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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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<title>Plan execution in Fortran (FFTW 3.3.10)</title>
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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-Modern-Fortran.html" rel="up" title="Calling FFTW from Modern Fortran">
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<span id="Plan-execution-in-Fortran"></span><div class="header">
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<p>
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Next: <a href="Allocating-aligned-memory-in-Fortran.html" accesskey="n" rel="next">Allocating aligned memory in Fortran</a>, Previous: <a href="FFTW-Fortran-type-reference.html" accesskey="p" rel="prev">FFTW Fortran type reference</a>, Up: <a href="Calling-FFTW-from-Modern-Fortran.html" accesskey="u" rel="up">Calling FFTW from Modern 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="Plan-execution-in-Fortran-1"></span><h3 class="section">7.4 Plan 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 modern Fortran is:
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</p><div class="example">
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<pre class="example"> call fftw_execute(plan)
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</pre></div>
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<span id="index-fftw_005fexecute-6"></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>fftw_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>fftw_execute</code>. As a consequence, certain compilers end up
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repositioning the call to <code>fftw_execute</code>, assuming incorrectly
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that it does nothing to the arrays.
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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>fftw_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 fftw_execute_dft(plan, in, out)
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</pre></div>
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<span id="index-fftw_005fexecute_005fdft-3"></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> <span id="index-fftw_005fexecute_005fdft_005fr2c-2"></span>
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<span id="index-fftw_005fexecute_005fdft_005fc2r-1"></span>
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<span id="index-fftw_005fexecute_005fr2r-1"></span>
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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>fftw_execute_dft</code>, Real-input (r2c) DFT plans should use use
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<code>fftw_execute_dft_r2c</code>, and real-output (c2r) DFT plans should
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use <code>fftw_execute_dft_c2r</code>. The various r2r plans should use
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<code>fftw_execute_r2r</code>. Fortunately, if you use the wrong one you
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will get a compile-time type-mismatch error (unlike legacy Fortran).
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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 tricky of these is the
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requirement that the new arrays have the same alignment as the
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original arrays; the best (and possibly only) way to guarantee this
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is to use the ‘<samp>fftw_alloc</samp>’ functions to allocate your arrays (see <a href="Allocating-aligned-memory-in-Fortran.html">Allocating aligned memory in Fortran</a>). Alternatively, you can
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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-2"></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="Allocating-aligned-memory-in-Fortran.html" accesskey="n" rel="next">Allocating aligned memory in Fortran</a>, Previous: <a href="FFTW-Fortran-type-reference.html" accesskey="p" rel="prev">FFTW Fortran type reference</a>, Up: <a href="Calling-FFTW-from-Modern-Fortran.html" accesskey="u" rel="up">Calling FFTW from Modern 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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