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6.6 KiB
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<!-- This manual is for FFTW
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(version 3.3.10, 10 December 2020).
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Copyright (C) 2003 Matteo Frigo.
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Copyright (C) 2003 Massachusetts Institute of Technology.
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Permission is granted to make and distribute verbatim copies of this
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manual provided the copyright notice and this permission notice are
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Permission is granted to copy and distribute modified versions of this
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entire resulting derived work is distributed under the terms of a
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<title>FFTW Execution in Fortran (FFTW 3.3.10)</title>
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<link href="index.html" rel="start" title="Top">
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Calling-FFTW-from-Legacy-Fortran.html" rel="up" title="Calling FFTW from Legacy Fortran">
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<link href="Fortran-Examples.html" rel="next" title="Fortran Examples">
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<span id="FFTW-Execution-in-Fortran"></span><div class="header">
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<p>
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Next: <a href="Fortran-Examples.html" accesskey="n" rel="next">Fortran Examples</a>, Previous: <a href="FFTW-Constants-in-Fortran.html" accesskey="p" rel="prev">FFTW Constants in Fortran</a>, Up: <a href="Calling-FFTW-from-Legacy-Fortran.html" accesskey="u" rel="up">Calling FFTW from Legacy Fortran</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<span id="FFTW-Execution-in-Fortran-1"></span><h3 class="section">8.3 FFTW Execution in Fortran</h3>
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<p>In C, in order to use a plan, one normally calls <code>fftw_execute</code>,
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which executes the plan to perform the transform on the input/output
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arrays passed when the plan was created (see <a href="Using-Plans.html">Using Plans</a>). The
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corresponding subroutine call in legacy Fortran is:
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</p><div class="example">
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<pre class="example"> call dfftw_execute(plan)
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</pre></div>
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<span id="index-dfftw_005fexecute"></span>
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<p>However, we have had reports that this causes problems with some
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recent optimizing Fortran compilers. The problem is, because the
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input/output arrays are not passed as explicit arguments to
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<code>dfftw_execute</code>, the semantics of Fortran (unlike C) allow the
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compiler to assume that the input/output arrays are not changed by
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<code>dfftw_execute</code>. As a consequence, certain compilers end up
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optimizing out or repositioning the call to <code>dfftw_execute</code>,
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assuming incorrectly that it does nothing.
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</p>
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<p>There are various workarounds to this, but the safest and simplest
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thing is to not use <code>dfftw_execute</code> in Fortran. Instead, use the
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functions described in <a href="New_002darray-Execute-Functions.html">New-array Execute Functions</a>, which take
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the input/output arrays as explicit arguments. For example, if the
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plan is for a complex-data DFT and was created for the arrays
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<code>in</code> and <code>out</code>, you would do:
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</p><div class="example">
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<pre class="example"> call dfftw_execute_dft(plan, in, out)
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</pre></div>
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<span id="index-dfftw_005fexecute_005fdft"></span>
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<p>There are a few things to be careful of, however:
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</p>
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<ul>
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<li> You must use the correct type of execute function, matching the way
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the plan was created. Complex DFT plans should use
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<code>dfftw_execute_dft</code>, Real-input (r2c) DFT plans should use use
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<code>dfftw_execute_dft_r2c</code>, and real-output (c2r) DFT plans should
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use <code>dfftw_execute_dft_c2r</code>. The various r2r plans should use
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<code>dfftw_execute_r2r</code>.
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</li><li> You should normally pass the same input/output arrays that were used when
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creating the plan. This is always safe.
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</li><li> <em>If</em> you pass <em>different</em> input/output arrays compared to
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those used when creating the plan, you must abide by all the
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restrictions of the new-array execute functions (see <a href="New_002darray-Execute-Functions.html">New-array Execute Functions</a>). The most difficult of these, in Fortran, is the
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requirement that the new arrays have the same alignment as the
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original arrays, because there seems to be no way in legacy Fortran to obtain
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guaranteed-aligned arrays (analogous to <code>fftw_malloc</code> in C). You
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can, of course, use the <code>FFTW_UNALIGNED</code> flag when creating the
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plan, in which case the plan does not depend on the alignment, but
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this may sacrifice substantial performance on architectures (like x86)
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with SIMD instructions (see <a href="SIMD-alignment-and-fftw_005fmalloc.html">SIMD alignment and fftw_malloc</a>).
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<span id="index-FFTW_005fUNALIGNED-3"></span>
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</li></ul>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Fortran-Examples.html" accesskey="n" rel="next">Fortran Examples</a>, Previous: <a href="FFTW-Constants-in-Fortran.html" accesskey="p" rel="prev">FFTW Constants in Fortran</a>, Up: <a href="Calling-FFTW-from-Legacy-Fortran.html" accesskey="u" rel="up">Calling FFTW from Legacy Fortran</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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