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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 37bf6b4a2645
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Chris@10 1 <html lang="en">
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Chris@10 3 <title>FFTW Execution in Fortran - FFTW 3.3.3</title>
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Chris@10 13 This manual is for FFTW
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Chris@10 48 <div class="node">
Chris@10 49 <a name="FFTW-Execution-in-Fortran"></a>
Chris@10 50 <p>
Chris@10 51 Next:&nbsp;<a rel="next" accesskey="n" href="Fortran-Examples.html#Fortran-Examples">Fortran Examples</a>,
Chris@10 52 Previous:&nbsp;<a rel="previous" accesskey="p" href="FFTW-Constants-in-Fortran.html#FFTW-Constants-in-Fortran">FFTW Constants in Fortran</a>,
Chris@10 53 Up:&nbsp;<a rel="up" accesskey="u" href="Calling-FFTW-from-Legacy-Fortran.html#Calling-FFTW-from-Legacy-Fortran">Calling FFTW from Legacy Fortran</a>
Chris@10 54 <hr>
Chris@10 55 </div>
Chris@10 56
Chris@10 57 <h3 class="section">8.3 FFTW Execution in Fortran</h3>
Chris@10 58
Chris@10 59 <p>In C, in order to use a plan, one normally calls <code>fftw_execute</code>,
Chris@10 60 which executes the plan to perform the transform on the input/output
Chris@10 61 arrays passed when the plan was created (see <a href="Using-Plans.html#Using-Plans">Using Plans</a>). The
Chris@10 62 corresponding subroutine call in legacy Fortran is:
Chris@10 63 <pre class="example"> call dfftw_execute(plan)
Chris@10 64 </pre>
Chris@10 65 <p><a name="index-dfftw_005fexecute-584"></a>
Chris@10 66 However, we have had reports that this causes problems with some
Chris@10 67 recent optimizing Fortran compilers. The problem is, because the
Chris@10 68 input/output arrays are not passed as explicit arguments to
Chris@10 69 <code>dfftw_execute</code>, the semantics of Fortran (unlike C) allow the
Chris@10 70 compiler to assume that the input/output arrays are not changed by
Chris@10 71 <code>dfftw_execute</code>. As a consequence, certain compilers end up
Chris@10 72 optimizing out or repositioning the call to <code>dfftw_execute</code>,
Chris@10 73 assuming incorrectly that it does nothing.
Chris@10 74
Chris@10 75 <p>There are various workarounds to this, but the safest and simplest
Chris@10 76 thing is to not use <code>dfftw_execute</code> in Fortran. Instead, use the
Chris@10 77 functions described in <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>, which take
Chris@10 78 the input/output arrays as explicit arguments. For example, if the
Chris@10 79 plan is for a complex-data DFT and was created for the arrays
Chris@10 80 <code>in</code> and <code>out</code>, you would do:
Chris@10 81 <pre class="example"> call dfftw_execute_dft(plan, in, out)
Chris@10 82 </pre>
Chris@10 83 <p><a name="index-dfftw_005fexecute_005fdft-585"></a>
Chris@10 84 There are a few things to be careful of, however:
Chris@10 85
Chris@10 86 <ul>
Chris@10 87 <li>You must use the correct type of execute function, matching the way
Chris@10 88 the plan was created. Complex DFT plans should use
Chris@10 89 <code>dfftw_execute_dft</code>, Real-input (r2c) DFT plans should use use
Chris@10 90 <code>dfftw_execute_dft_r2c</code>, and real-output (c2r) DFT plans should
Chris@10 91 use <code>dfftw_execute_dft_c2r</code>. The various r2r plans should use
Chris@10 92 <code>dfftw_execute_r2r</code>.
Chris@10 93
Chris@10 94 <li>You should normally pass the same input/output arrays that were used when
Chris@10 95 creating the plan. This is always safe.
Chris@10 96
Chris@10 97 <li><em>If</em> you pass <em>different</em> input/output arrays compared to
Chris@10 98 those used when creating the plan, you must abide by all the
Chris@10 99 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@10 100 requirement that the new arrays have the same alignment as the
Chris@10 101 original arrays, because there seems to be no way in legacy Fortran to obtain
Chris@10 102 guaranteed-aligned arrays (analogous to <code>fftw_malloc</code> in C). You
Chris@10 103 can, of course, use the <code>FFTW_UNALIGNED</code> flag when creating the
Chris@10 104 plan, in which case the plan does not depend on the alignment, but
Chris@10 105 this may sacrifice substantial performance on architectures (like x86)
Chris@10 106 with SIMD instructions (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>).
Chris@10 107 <a name="index-FFTW_005fUNALIGNED-586"></a>
Chris@10 108 </ul>
Chris@10 109
Chris@10 110 <!-- -->
Chris@10 111 </body></html>
Chris@10 112