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1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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2 <html>
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3 <!-- This manual is for FFTW
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4 (version 3.3.5, 30 July 2016).
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5
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6 Copyright (C) 2003 Matteo Frigo.
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7
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8 Copyright (C) 2003 Massachusetts Institute of Technology.
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9
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10 Permission is granted to make and distribute verbatim copies of this
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11 manual provided the copyright notice and this permission notice are
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12 preserved on all copies.
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13
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14 Permission is granted to copy and distribute modified versions of this
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15 manual under the conditions for verbatim copying, provided that the
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16 entire resulting derived work is distributed under the terms of a
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17 permission notice identical to this one.
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18
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19 Permission is granted to copy and distribute translations of this manual
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20 into another language, under the above conditions for modified versions,
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21 except that this permission notice may be stated in a translation
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22 approved by the Free Software Foundation. -->
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23 <!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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24 <head>
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25 <title>FFTW 3.3.5: FFTW Execution in Fortran</title>
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26
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27 <meta name="description" content="FFTW 3.3.5: FFTW Execution in Fortran">
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28 <meta name="keywords" content="FFTW 3.3.5: FFTW Execution in Fortran">
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33 <link href="index.html#Top" rel="start" title="Top">
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34 <link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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35 <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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36 <link href="Calling-FFTW-from-Legacy-Fortran.html#Calling-FFTW-from-Legacy-Fortran" rel="up" title="Calling FFTW from Legacy Fortran">
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37 <link href="Fortran-Examples.html#Fortran-Examples" rel="next" title="Fortran Examples">
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38 <link href="FFTW-Constants-in-Fortran.html#FFTW-Constants-in-Fortran" rel="prev" title="FFTW Constants in Fortran">
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65 -->
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66 </style>
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67
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68
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69 </head>
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70
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71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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72 <a name="FFTW-Execution-in-Fortran"></a>
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73 <div class="header">
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74 <p>
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75 Next: <a href="Fortran-Examples.html#Fortran-Examples" accesskey="n" rel="next">Fortran Examples</a>, Previous: <a href="FFTW-Constants-in-Fortran.html#FFTW-Constants-in-Fortran" accesskey="p" rel="prev">FFTW Constants in Fortran</a>, Up: <a href="Calling-FFTW-from-Legacy-Fortran.html#Calling-FFTW-from-Legacy-Fortran" accesskey="u" rel="up">Calling FFTW from Legacy Fortran</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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76 </div>
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77 <hr>
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78 <a name="FFTW-Execution-in-Fortran-1"></a>
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79 <h3 class="section">8.3 FFTW Execution in Fortran</h3>
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80
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81 <p>In C, in order to use a plan, one normally calls <code>fftw_execute</code>,
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82 which executes the plan to perform the transform on the input/output
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83 arrays passed when the plan was created (see <a href="Using-Plans.html#Using-Plans">Using Plans</a>). The
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84 corresponding subroutine call in legacy Fortran is:
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85 </p><div class="example">
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86 <pre class="example"> call dfftw_execute(plan)
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87 </pre></div>
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88 <a name="index-dfftw_005fexecute"></a>
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89
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90 <p>However, we have had reports that this causes problems with some
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91 recent optimizing Fortran compilers. The problem is, because the
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92 input/output arrays are not passed as explicit arguments to
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93 <code>dfftw_execute</code>, the semantics of Fortran (unlike C) allow the
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94 compiler to assume that the input/output arrays are not changed by
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95 <code>dfftw_execute</code>. As a consequence, certain compilers end up
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96 optimizing out or repositioning the call to <code>dfftw_execute</code>,
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97 assuming incorrectly that it does nothing.
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98 </p>
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99 <p>There are various workarounds to this, but the safest and simplest
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100 thing is to not use <code>dfftw_execute</code> in Fortran. Instead, use the
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101 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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102 the input/output arrays as explicit arguments. For example, if the
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103 plan is for a complex-data DFT and was created for the arrays
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104 <code>in</code> and <code>out</code>, you would do:
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105 </p><div class="example">
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106 <pre class="example"> call dfftw_execute_dft(plan, in, out)
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107 </pre></div>
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108 <a name="index-dfftw_005fexecute_005fdft"></a>
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109
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110 <p>There are a few things to be careful of, however:
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111 </p>
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112 <ul>
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113 <li> You must use the correct type of execute function, matching the way
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114 the plan was created. Complex DFT plans should use
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115 <code>dfftw_execute_dft</code>, Real-input (r2c) DFT plans should use use
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116 <code>dfftw_execute_dft_r2c</code>, and real-output (c2r) DFT plans should
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117 use <code>dfftw_execute_dft_c2r</code>. The various r2r plans should use
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118 <code>dfftw_execute_r2r</code>.
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119
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120 </li><li> You should normally pass the same input/output arrays that were used when
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121 creating the plan. This is always safe.
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122
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123 </li><li> <em>If</em> you pass <em>different</em> input/output arrays compared to
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124 those used when creating the plan, you must abide by all the
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125 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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126 requirement that the new arrays have the same alignment as the
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127 original arrays, because there seems to be no way in legacy Fortran to obtain
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128 guaranteed-aligned arrays (analogous to <code>fftw_malloc</code> in C). You
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129 can, of course, use the <code>FFTW_UNALIGNED</code> flag when creating the
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130 plan, in which case the plan does not depend on the alignment, but
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131 this may sacrifice substantial performance on architectures (like x86)
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132 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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133 <a name="index-FFTW_005fUNALIGNED-3"></a>
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134
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135 </li></ul>
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136
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137 <hr>
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138 <div class="header">
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139 <p>
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140 Next: <a href="Fortran-Examples.html#Fortran-Examples" accesskey="n" rel="next">Fortran Examples</a>, Previous: <a href="FFTW-Constants-in-Fortran.html#FFTW-Constants-in-Fortran" accesskey="p" rel="prev">FFTW Constants in Fortran</a>, Up: <a href="Calling-FFTW-from-Legacy-Fortran.html#Calling-FFTW-from-Legacy-Fortran" accesskey="u" rel="up">Calling FFTW from Legacy Fortran</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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141 </div>
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142
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143
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144
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145 </body>
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146 </html>
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