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author Chris Cannam
date Tue, 18 Oct 2016 13:40:26 +0100
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72 <a name="Plan-execution-in-Fortran"></a>
73 <div class="header">
74 <p>
75 Next: <a href="Allocating-aligned-memory-in-Fortran.html#Allocating-aligned-memory-in-Fortran" accesskey="n" rel="next">Allocating aligned memory in Fortran</a>, Previous: <a href="FFTW-Fortran-type-reference.html#FFTW-Fortran-type-reference" accesskey="p" rel="prev">FFTW Fortran type reference</a>, Up: <a href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran" accesskey="u" rel="up">Calling FFTW from Modern Fortran</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
76 </div>
77 <hr>
78 <a name="Plan-execution-in-Fortran-1"></a>
79 <h3 class="section">7.4 Plan execution in Fortran</h3>
80
81 <p>In C, in order to use a plan, one normally calls <code>fftw_execute</code>,
82 which executes the plan to perform the transform on the input/output
83 arrays passed when the plan was created (see <a href="Using-Plans.html#Using-Plans">Using Plans</a>). The
84 corresponding subroutine call in modern Fortran is:
85 </p><div class="example">
86 <pre class="example"> call fftw_execute(plan)
87 </pre></div>
88 <a name="index-fftw_005fexecute-6"></a>
89
90 <p>However, we have had reports that this causes problems with some
91 recent optimizing Fortran compilers. The problem is, because the
92 input/output arrays are not passed as explicit arguments to
93 <code>fftw_execute</code>, the semantics of Fortran (unlike C) allow the
94 compiler to assume that the input/output arrays are not changed by
95 <code>fftw_execute</code>. As a consequence, certain compilers end up
96 repositioning the call to <code>fftw_execute</code>, assuming incorrectly
97 that it does nothing to the arrays.
98 </p>
99 <p>There are various workarounds to this, but the safest and simplest
100 thing is to not use <code>fftw_execute</code> in Fortran. Instead, use the
101 functions described in <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>, which take
102 the input/output arrays as explicit arguments. For example, if the
103 plan is for a complex-data DFT and was created for the arrays
104 <code>in</code> and <code>out</code>, you would do:
105 </p><div class="example">
106 <pre class="example"> call fftw_execute_dft(plan, in, out)
107 </pre></div>
108 <a name="index-fftw_005fexecute_005fdft-3"></a>
109
110 <p>There are a few things to be careful of, however:
111 </p>
112 <ul>
113 <li> <a name="index-fftw_005fexecute_005fdft_005fr2c-2"></a>
114 <a name="index-fftw_005fexecute_005fdft_005fc2r-1"></a>
115 <a name="index-fftw_005fexecute_005fr2r-1"></a>
116 You must use the correct type of execute function, matching the way
117 the plan was created. Complex DFT plans should use
118 <code>fftw_execute_dft</code>, Real-input (r2c) DFT plans should use use
119 <code>fftw_execute_dft_r2c</code>, and real-output (c2r) DFT plans should
120 use <code>fftw_execute_dft_c2r</code>. The various r2r plans should use
121 <code>fftw_execute_r2r</code>. Fortunately, if you use the wrong one you
122 will get a compile-time type-mismatch error (unlike legacy Fortran).
123
124 </li><li> You should normally pass the same input/output arrays that were used when
125 creating the plan. This is always safe.
126
127 </li><li> <em>If</em> you pass <em>different</em> input/output arrays compared to
128 those used when creating the plan, you must abide by all the
129 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
130 requirement that the new arrays have the same alignment as the
131 original arrays; the best (and possibly only) way to guarantee this
132 is to use the &lsquo;<samp>fftw_alloc</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
133 use the <code>FFTW_UNALIGNED</code> flag when creating the
134 plan, in which case the plan does not depend on the alignment, but
135 this may sacrifice substantial performance on architectures (like x86)
136 with SIMD instructions (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>).
137 <a name="index-FFTW_005fUNALIGNED-2"></a>
138
139 </li></ul>
140
141 <hr>
142 <div class="header">
143 <p>
144 Next: <a href="Allocating-aligned-memory-in-Fortran.html#Allocating-aligned-memory-in-Fortran" accesskey="n" rel="next">Allocating aligned memory in Fortran</a>, Previous: <a href="FFTW-Fortran-type-reference.html#FFTW-Fortran-type-reference" accesskey="p" rel="prev">FFTW Fortran type reference</a>, Up: <a href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran" accesskey="u" rel="up">Calling FFTW from Modern Fortran</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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