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Add FFTW 3.3.8 source, and a Linux build
author Chris Cannam <cannam@all-day-breakfast.com>
date Tue, 19 Nov 2019 14:52:55 +0000
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cannam@167 1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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cannam@167 3 <!-- This manual is for FFTW
cannam@167 4 (version 3.3.8, 24 May 2018).
cannam@167 5
cannam@167 6 Copyright (C) 2003 Matteo Frigo.
cannam@167 7
cannam@167 8 Copyright (C) 2003 Massachusetts Institute of Technology.
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cannam@167 10 Permission is granted to make and distribute verbatim copies of this
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cannam@167 24 <head>
cannam@167 25 <title>FFTW 3.3.8: Plan execution in Fortran</title>
cannam@167 26
cannam@167 27 <meta name="description" content="FFTW 3.3.8: Plan execution in Fortran">
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cannam@167 36 <link href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran" rel="up" title="Calling FFTW from Modern Fortran">
cannam@167 37 <link href="Allocating-aligned-memory-in-Fortran.html#Allocating-aligned-memory-in-Fortran" rel="next" title="Allocating aligned memory in Fortran">
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cannam@167 68 </head>
cannam@167 69
cannam@167 70 <body lang="en">
cannam@167 71 <a name="Plan-execution-in-Fortran"></a>
cannam@167 72 <div class="header">
cannam@167 73 <p>
cannam@167 74 Next: <a href="Allocating-aligned-memory-in-Fortran.html#Allocating-aligned-memory-in-Fortran" accesskey="n" rel="next">Allocating aligned memory in Fortran</a>, Previous: <a href="FFTW-Fortran-type-reference.html#FFTW-Fortran-type-reference" accesskey="p" rel="prev">FFTW Fortran type reference</a>, Up: <a href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran" accesskey="u" rel="up">Calling FFTW from Modern 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@167 75 </div>
cannam@167 76 <hr>
cannam@167 77 <a name="Plan-execution-in-Fortran-1"></a>
cannam@167 78 <h3 class="section">7.4 Plan execution in Fortran</h3>
cannam@167 79
cannam@167 80 <p>In C, in order to use a plan, one normally calls <code>fftw_execute</code>,
cannam@167 81 which executes the plan to perform the transform on the input/output
cannam@167 82 arrays passed when the plan was created (see <a href="Using-Plans.html#Using-Plans">Using Plans</a>). The
cannam@167 83 corresponding subroutine call in modern Fortran is:
cannam@167 84 </p><div class="example">
cannam@167 85 <pre class="example"> call fftw_execute(plan)
cannam@167 86 </pre></div>
cannam@167 87 <a name="index-fftw_005fexecute-6"></a>
cannam@167 88
cannam@167 89 <p>However, we have had reports that this causes problems with some
cannam@167 90 recent optimizing Fortran compilers. The problem is, because the
cannam@167 91 input/output arrays are not passed as explicit arguments to
cannam@167 92 <code>fftw_execute</code>, the semantics of Fortran (unlike C) allow the
cannam@167 93 compiler to assume that the input/output arrays are not changed by
cannam@167 94 <code>fftw_execute</code>. As a consequence, certain compilers end up
cannam@167 95 repositioning the call to <code>fftw_execute</code>, assuming incorrectly
cannam@167 96 that it does nothing to the arrays.
cannam@167 97 </p>
cannam@167 98 <p>There are various workarounds to this, but the safest and simplest
cannam@167 99 thing is to not use <code>fftw_execute</code> in Fortran. Instead, use the
cannam@167 100 functions described in <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>, which take
cannam@167 101 the input/output arrays as explicit arguments. For example, if the
cannam@167 102 plan is for a complex-data DFT and was created for the arrays
cannam@167 103 <code>in</code> and <code>out</code>, you would do:
cannam@167 104 </p><div class="example">
cannam@167 105 <pre class="example"> call fftw_execute_dft(plan, in, out)
cannam@167 106 </pre></div>
cannam@167 107 <a name="index-fftw_005fexecute_005fdft-3"></a>
cannam@167 108
cannam@167 109 <p>There are a few things to be careful of, however:
cannam@167 110 </p>
cannam@167 111 <ul>
cannam@167 112 <li> <a name="index-fftw_005fexecute_005fdft_005fr2c-2"></a>
cannam@167 113 <a name="index-fftw_005fexecute_005fdft_005fc2r-1"></a>
cannam@167 114 <a name="index-fftw_005fexecute_005fr2r-1"></a>
cannam@167 115 You must use the correct type of execute function, matching the way
cannam@167 116 the plan was created. Complex DFT plans should use
cannam@167 117 <code>fftw_execute_dft</code>, Real-input (r2c) DFT plans should use use
cannam@167 118 <code>fftw_execute_dft_r2c</code>, and real-output (c2r) DFT plans should
cannam@167 119 use <code>fftw_execute_dft_c2r</code>. The various r2r plans should use
cannam@167 120 <code>fftw_execute_r2r</code>. Fortunately, if you use the wrong one you
cannam@167 121 will get a compile-time type-mismatch error (unlike legacy Fortran).
cannam@167 122
cannam@167 123 </li><li> You should normally pass the same input/output arrays that were used when
cannam@167 124 creating the plan. This is always safe.
cannam@167 125
cannam@167 126 </li><li> <em>If</em> you pass <em>different</em> input/output arrays compared to
cannam@167 127 those used when creating the plan, you must abide by all the
cannam@167 128 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 tricky of these is the
cannam@167 129 requirement that the new arrays have the same alignment as the
cannam@167 130 original arrays; the best (and possibly only) way to guarantee this
cannam@167 131 is to use the &lsquo;<samp>fftw_alloc</samp>&rsquo; functions to allocate your arrays (see <a href="Allocating-aligned-memory-in-Fortran.html#Allocating-aligned-memory-in-Fortran">Allocating aligned memory in Fortran</a>). Alternatively, you can
cannam@167 132 use the <code>FFTW_UNALIGNED</code> flag when creating the
cannam@167 133 plan, in which case the plan does not depend on the alignment, but
cannam@167 134 this may sacrifice substantial performance on architectures (like x86)
cannam@167 135 with SIMD instructions (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>).
cannam@167 136 <a name="index-FFTW_005fUNALIGNED-2"></a>
cannam@167 137
cannam@167 138 </li></ul>
cannam@167 139
cannam@167 140 <hr>
cannam@167 141 <div class="header">
cannam@167 142 <p>
cannam@167 143 Next: <a href="Allocating-aligned-memory-in-Fortran.html#Allocating-aligned-memory-in-Fortran" accesskey="n" rel="next">Allocating aligned memory in Fortran</a>, Previous: <a href="FFTW-Fortran-type-reference.html#FFTW-Fortran-type-reference" accesskey="p" rel="prev">FFTW Fortran type reference</a>, Up: <a href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran" accesskey="u" rel="up">Calling FFTW from Modern 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@167 144 </div>
cannam@167 145
cannam@167 146
cannam@167 147
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