Mercurial > hg > sv-dependency-builds

comparison src/fftw-3.3.5/doc/html/FFTW-Execution-in-Fortran.html @ 42:2cd0e3b3e1fd

Find changesets by keywords (author, files, the commit message), revision number or hash, or revset expression.
Current fftw source
author Chris Cannam
date Tue, 18 Oct 2016 13:40:26 +0100
parents
children
comparison
equal deleted inserted replaced
41:481f5f8c5634 42:2cd0e3b3e1fd
1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
2 <html>
3 <!-- This manual is for FFTW
4 (version 3.3.5, 30 July 2016).
5
6 Copyright (C) 2003 Matteo Frigo.
7
8 Copyright (C) 2003 Massachusetts Institute of Technology.
9
10 Permission is granted to make and distribute verbatim copies of this
11 manual provided the copyright notice and this permission notice are
12 preserved on all copies.
13
14 Permission is granted to copy and distribute modified versions of this
15 manual under the conditions for verbatim copying, provided that the
16 entire resulting derived work is distributed under the terms of a
17 permission notice identical to this one.
18
19 Permission is granted to copy and distribute translations of this manual
20 into another language, under the above conditions for modified versions,
21 except that this permission notice may be stated in a translation
22 approved by the Free Software Foundation. -->
23 <!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
24 <head>
25 <title>FFTW 3.3.5: FFTW Execution in Fortran</title>
26
27 <meta name="description" content="FFTW 3.3.5: FFTW Execution in Fortran">
28 <meta name="keywords" content="FFTW 3.3.5: FFTW Execution in Fortran">
29 <meta name="resource-type" content="document">
30 <meta name="distribution" content="global">
31 <meta name="Generator" content="makeinfo">
32 <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
33 <link href="index.html#Top" rel="start" title="Top">
34 <link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
35 <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
36 <link href="Calling-FFTW-from-Legacy-Fortran.html#Calling-FFTW-from-Legacy-Fortran" rel="up" title="Calling FFTW from Legacy Fortran">
37 <link href="Fortran-Examples.html#Fortran-Examples" rel="next" title="Fortran Examples">
38 <link href="FFTW-Constants-in-Fortran.html#FFTW-Constants-in-Fortran" rel="prev" title="FFTW Constants in Fortran">
39 <style type="text/css">
40 <!--
41 a.summary-letter {text-decoration: none}
42 blockquote.smallquotation {font-size: smaller}
43 div.display {margin-left: 3.2em}
44 div.example {margin-left: 3.2em}
45 div.indentedblock {margin-left: 3.2em}
46 div.lisp {margin-left: 3.2em}
47 div.smalldisplay {margin-left: 3.2em}
48 div.smallexample {margin-left: 3.2em}
49 div.smallindentedblock {margin-left: 3.2em; font-size: smaller}
50 div.smalllisp {margin-left: 3.2em}
51 kbd {font-style:oblique}
52 pre.display {font-family: inherit}
53 pre.format {font-family: inherit}
54 pre.menu-comment {font-family: serif}
55 pre.menu-preformatted {font-family: serif}
56 pre.smalldisplay {font-family: inherit; font-size: smaller}
57 pre.smallexample {font-size: smaller}
58 pre.smallformat {font-family: inherit; font-size: smaller}
59 pre.smalllisp {font-size: smaller}
60 span.nocodebreak {white-space:nowrap}
61 span.nolinebreak {white-space:nowrap}
62 span.roman {font-family:serif; font-weight:normal}
63 span.sansserif {font-family:sans-serif; font-weight:normal}
64 ul.no-bullet {list-style: none}
65 -->
66 </style>
67
68
69 </head>
70
71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
72 <a name="FFTW-Execution-in-Fortran"></a>
73 <div class="header">
74 <p>
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>
76 </div>
77 <hr>
78 <a name="FFTW-Execution-in-Fortran-1"></a>
79 <h3 class="section">8.3 FFTW Execution in Fortran</h3>
80
81 <p>In C, in order to use a plan, one normally calls <code>fftw_execute</code>,
82 which executes the plan to perform the transform on the input/output
83 arrays passed when the plan was created (see <a href="Using-Plans.html#Using-Plans">Using Plans</a>). The
84 corresponding subroutine call in legacy Fortran is:
85 </p><div class="example">
86 <pre class="example"> call dfftw_execute(plan)
87 </pre></div>
88 <a name="index-dfftw_005fexecute"></a>
89
90 <p>However, we have had reports that this causes problems with some
91 recent optimizing Fortran compilers. The problem is, because the
92 input/output arrays are not passed as explicit arguments to
93 <code>dfftw_execute</code>, the semantics of Fortran (unlike C) allow the
94 compiler to assume that the input/output arrays are not changed by
95 <code>dfftw_execute</code>. As a consequence, certain compilers end up
96 optimizing out or repositioning the call to <code>dfftw_execute</code>,
97 assuming incorrectly that it does nothing.
98 </p>
99 <p>There are various workarounds to this, but the safest and simplest
100 thing is to not use <code>dfftw_execute</code> in Fortran. Instead, use the
101 functions described in <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>, which take
102 the input/output arrays as explicit arguments. For example, if the
103 plan is for a complex-data DFT and was created for the arrays
104 <code>in</code> and <code>out</code>, you would do:
105 </p><div class="example">
106 <pre class="example"> call dfftw_execute_dft(plan, in, out)
107 </pre></div>
108 <a name="index-dfftw_005fexecute_005fdft"></a>
109
110 <p>There are a few things to be careful of, however:
111 </p>
112 <ul>
113 <li> You must use the correct type of execute function, matching the way
114 the plan was created. Complex DFT plans should use
115 <code>dfftw_execute_dft</code>, Real-input (r2c) DFT plans should use use
116 <code>dfftw_execute_dft_r2c</code>, and real-output (c2r) DFT plans should
117 use <code>dfftw_execute_dft_c2r</code>. The various r2r plans should use
118 <code>dfftw_execute_r2r</code>.
119
120 </li><li> You should normally pass the same input/output arrays that were used when
121 creating the plan. This is always safe.
122
123 </li><li> <em>If</em> you pass <em>different</em> input/output arrays compared to
124 those used when creating the plan, you must abide by all the
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
126 requirement that the new arrays have the same alignment as the
127 original arrays, because there seems to be no way in legacy Fortran to obtain
128 guaranteed-aligned arrays (analogous to <code>fftw_malloc</code> in C). You
129 can, of course, use the <code>FFTW_UNALIGNED</code> flag when creating the
130 plan, in which case the plan does not depend on the alignment, but
131 this may sacrifice substantial performance on architectures (like x86)
132 with SIMD instructions (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>).
133 <a name="index-FFTW_005fUNALIGNED-3"></a>
134
135 </li></ul>
136
137 <hr>
138 <div class="header">
139 <p>
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>
141 </div>
142
143
144
145 </body>
146 </html>