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date Wed, 20 Mar 2013 15:35:50 +0000
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3 <title>Plan execution in Fortran - FFTW 3.3.3</title>
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49 <a name="Plan-execution-in-Fortran"></a>
50 <p>
51 Next:&nbsp;<a rel="next" accesskey="n" href="Allocating-aligned-memory-in-Fortran.html#Allocating-aligned-memory-in-Fortran">Allocating aligned memory in Fortran</a>,
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56
57 <h3 class="section">7.4 Plan execution in Fortran</h3>
58
59 <p>In C, in order to use a plan, one normally calls <code>fftw_execute</code>,
60 which executes the plan to perform the transform on the input/output
61 arrays passed when the plan was created (see <a href="Using-Plans.html#Using-Plans">Using Plans</a>). The
62 corresponding subroutine call in modern Fortran is:
63 <pre class="example"> call fftw_execute(plan)
64 </pre>
65 <p><a name="index-fftw_005fexecute-554"></a>
66 However, we have had reports that this causes problems with some
67 recent optimizing Fortran compilers. The problem is, because the
68 input/output arrays are not passed as explicit arguments to
69 <code>fftw_execute</code>, the semantics of Fortran (unlike C) allow the
70 compiler to assume that the input/output arrays are not changed by
71 <code>fftw_execute</code>. As a consequence, certain compilers end up
72 repositioning the call to <code>fftw_execute</code>, assuming incorrectly
73 that it does nothing to the arrays.
74
75 <p>There are various workarounds to this, but the safest and simplest
76 thing is to not use <code>fftw_execute</code> in Fortran. Instead, use the
77 functions described in <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>, which take
78 the input/output arrays as explicit arguments. For example, if the
79 plan is for a complex-data DFT and was created for the arrays
80 <code>in</code> and <code>out</code>, you would do:
81 <pre class="example"> call fftw_execute_dft(plan, in, out)
82 </pre>
83 <p><a name="index-fftw_005fexecute_005fdft-555"></a>
84 There are a few things to be careful of, however:
85
86 <ul>
87 <li><a name="index-fftw_005fexecute_005fdft_005fr2c-556"></a><a name="index-fftw_005fexecute_005fdft_005fc2r-557"></a><a name="index-fftw_005fexecute_005fr2r-558"></a>You must use the correct type of execute function, matching the way
88 the plan was created. Complex DFT plans should use
89 <code>fftw_execute_dft</code>, Real-input (r2c) DFT plans should use use
90 <code>fftw_execute_dft_r2c</code>, and real-output (c2r) DFT plans should
91 use <code>fftw_execute_dft_c2r</code>. The various r2r plans should use
92 <code>fftw_execute_r2r</code>. Fortunately, if you use the wrong one you
93 will get a compile-time type-mismatch error (unlike legacy Fortran).
94
95 <li>You should normally pass the same input/output arrays that were used when
96 creating the plan. This is always safe.
97
98 <li><em>If</em> you pass <em>different</em> input/output arrays compared to
99 those used when creating the plan, you must abide by all the
100 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
101 requirement that the new arrays have the same alignment as the
102 original arrays; the best (and possibly only) way to guarantee this
103 is to use the &lsquo;<samp><span class="samp">fftw_alloc</span></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
104 use the <code>FFTW_UNALIGNED</code> flag when creating the
105 plan, in which case the plan does not depend on the alignment, but
106 this may sacrifice substantial performance on architectures (like x86)
107 with SIMD instructions (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>).
108 <a name="index-FFTW_005fUNALIGNED-559"></a>
109 </ul>
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