlike legacy Fortran).
Chris@10 94
Chris@10 95 <li>You should normally pass the same input/output arrays that were used when
Chris@10 96 creating the plan. This is always safe.
Chris@10 97
Chris@10 98 <li><em>If</em> you pass <em>different</em> input/output arrays compared to
Chris@10 99 those used when creating the plan, you must abide by all the
Chris@10 100 restrictions of the new-array execute functions (see <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>). The most tricky of these is the
Chris@10 101 requirement that the new arrays have the same alignment as the
Chris@10 102 original arrays; the best (and possibly only) way to guarantee this
Chris@10 103 is to use the &lsquo;<samp><span class="samp">fftw_alloc</span></samp>&rsquo; functions to allocate your arrays (see <a href="Allocating-aligned-memory-in-Fortran.html#Allocating-aligned-memory-in-Fortran">Allocating aligned memory in Fortran</a>). Alternatively, you can
Chris@10 104 use the <code>FFTW_UNALIGNED</code> flag when creating the
Chris@10 105 plan, in which case the plan does not depend on the alignment, but
Chris@10 106 this may sacrifice substantial performance on architectures (like x86)
Chris@10 107 with SIMD instructions (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>).
Chris@10 108 <a name="index-FFTW_005fUNALIGNED-559"></a>
Chris@10 109 </ul>
Chris@10 110
Chris@10 111 <!-- -->
Chris@10 112 </body></html>
Chris@10 113