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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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6 Copyright (C) 2003 Matteo Frigo.
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25 <title>FFTW 3.3.8: Plan execution in Fortran</title>
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71 <a name="Plan-execution-in-Fortran"></a>
72 <div class="header">
73 <p>
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>
75 </div>
76 <hr>
77 <a name="Plan-execution-in-Fortran-1"></a>
78 <h3 class="section">7.4 Plan 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 modern Fortran is:
84 </p><div class="example">
85 <pre class="example"> call fftw_execute(plan)
86 </pre></div>
87 <a name="index-fftw_005fexecute-6"></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>fftw_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>fftw_execute</code>. As a consequence, certain compilers end up
95 repositioning the call to <code>fftw_execute</code>, assuming incorrectly
96 that it does nothing to the arrays.
97 </p>
98 <p>There are various workarounds to this, but the safest and simplest
99 thing is to not use <code>fftw_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 fftw_execute_dft(plan, in, out)
106 </pre></div>
107 <a name="index-fftw_005fexecute_005fdft-3"></a>
108
109 <p>There are a few things to be careful of, however:
110 </p>
111 <ul>
112 <li> <a name="index-fftw_005fexecute_005fdft_005fr2c-2"></a>
113 <a name="index-fftw_005fexecute_005fdft_005fc2r-1"></a>
114 <a name="index-fftw_005fexecute_005fr2r-1"></a>
115 You must use the correct type of execute function, matching the way
116 the plan was created. Complex DFT plans should use
117 <code>fftw_execute_dft</code>, Real-input (r2c) DFT plans should use use
118 <code>fftw_execute_dft_r2c</code>, and real-output (c2r) DFT plans should
119 use <code>fftw_execute_dft_c2r</code>. The various r2r plans should use
120 <code>fftw_execute_r2r</code>. Fortunately, if you use the wrong one you
121 will get a compile-time type-mismatch error (unlike legacy Fortran).
122
123 </li><li> You should normally pass the same input/output arrays that were used when
124 creating the plan. This is always safe.
125
126 </li><li> <em>If</em> you pass <em>different</em> input/output arrays compared to
127 those used when creating the plan, you must abide by all the
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
129 requirement that the new arrays have the same alignment as the
130 original arrays; the best (and possibly only) way to guarantee this
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
132 use the <code>FFTW_UNALIGNED</code> flag when creating the
133 plan, in which case the plan does not depend on the alignment, but
134 this may sacrifice substantial performance on architectures (like x86)
135 with SIMD instructions (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>).
136 <a name="index-FFTW_005fUNALIGNED-2"></a>
137
138 </li></ul>
139
140 <hr>
141 <div class="header">
142 <p>
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>
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