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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 d0c2a83c1364
children
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Chris@82 24 <head>
Chris@82 25 <title>FFTW 3.3.8: Plan execution in Fortran</title>
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Chris@82 69
Chris@82 70 <body lang="en">
Chris@82 71 <a name="Plan-execution-in-Fortran"></a>
Chris@82 72 <div class="header">
Chris@82 73 <p>
Chris@82 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>
Chris@82 75 </div>
Chris@82 76 <hr>
Chris@82 77 <a name="Plan-execution-in-Fortran-1"></a>
Chris@82 78 <h3 class="section">7.4 Plan execution in Fortran</h3>
Chris@82 79
Chris@82 80 <p>In C, in order to use a plan, one normally calls <code>fftw_execute</code>,
Chris@82 81 which executes the plan to perform the transform on the input/output
Chris@82 82 arrays passed when the plan was created (see <a href="Using-Plans.html#Using-Plans">Using Plans</a>). The
Chris@82 83 corresponding subroutine call in modern Fortran is:
Chris@82 84 </p><div class="example">
Chris@82 85 <pre class="example"> call fftw_execute(plan)
Chris@82 86 </pre></div>
Chris@82 87 <a name="index-fftw_005fexecute-6"></a>
Chris@82 88
Chris@82 89 <p>However, we have had reports that this causes problems with some
Chris@82 90 recent optimizing Fortran compilers. The problem is, because the
Chris@82 91 input/output arrays are not passed as explicit arguments to
Chris@82 92 <code>fftw_execute</code>, the semantics of Fortran (unlike C) allow the
Chris@82 93 compiler to assume that the input/output arrays are not changed by
Chris@82 94 <code>fftw_execute</code>. As a consequence, certain compilers end up
Chris@82 95 repositioning the call to <code>fftw_execute</code>, assuming incorrectly
Chris@82 96 that it does nothing to the arrays.
Chris@82 97 </p>
Chris@82 98 <p>There are various workarounds to this, but the safest and simplest
Chris@82 99 thing is to not use <code>fftw_execute</code> in Fortran. Instead, use the
Chris@82 100 functions described in <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>, which take
Chris@82 101 the input/output arrays as explicit arguments. For example, if the
Chris@82 102 plan is for a complex-data DFT and was created for the arrays
Chris@82 103 <code>in</code> and <code>out</code>, you would do:
Chris@82 104 </p><div class="example">
Chris@82 105 <pre class="example"> call fftw_execute_dft(plan, in, out)
Chris@82 106 </pre></div>
Chris@82 107 <a name="index-fftw_005fexecute_005fdft-3"></a>
Chris@82 108
Chris@82 109 <p>There are a few things to be careful of, however:
Chris@82 110 </p>
Chris@82 111 <ul>
Chris@82 112 <li> <a name="index-fftw_005fexecute_005fdft_005fr2c-2"></a>
Chris@82 113 <a name="index-fftw_005fexecute_005fdft_005fc2r-1"></a>
Chris@82 114 <a name="index-fftw_005fexecute_005fr2r-1"></a>
Chris@82 115 You must use the correct type of execute function, matching the way
Chris@82 116 the plan was created. Complex DFT plans should use
Chris@82 117 <code>fftw_execute_dft</code>, Real-input (r2c) DFT plans should use use
Chris@82 118 <code>fftw_execute_dft_r2c</code>, and real-output (c2r) DFT plans should
Chris@82 119 use <code>fftw_execute_dft_c2r</code>. The various r2r plans should use
Chris@82 120 <code>fftw_execute_r2r</code>. Fortunately, if you use the wrong one you
Chris@82 121 will get a compile-time type-mismatch error (unlike legacy Fortran).
Chris@82 122
Chris@82 123 </li><li> You should normally pass the same input/output arrays that were used when
Chris@82 124 creating the plan. This is always safe.
Chris@82 125
Chris@82 126 </li><li> <em>If</em> you pass <em>different</em> input/output arrays compared to
Chris@82 127 those used when creating the plan, you must abide by all the
Chris@82 128 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
Chris@82 129 requirement that the new arrays have the same alignment as the
Chris@82 130 original arrays; the best (and possibly only) way to guarantee this
Chris@82 131 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
Chris@82 132 use the <code>FFTW_UNALIGNED</code> flag when creating the
Chris@82 133 plan, in which case the plan does not depend on the alignment, but
Chris@82 134 this may sacrifice substantial performance on architectures (like x86)
Chris@82 135 with SIMD instructions (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>).
Chris@82 136 <a name="index-FFTW_005fUNALIGNED-2"></a>
Chris@82 137
Chris@82 138 </li></ul>
Chris@82 139
Chris@82 140 <hr>
Chris@82 141 <div class="header">
Chris@82 142 <p>
Chris@82 143 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>
Chris@82 144 </div>
Chris@82 145
Chris@82 146
Chris@82 147
Chris@82 148 </body>
Chris@82 149 </html>