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First commit. VS2013, Codeblocks and Mac OSX configuration
author Geogaddi\David <d.m.ronan@qmul.ac.uk>
date Thu, 09 Jul 2015 01:12:16 +0100
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d@0 3 <title>FFTW Execution in Fortran - FFTW 3.2.1</title>
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d@0 13 This manual is for FFTW
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d@0 50 <a name="FFTW-Execution-in-Fortran"></a>
d@0 51 Next:&nbsp;<a rel="next" accesskey="n" href="Fortran-Examples.html#Fortran-Examples">Fortran Examples</a>,
d@0 52 Previous:&nbsp;<a rel="previous" accesskey="p" href="FFTW-Constants-in-Fortran.html#FFTW-Constants-in-Fortran">FFTW Constants in Fortran</a>,
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d@0 55 </div>
d@0 56
d@0 57 <h3 class="section">7.3 FFTW Execution in Fortran</h3>
d@0 58
d@0 59 <p>In C, in order to use a plan, one normally calls <code>fftw_execute</code>,
d@0 60 which executes the plan to perform the transform on the input/output
d@0 61 arrays passed when the plan was created (see <a href="Using-Plans.html#Using-Plans">Using Plans</a>). The
d@0 62 corresponding subroutine call in Fortran is:
d@0 63 <pre class="example"> call dfftw_execute(plan)
d@0 64 </pre>
d@0 65 <p><a name="index-dfftw_005fexecute-338"></a>
d@0 66 However, we have had reports that this causes problems with some
d@0 67 recent optimizing Fortran compilers. The problem is, because the
d@0 68 input/output arrays are not passed as explicit arguments to
d@0 69 <code>dfftw_execute</code>, the semantics of Fortran (unlike C) allow the
d@0 70 compiler to assume that the input/output arrays are not changed by
d@0 71 <code>dfftw_execute</code>. As a consequence, certain compilers end up
d@0 72 optimizing out or repositioning the call to <code>dfftw_execute</code>,
d@0 73 assuming incorrectly that it does nothing.
d@0 74
d@0 75 <p>There are various workarounds to this, but the safest and simplest
d@0 76 thing is to not use <code>dfftw_execute</code> in Fortran. Instead, use the
d@0 77 functions described in <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>, which take
d@0 78 the input/output arrays as explicit arguments. For example, if the
d@0 79 plan is for a complex-data DFT and was created for the arrays
d@0 80 <code>in</code> and <code>out</code>, you would do:
d@0 81 <pre class="example"> call dfftw_execute_dft(plan, in, out)
d@0 82 </pre>
d@0 83 <p><a name="index-dfftw_005fexecute_005fdft-339"></a>
d@0 84 There are a few things to be careful of, however:
d@0 85
d@0 86 <ul>
d@0 87 <li>You must use the correct type of execute function, matching the way
d@0 88 the plan was created. Complex DFT plans should use
d@0 89 <code>dfftw_execute_dft</code>, Real-input (r2c) DFT plans should use use
d@0 90 <code>dfftw_execute_dft_r2c</code>, and real-output (c2r) DFT plans should
d@0 91 use <code>dfftw_execute_dft_c2r</code>. The various r2r plans should use
d@0 92 <code>dfftw_execute_r2r</code>.
d@0 93
d@0 94 <li>You should normally pass the same input/output arrays that were used when
d@0 95 creating the plan. This is always safe.
d@0 96
d@0 97 <li><em>If</em> you pass <em>different</em> input/output arrays compared to
d@0 98 those used when creating the plan, you must abide by all the
d@0 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
d@0 100 requirement that the new arrays have the same alignment as the
d@0 101 original arrays, because there seems to be no way in Fortran to obtain
d@0 102 guaranteed-aligned arrays (analogous to <code>fftw_malloc</code> in C). You
d@0 103 can, of course, use the <code>FFTW_UNALIGNED</code> flag when creating the
d@0 104 plan, in which case the plan does not depend on the alignment, but
d@0 105 this may sacrifice substantial performance on architectures (like x86)
d@0 106 with SIMD instructions (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>).
d@0 107 <a name="index-FFTW_005fUNALIGNED-340"></a>
d@0 108 </ul>
d@0 109
d@0 110 <!-- -->
d@0 111 </body></html>
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