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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
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Chris@42 6 Copyright (C) 2003 Matteo Frigo.
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Chris@42 24 <head>
Chris@42 25 <title>FFTW 3.3.5: Plan execution in Fortran</title>
Chris@42 26
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Chris@42 70
Chris@42 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
Chris@42 72 <a name="Plan-execution-in-Fortran"></a>
Chris@42 73 <div class="header">
Chris@42 74 <p>
Chris@42 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>
Chris@42 76 </div>
Chris@42 77 <hr>
Chris@42 78 <a name="Plan-execution-in-Fortran-1"></a>
Chris@42 79 <h3 class="section">7.4 Plan 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 modern Fortran is:
Chris@42 85 </p><div class="example">
Chris@42 86 <pre class="example"> call fftw_execute(plan)
Chris@42 87 </pre></div>
Chris@42 88 <a name="index-fftw_005fexecute-6"></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>fftw_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>fftw_execute</code>. As a consequence, certain compilers end up
Chris@42 96 repositioning the call to <code>fftw_execute</code>, assuming incorrectly
Chris@42 97 that it does nothing to the arrays.
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>fftw_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 fftw_execute_dft(plan, in, out)
Chris@42 107 </pre></div>
Chris@42 108 <a name="index-fftw_005fexecute_005fdft-3"></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> <a name="index-fftw_005fexecute_005fdft_005fr2c-2"></a>
Chris@42 114 <a name="index-fftw_005fexecute_005fdft_005fc2r-1"></a>
Chris@42 115 <a name="index-fftw_005fexecute_005fr2r-1"></a>
Chris@42 116 You must use the correct type of execute function, matching the way
Chris@42 117 the plan was created. Complex DFT plans should use
Chris@42 118 <code>fftw_execute_dft</code>, Real-input (r2c) DFT plans should use use
Chris@42 119 <code>fftw_execute_dft_r2c</code>, and real-output (c2r) DFT plans should
Chris@42 120 use <code>fftw_execute_dft_c2r</code>. The various r2r plans should use
Chris@42 121 <code>fftw_execute_r2r</code>. Fortunately, if you use the wrong one you
Chris@42 122 will get a compile-time type-mismatch error (unlike legacy Fortran).
Chris@42 123
Chris@42 124 </li><li> You should normally pass the same input/output arrays that were used when
Chris@42 125 creating the plan. This is always safe.
Chris@42 126
Chris@42 127 </li><li> <em>If</em> you pass <em>different</em> input/output arrays compared to
Chris@42 128 those used when creating the plan, you must abide by all the
Chris@42 129 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
Chris@42 130 requirement that the new arrays have the same alignment as the
Chris@42 131 original arrays; the best (and possibly only) way to guarantee this
Chris@42 132 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
Chris@42 133 use the <code>FFTW_UNALIGNED</code> flag when creating the
Chris@42 134 plan, in which case the plan does not depend on the alignment, but
Chris@42 135 this may sacrifice substantial performance on architectures (like x86)
Chris@42 136 with SIMD instructions (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>).
Chris@42 137 <a name="index-FFTW_005fUNALIGNED-2"></a>
Chris@42 138
Chris@42 139 </li></ul>
Chris@42 140
Chris@42 141 <hr>
Chris@42 142 <div class="header">
Chris@42 143 <p>
Chris@42 144 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>
Chris@42 145 </div>
Chris@42 146
Chris@42 147
Chris@42 148
Chris@42 149 </body>
Chris@42 150 </html>