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