Mercurial > hg > sv-dependency-builds

annotate src/fftw-3.3.5/doc/html/FFTW-Execution-in-Fortran.html @ 83:ae30d91d2ffe

Find changesets by keywords (author, files, the commit message), revision number or hash, or revset expression.
Replace these with versions built using an older toolset (so as to avoid ABI compatibilities when linking on Ubuntu 14.04 for packaging purposes)
author Chris Cannam
date Fri, 07 Feb 2020 11:51:13 +0000
parents 2cd0e3b3e1fd
children
rev   line source
Chris@42 1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
Chris@42 2 <html>
Chris@42 3 <!-- This manual is for FFTW
Chris@42 4 (version 3.3.5, 30 July 2016).
Chris@42 5
Chris@42 6 Copyright (C) 2003 Matteo Frigo.
Chris@42 7
Chris@42 8 Copyright (C) 2003 Massachusetts Institute of Technology.
Chris@42 9
Chris@42 10 Permission is granted to make and distribute verbatim copies of this
Chris@42 11 manual provided the copyright notice and this permission notice are
Chris@42 12 preserved on all copies.
Chris@42 13
Chris@42 14 Permission is granted to copy and distribute modified versions of this
Chris@42 15 manual under the conditions for verbatim copying, provided that the
Chris@42 16 entire resulting derived work is distributed under the terms of a
Chris@42 17 permission notice identical to this one.
Chris@42 18
Chris@42 19 Permission is granted to copy and distribute translations of this manual
Chris@42 20 into another language, under the above conditions for modified versions,
Chris@42 21 except that this permission notice may be stated in a translation
Chris@42 22 approved by the Free Software Foundation. -->
Chris@42 23 <!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
Chris@42 24 <head>
Chris@42 25 <title>FFTW 3.3.5: FFTW Execution in Fortran</title>
Chris@42 26
Chris@42 27 <meta name="description" content="FFTW 3.3.5: FFTW Execution in Fortran">
Chris@42 28 <meta name="keywords" content="FFTW 3.3.5: FFTW Execution in Fortran">
Chris@42 29 <meta name="resource-type" content="document">
Chris@42 30 <meta name="distribution" content="global">
Chris@42 31 <meta name="Generator" content="makeinfo">
Chris@42 32 <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
Chris@42 33 <link href="index.html#Top" rel="start" title="Top">
Chris@42 34 <link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
Chris@42 35 <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
Chris@42 36 <link href="Calling-FFTW-from-Legacy-Fortran.html#Calling-FFTW-from-Legacy-Fortran" rel="up" title="Calling FFTW from Legacy Fortran">
Chris@42 37 <link href="Fortran-Examples.html#Fortran-Examples" rel="next" title="Fortran Examples">
Chris@42 38 <link href="FFTW-Constants-in-Fortran.html#FFTW-Constants-in-Fortran" rel="prev" title="FFTW Constants in Fortran">
Chris@42 39 <style type="text/css">
Chris@42 40 <!--
Chris@42 41 a.summary-letter {text-decoration: none}
Chris@42 42 blockquote.smallquotation {font-size: smaller}
Chris@42 43 div.display {margin-left: 3.2em}
Chris@42 44 div.example {margin-left: 3.2em}
Chris@42 45 div.indentedblock {margin-left: 3.2em}
Chris@42 46 div.lisp {margin-left: 3.2em}
Chris@42 47 div.smalldisplay {margin-left: 3.2em}
Chris@42 48 div.smallexample {margin-left: 3.2em}
Chris@42 49 div.smallindentedblock {margin-left: 3.2em; font-size: smaller}
Chris@42 50 div.smalllisp {margin-left: 3.2em}
Chris@42 51 kbd {font-style:oblique}
Chris@42 52 pre.display {font-family: inherit}
Chris@42 53 pre.format {font-family: inherit}
Chris@42 54 pre.menu-comment {font-family: serif}
Chris@42 55 pre.menu-preformatted {font-family: serif}
Chris@42 56 pre.smalldisplay {font-family: inherit; font-size: smaller}
Chris@42 57 pre.smallexample {font-size: smaller}
Chris@42 58 pre.smallformat {font-family: inherit; font-size: smaller}
Chris@42 59 pre.smalllisp {font-size: smaller}
Chris@42 60 span.nocodebreak {white-space:nowrap}
Chris@42 61 span.nolinebreak {white-space:nowrap}
Chris@42 62 span.roman {font-family:serif; font-weight:normal}
Chris@42 63 span.sansserif {font-family:sans-serif; font-weight:normal}
Chris@42 64 ul.no-bullet {list-style: none}
Chris@42 65 -->
Chris@42 66 </style>
Chris@42 67
Chris@42 68
Chris@42 69 </head>
Chris@42 70
Chris@42 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
Chris@42 72 <a name="FFTW-Execution-in-Fortran"></a>
Chris@42 73 <div class="header">
Chris@42 74 <p>
Chris@42 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>
Chris@42 76 </div>
Chris@42 77 <hr>
Chris@42 78 <a name="FFTW-Execution-in-Fortran-1"></a>
Chris@42 79 <h3 class="section">8.3 FFTW Execution in Fortran</h3>
Chris@42 80
Chris@42 81 <p>In C, in order to use a plan, one normally calls <code>fftw_execute</code>,
Chris@42 82 which executes the plan to perform the transform on the input/output
Chris@42 83 arrays passed when the plan was created (see <a href="Using-Plans.html#Using-Plans">Using Plans</a>). The
Chris@42 84 corresponding subroutine call in legacy Fortran is:
Chris@42 85 </p><div class="example">
Chris@42 86 <pre class="example"> call dfftw_execute(plan)
Chris@42 87 </pre></div>
Chris@42 88 <a name="index-dfftw_005fexecute"></a>
Chris@42 89
Chris@42 90 <p>However, we have had reports that this causes problems with some
Chris@42 91 recent optimizing Fortran compilers. The problem is, because the
Chris@42 92 input/output arrays are not passed as explicit arguments to
Chris@42 93 <code>dfftw_execute</code>, the semantics of Fortran (unlike C) allow the
Chris@42 94 compiler to assume that the input/output arrays are not changed by
Chris@42 95 <code>dfftw_execute</code>. As a consequence, certain compilers end up
Chris@42 96 optimizing out or repositioning the call to <code>dfftw_execute</code>,
Chris@42 97 assuming incorrectly that it does nothing.
Chris@42 98 </p>
Chris@42 99 <p>There are various workarounds to this, but the safest and simplest
Chris@42 100 thing is to not use <code>dfftw_execute</code> in Fortran. Instead, use the
Chris@42 101 functions described in <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>, which take
Chris@42 102 the input/output arrays as explicit arguments. For example, if the
Chris@42 103 plan is for a complex-data DFT and was created for the arrays
Chris@42 104 <code>in</code> and <code>out</code>, you would do:
Chris@42 105 </p><div class="example">
Chris@42 106 <pre class="example"> call dfftw_execute_dft(plan, in, out)
Chris@42 107 </pre></div>
Chris@42 108 <a name="index-dfftw_005fexecute_005fdft"></a>
Chris@42 109
Chris@42 110 <p>There are a few things to be careful of, however:
Chris@42 111 </p>
Chris@42 112 <ul>
Chris@42 113 <li> You must use the correct type of execute function, matching the way
Chris@42 114 the plan was created. Complex DFT plans should use
Chris@42 115 <code>dfftw_execute_dft</code>, Real-input (r2c) DFT plans should use use
Chris@42 116 <code>dfftw_execute_dft_r2c</code>, and real-output (c2r) DFT plans should
Chris@42 117 use <code>dfftw_execute_dft_c2r</code>. The various r2r plans should use
Chris@42 118 <code>dfftw_execute_r2r</code>.
Chris@42 119
Chris@42 120 </li><li> You should normally pass the same input/output arrays that were used when
Chris@42 121 creating the plan. This is always safe.
Chris@42 122
Chris@42 123 </li><li> <em>If</em> you pass <em>different</em> input/output arrays compared to
Chris@42 124 those used when creating the plan, you must abide by all the
Chris@42 125 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
Chris@42 126 requirement that the new arrays have the same alignment as the
Chris@42 127 original arrays, because there seems to be no way in legacy Fortran to obtain
Chris@42 128 guaranteed-aligned arrays (analogous to <code>fftw_malloc</code> in C). You
Chris@42 129 can, of course, use the <code>FFTW_UNALIGNED</code> flag when creating the
Chris@42 130 plan, in which case the plan does not depend on the alignment, but
Chris@42 131 this may sacrifice substantial performance on architectures (like x86)
Chris@42 132 with SIMD instructions (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>).
Chris@42 133 <a name="index-FFTW_005fUNALIGNED-3"></a>
Chris@42 134
Chris@42 135 </li></ul>
Chris@42 136
Chris@42 137 <hr>
Chris@42 138 <div class="header">
Chris@42 139 <p>
Chris@42 140 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>
Chris@42 141 </div>
Chris@42 142
Chris@42 143
Chris@42 144
Chris@42 145 </body>
Chris@42 146 </html>