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Add null config files
author Chris Cannam <cannam@all-day-breakfast.com>
date Mon, 02 Mar 2020 14:03:47 +0000
parents 7867fa7e1b6b
children
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cannam@127 1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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cannam@127 3 <!-- This manual is for FFTW
cannam@127 4 (version 3.3.5, 30 July 2016).
cannam@127 5
cannam@127 6 Copyright (C) 2003 Matteo Frigo.
cannam@127 7
cannam@127 8 Copyright (C) 2003 Massachusetts Institute of Technology.
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cannam@127 24 <head>
cannam@127 25 <title>FFTW 3.3.5: FFTW Execution in Fortran</title>
cannam@127 26
cannam@127 27 <meta name="description" content="FFTW 3.3.5: FFTW Execution in Fortran">
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cannam@127 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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cannam@127 69 </head>
cannam@127 70
cannam@127 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
cannam@127 72 <a name="FFTW-Execution-in-Fortran"></a>
cannam@127 73 <div class="header">
cannam@127 74 <p>
cannam@127 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> &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>
cannam@127 76 </div>
cannam@127 77 <hr>
cannam@127 78 <a name="FFTW-Execution-in-Fortran-1"></a>
cannam@127 79 <h3 class="section">8.3 FFTW Execution in Fortran</h3>
cannam@127 80
cannam@127 81 <p>In C, in order to use a plan, one normally calls <code>fftw_execute</code>,
cannam@127 82 which executes the plan to perform the transform on the input/output
cannam@127 83 arrays passed when the plan was created (see <a href="Using-Plans.html#Using-Plans">Using Plans</a>). The
cannam@127 84 corresponding subroutine call in legacy Fortran is:
cannam@127 85 </p><div class="example">
cannam@127 86 <pre class="example"> call dfftw_execute(plan)
cannam@127 87 </pre></div>
cannam@127 88 <a name="index-dfftw_005fexecute"></a>
cannam@127 89
cannam@127 90 <p>However, we have had reports that this causes problems with some
cannam@127 91 recent optimizing Fortran compilers. The problem is, because the
cannam@127 92 input/output arrays are not passed as explicit arguments to
cannam@127 93 <code>dfftw_execute</code>, the semantics of Fortran (unlike C) allow the
cannam@127 94 compiler to assume that the input/output arrays are not changed by
cannam@127 95 <code>dfftw_execute</code>. As a consequence, certain compilers end up
cannam@127 96 optimizing out or repositioning the call to <code>dfftw_execute</code>,
cannam@127 97 assuming incorrectly that it does nothing.
cannam@127 98 </p>
cannam@127 99 <p>There are various workarounds to this, but the safest and simplest
cannam@127 100 thing is to not use <code>dfftw_execute</code> in Fortran. Instead, use the
cannam@127 101 functions described in <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>, which take
cannam@127 102 the input/output arrays as explicit arguments. For example, if the
cannam@127 103 plan is for a complex-data DFT and was created for the arrays
cannam@127 104 <code>in</code> and <code>out</code>, you would do:
cannam@127 105 </p><div class="example">
cannam@127 106 <pre class="example"> call dfftw_execute_dft(plan, in, out)
cannam@127 107 </pre></div>
cannam@127 108 <a name="index-dfftw_005fexecute_005fdft"></a>
cannam@127 109
cannam@127 110 <p>There are a few things to be careful of, however:
cannam@127 111 </p>
cannam@127 112 <ul>
cannam@127 113 <li> You must use the correct type of execute function, matching the way
cannam@127 114 the plan was created. Complex DFT plans should use
cannam@127 115 <code>dfftw_execute_dft</code>, Real-input (r2c) DFT plans should use use
cannam@127 116 <code>dfftw_execute_dft_r2c</code>, and real-output (c2r) DFT plans should
cannam@127 117 use <code>dfftw_execute_dft_c2r</code>. The various r2r plans should use
cannam@127 118 <code>dfftw_execute_r2r</code>.
cannam@127 119
cannam@127 120 </li><li> You should normally pass the same input/output arrays that were used when
cannam@127 121 creating the plan. This is always safe.
cannam@127 122
cannam@127 123 </li><li> <em>If</em> you pass <em>different</em> input/output arrays compared to
cannam@127 124 those used when creating the plan, you must abide by all the
cannam@127 125 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
cannam@127 126 requirement that the new arrays have the same alignment as the
cannam@127 127 original arrays, because there seems to be no way in legacy Fortran to obtain
cannam@127 128 guaranteed-aligned arrays (analogous to <code>fftw_malloc</code> in C). You
cannam@127 129 can, of course, use the <code>FFTW_UNALIGNED</code> flag when creating the
cannam@127 130 plan, in which case the plan does not depend on the alignment, but
cannam@127 131 this may sacrifice substantial performance on architectures (like x86)
cannam@127 132 with SIMD instructions (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>).
cannam@127 133 <a name="index-FFTW_005fUNALIGNED-3"></a>
cannam@127 134
cannam@127 135 </li></ul>
cannam@127 136
cannam@127 137 <hr>
cannam@127 138 <div class="header">
cannam@127 139 <p>
cannam@127 140 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> &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>
cannam@127 141 </div>
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cannam@127 143
cannam@127 144
cannam@127 145 </body>
cannam@127 146 </html>