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Add FFTW 3.3.8 source, and a Linux build
author Chris Cannam
date Tue, 19 Nov 2019 14:52:55 +0000
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Chris@82 1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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Chris@82 3 <!-- This manual is for FFTW
Chris@82 4 (version 3.3.8, 24 May 2018).
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Chris@82 6 Copyright (C) 2003 Matteo Frigo.
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Chris@82 8 Copyright (C) 2003 Massachusetts Institute of Technology.
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Chris@82 24 <head>
Chris@82 25 <title>FFTW 3.3.8: FFTW Execution in Fortran</title>
Chris@82 26
Chris@82 27 <meta name="description" content="FFTW 3.3.8: FFTW Execution in Fortran">
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Chris@82 35 <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
Chris@82 36 <link href="Calling-FFTW-from-Legacy-Fortran.html#Calling-FFTW-from-Legacy-Fortran" rel="up" title="Calling FFTW from Legacy Fortran">
Chris@82 37 <link href="Fortran-Examples.html#Fortran-Examples" rel="next" title="Fortran Examples">
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Chris@82 68 </head>
Chris@82 69
Chris@82 70 <body lang="en">
Chris@82 71 <a name="FFTW-Execution-in-Fortran"></a>
Chris@82 72 <div class="header">
Chris@82 73 <p>
Chris@82 74 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> &nbsp; [<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>
Chris@82 75 </div>
Chris@82 76 <hr>
Chris@82 77 <a name="FFTW-Execution-in-Fortran-1"></a>
Chris@82 78 <h3 class="section">8.3 FFTW Execution in Fortran</h3>
Chris@82 79
Chris@82 80 <p>In C, in order to use a plan, one normally calls <code>fftw_execute</code>,
Chris@82 81 which executes the plan to perform the transform on the input/output
Chris@82 82 arrays passed when the plan was created (see <a href="Using-Plans.html#Using-Plans">Using Plans</a>). The
Chris@82 83 corresponding subroutine call in legacy Fortran is:
Chris@82 84 </p><div class="example">
Chris@82 85 <pre class="example"> call dfftw_execute(plan)
Chris@82 86 </pre></div>
Chris@82 87 <a name="index-dfftw_005fexecute"></a>
Chris@82 88
Chris@82 89 <p>However, we have had reports that this causes problems with some
Chris@82 90 recent optimizing Fortran compilers. The problem is, because the
Chris@82 91 input/output arrays are not passed as explicit arguments to
Chris@82 92 <code>dfftw_execute</code>, the semantics of Fortran (unlike C) allow the
Chris@82 93 compiler to assume that the input/output arrays are not changed by
Chris@82 94 <code>dfftw_execute</code>. As a consequence, certain compilers end up
Chris@82 95 optimizing out or repositioning the call to <code>dfftw_execute</code>,
Chris@82 96 assuming incorrectly that it does nothing.
Chris@82 97 </p>
Chris@82 98 <p>There are various workarounds to this, but the safest and simplest
Chris@82 99 thing is to not use <code>dfftw_execute</code> in Fortran. Instead, use the
Chris@82 100 functions described in <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>, which take
Chris@82 101 the input/output arrays as explicit arguments. For example, if the
Chris@82 102 plan is for a complex-data DFT and was created for the arrays
Chris@82 103 <code>in</code> and <code>out</code>, you would do:
Chris@82 104 </p><div class="example">
Chris@82 105 <pre class="example"> call dfftw_execute_dft(plan, in, out)
Chris@82 106 </pre></div>
Chris@82 107 <a name="index-dfftw_005fexecute_005fdft"></a>
Chris@82 108
Chris@82 109 <p>There are a few things to be careful of, however:
Chris@82 110 </p>
Chris@82 111 <ul>
Chris@82 112 <li> You must use the correct type of execute function, matching the way
Chris@82 113 the plan was created. Complex DFT plans should use
Chris@82 114 <code>dfftw_execute_dft</code>, Real-input (r2c) DFT plans should use use
Chris@82 115 <code>dfftw_execute_dft_r2c</code>, and real-output (c2r) DFT plans should
Chris@82 116 use <code>dfftw_execute_dft_c2r</code>. The various r2r plans should use
Chris@82 117 <code>dfftw_execute_r2r</code>.
Chris@82 118
Chris@82 119 </li><li> You should normally pass the same input/output arrays that were used when
Chris@82 120 creating the plan. This is always safe.
Chris@82 121
Chris@82 122 </li><li> <em>If</em> you pass <em>different</em> input/output arrays compared to
Chris@82 123 those used when creating the plan, you must abide by all the
Chris@82 124 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
Chris@82 125 requirement that the new arrays have the same alignment as the
Chris@82 126 original arrays, because there seems to be no way in legacy Fortran to obtain
Chris@82 127 guaranteed-aligned arrays (analogous to <code>fftw_malloc</code> in C). You
Chris@82 128 can, of course, use the <code>FFTW_UNALIGNED</code> flag when creating the
Chris@82 129 plan, in which case the plan does not depend on the alignment, but
Chris@82 130 this may sacrifice substantial performance on architectures (like x86)
Chris@82 131 with SIMD instructions (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>).
Chris@82 132 <a name="index-FFTW_005fUNALIGNED-3"></a>
Chris@82 133
Chris@82 134 </li></ul>
Chris@82 135
Chris@82 136 <hr>
Chris@82 137 <div class="header">
Chris@82 138 <p>
Chris@82 139 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> &nbsp; [<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>
Chris@82 140 </div>
Chris@82 141
Chris@82 142
Chris@82 143
Chris@82 144 </body>
Chris@82 145 </html>