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Current fftw source
author Chris Cannam <cannam@all-day-breakfast.com>
date Tue, 18 Oct 2016 13:40:26 +0100
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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.
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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: Plan execution in Fortran</title>
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cannam@127 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
cannam@127 72 <a name="Plan-execution-in-Fortran"></a>
cannam@127 73 <div class="header">
cannam@127 74 <p>
cannam@127 75 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@127 76 </div>
cannam@127 77 <hr>
cannam@127 78 <a name="Plan-execution-in-Fortran-1"></a>
cannam@127 79 <h3 class="section">7.4 Plan 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 modern Fortran is:
cannam@127 85 </p><div class="example">
cannam@127 86 <pre class="example"> call fftw_execute(plan)
cannam@127 87 </pre></div>
cannam@127 88 <a name="index-fftw_005fexecute-6"></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>fftw_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>fftw_execute</code>. As a consequence, certain compilers end up
cannam@127 96 repositioning the call to <code>fftw_execute</code>, assuming incorrectly
cannam@127 97 that it does nothing to the arrays.
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>fftw_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 fftw_execute_dft(plan, in, out)
cannam@127 107 </pre></div>
cannam@127 108 <a name="index-fftw_005fexecute_005fdft-3"></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> <a name="index-fftw_005fexecute_005fdft_005fr2c-2"></a>
cannam@127 114 <a name="index-fftw_005fexecute_005fdft_005fc2r-1"></a>
cannam@127 115 <a name="index-fftw_005fexecute_005fr2r-1"></a>
cannam@127 116 You must use the correct type of execute function, matching the way
cannam@127 117 the plan was created. Complex DFT plans should use
cannam@127 118 <code>fftw_execute_dft</code>, Real-input (r2c) DFT plans should use use
cannam@127 119 <code>fftw_execute_dft_r2c</code>, and real-output (c2r) DFT plans should
cannam@127 120 use <code>fftw_execute_dft_c2r</code>. The various r2r plans should use
cannam@127 121 <code>fftw_execute_r2r</code>. Fortunately, if you use the wrong one you
cannam@127 122 will get a compile-time type-mismatch error (unlike legacy Fortran).
cannam@127 123
cannam@127 124 </li><li> You should normally pass the same input/output arrays that were used when
cannam@127 125 creating the plan. This is always safe.
cannam@127 126
cannam@127 127 </li><li> <em>If</em> you pass <em>different</em> input/output arrays compared to
cannam@127 128 those used when creating the plan, you must abide by all the
cannam@127 129 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@127 130 requirement that the new arrays have the same alignment as the
cannam@127 131 original arrays; the best (and possibly only) way to guarantee this
cannam@127 132 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@127 133 use the <code>FFTW_UNALIGNED</code> flag when creating the
cannam@127 134 plan, in which case the plan does not depend on the alignment, but
cannam@127 135 this may sacrifice substantial performance on architectures (like x86)
cannam@127 136 with SIMD instructions (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>).
cannam@127 137 <a name="index-FFTW_005fUNALIGNED-2"></a>
cannam@127 138
cannam@127 139 </li></ul>
cannam@127 140
cannam@127 141 <hr>
cannam@127 142 <div class="header">
cannam@127 143 <p>
cannam@127 144 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@127 145 </div>
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