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1 <html lang="en">
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2 <head>
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3 <title>FFTW Execution in Fortran - FFTW 3.3.3</title>
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4 <meta http-equiv="Content-Type" content="text/html">
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5 <meta name="description" content="FFTW 3.3.3">
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6 <meta name="generator" content="makeinfo 4.13">
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7 <link title="Top" rel="start" href="index.html#Top">
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8 <link rel="up" href="Calling-FFTW-from-Legacy-Fortran.html#Calling-FFTW-from-Legacy-Fortran" title="Calling FFTW from Legacy Fortran">
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9 <link rel="prev" href="FFTW-Constants-in-Fortran.html#FFTW-Constants-in-Fortran" title="FFTW Constants in Fortran">
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10 <link rel="next" href="Fortran-Examples.html#Fortran-Examples" title="Fortran Examples">
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11 <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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12 <!--
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13 This manual is for FFTW
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14 (version 3.3.3, 25 November 2012).
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15
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16 Copyright (C) 2003 Matteo Frigo.
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17
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18 Copyright (C) 2003 Massachusetts Institute of Technology.
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19
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20 Permission is granted to make and distribute verbatim copies of
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21 this manual provided the copyright notice and this permission
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22 notice are preserved on all copies.
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23
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24 Permission is granted to copy and distribute modified versions of
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25 this manual under the conditions for verbatim copying, provided
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26 that the entire resulting derived work is distributed under the
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27 terms of a permission notice identical to this one.
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28
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29 Permission is granted to copy and distribute translations of this
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30 manual into another language, under the above conditions for
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31 modified versions, except that this permission notice may be
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32 stated in a translation approved by the Free Software Foundation.
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33 -->
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45 --></style>
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46 </head>
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47 <body>
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48 <div class="node">
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49 <a name="FFTW-Execution-in-Fortran"></a>
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50 <p>
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51 Next: <a rel="next" accesskey="n" href="Fortran-Examples.html#Fortran-Examples">Fortran Examples</a>,
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52 Previous: <a rel="previous" accesskey="p" href="FFTW-Constants-in-Fortran.html#FFTW-Constants-in-Fortran">FFTW Constants in Fortran</a>,
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53 Up: <a rel="up" accesskey="u" href="Calling-FFTW-from-Legacy-Fortran.html#Calling-FFTW-from-Legacy-Fortran">Calling FFTW from Legacy Fortran</a>
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54 <hr>
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55 </div>
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56
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57 <h3 class="section">8.3 FFTW Execution in Fortran</h3>
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58
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59 <p>In C, in order to use a plan, one normally calls <code>fftw_execute</code>,
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60 which executes the plan to perform the transform on the input/output
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61 arrays passed when the plan was created (see <a href="Using-Plans.html#Using-Plans">Using Plans</a>). The
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62 corresponding subroutine call in legacy Fortran is:
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63 <pre class="example"> call dfftw_execute(plan)
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64 </pre>
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65 <p><a name="index-dfftw_005fexecute-584"></a>
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66 However, we have had reports that this causes problems with some
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67 recent optimizing Fortran compilers. The problem is, because the
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68 input/output arrays are not passed as explicit arguments to
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69 <code>dfftw_execute</code>, the semantics of Fortran (unlike C) allow the
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70 compiler to assume that the input/output arrays are not changed by
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71 <code>dfftw_execute</code>. As a consequence, certain compilers end up
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72 optimizing out or repositioning the call to <code>dfftw_execute</code>,
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73 assuming incorrectly that it does nothing.
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74
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75 <p>There are various workarounds to this, but the safest and simplest
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76 thing is to not use <code>dfftw_execute</code> in Fortran. Instead, use the
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77 functions described in <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>, which take
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78 the input/output arrays as explicit arguments. For example, if the
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79 plan is for a complex-data DFT and was created for the arrays
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80 <code>in</code> and <code>out</code>, you would do:
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81 <pre class="example"> call dfftw_execute_dft(plan, in, out)
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82 </pre>
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83 <p><a name="index-dfftw_005fexecute_005fdft-585"></a>
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84 There are a few things to be careful of, however:
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85
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86 <ul>
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87 <li>You must use the correct type of execute function, matching the way
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88 the plan was created. Complex DFT plans should use
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89 <code>dfftw_execute_dft</code>, Real-input (r2c) DFT plans should use use
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90 <code>dfftw_execute_dft_r2c</code>, and real-output (c2r) DFT plans should
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91 use <code>dfftw_execute_dft_c2r</code>. The various r2r plans should use
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92 <code>dfftw_execute_r2r</code>.
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93
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94 <li>You should normally pass the same input/output arrays that were used when
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95 creating the plan. This is always safe.
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96
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97 <li><em>If</em> you pass <em>different</em> input/output arrays compared to
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98 those used when creating the plan, you must abide by all the
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99 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 difficult of these, in Fortran, is the
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100 requirement that the new arrays have the same alignment as the
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101 original arrays, because there seems to be no way in legacy Fortran to obtain
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102 guaranteed-aligned arrays (analogous to <code>fftw_malloc</code> in C). You
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103 can, of course, use the <code>FFTW_UNALIGNED</code> flag when creating the
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104 plan, in which case the plan does not depend on the alignment, but
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105 this may sacrifice substantial performance on architectures (like x86)
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106 with SIMD instructions (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>).
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107 <a name="index-FFTW_005fUNALIGNED-586"></a>
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108 </ul>
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109
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110 <!-- -->
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111 </body></html>
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112
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