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Replace these with versions built using an older toolset (so as to avoid ABI compatibilities when linking on Ubuntu 14.04 for packaging purposes)
author Chris Cannam
date Fri, 07 Feb 2020 11:51:13 +0000
parents d0c2a83c1364
children
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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">
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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>