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Add FFTW 3.3.8 source, and a Linux build
author Chris Cannam <cannam@all-day-breakfast.com>
date Tue, 19 Nov 2019 14:52:55 +0000
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3 <!-- This manual is for FFTW
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6 Copyright (C) 2003 Matteo Frigo.
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8 Copyright (C) 2003 Massachusetts Institute of Technology.
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25 <title>FFTW 3.3.8: FFTW MPI Fortran Interface</title>
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36 <link href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" rel="up" title="Distributed-memory FFTW with MPI">
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71 <a name="FFTW-MPI-Fortran-Interface"></a>
72 <div class="header">
73 <p>
74 Previous: <a href="FFTW-MPI-Reference.html#FFTW-MPI-Reference" accesskey="p" rel="prev">FFTW MPI Reference</a>, Up: <a href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" accesskey="u" rel="up">Distributed-memory FFTW with MPI</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>
75 </div>
76 <hr>
77 <a name="FFTW-MPI-Fortran-Interface-1"></a>
78 <h3 class="section">6.13 FFTW MPI Fortran Interface</h3>
79 <a name="index-Fortran-interface-1"></a>
80
81 <a name="index-iso_005fc_005fbinding"></a>
82 <p>The FFTW MPI interface is callable from modern Fortran compilers
83 supporting the Fortran 2003 <code>iso_c_binding</code> standard for calling
84 C functions. As described in <a href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran">Calling FFTW from Modern Fortran</a>,
85 this means that you can directly call FFTW&rsquo;s C interface from Fortran
86 with only minor changes in syntax. There are, however, a few things
87 specific to the MPI interface to keep in mind:
88 </p>
89 <ul>
90 <li> Instead of including <code>fftw3.f03</code> as in <a href="Overview-of-Fortran-interface.html#Overview-of-Fortran-interface">Overview of Fortran interface</a>, you should <code>include 'fftw3-mpi.f03'</code> (after
91 <code>use, intrinsic :: iso_c_binding</code> as before). The
92 <code>fftw3-mpi.f03</code> file includes <code>fftw3.f03</code>, so you should
93 <em>not</em> <code>include</code> them both yourself. (You will also want to
94 include the MPI header file, usually via <code>include 'mpif.h'</code> or
95 similar, although though this is not needed by <code>fftw3-mpi.f03</code>
96 <i>per se</i>.) (To use the &lsquo;<samp>fftwl_</samp>&rsquo; <code>long double</code> extended-precision routines in supporting compilers, you should include <code>fftw3f-mpi.f03</code> in <em>addition</em> to <code>fftw3-mpi.f03</code>. See <a href="Extended-and-quadruple-precision-in-Fortran.html#Extended-and-quadruple-precision-in-Fortran">Extended and quadruple precision in Fortran</a>.)
97
98 </li><li> Because of the different storage conventions between C and Fortran,
99 you reverse the order of your array dimensions when passing them to
100 FFTW (see <a href="Reversing-array-dimensions.html#Reversing-array-dimensions">Reversing array dimensions</a>). This is merely a
101 difference in notation and incurs no performance overhead. However,
102 it means that, whereas in C the <em>first</em> dimension is distributed,
103 in Fortran the <em>last</em> dimension of your array is distributed.
104
105 </li><li> <a name="index-MPI-communicator-3"></a>
106 In Fortran, communicators are stored as <code>integer</code> types; there is
107 no <code>MPI_Comm</code> type, nor is there any way to access a C
108 <code>MPI_Comm</code>. Fortunately, this is taken care of for you by the
109 FFTW Fortran interface: whenever the C interface expects an
110 <code>MPI_Comm</code> type, you should pass the Fortran communicator as an
111 <code>integer</code>.<a name="DOCF8" href="#FOOT8"><sup>8</sup></a>
112
113 </li><li> Because you need to call the &lsquo;<samp>local_size</samp>&rsquo; function to find out
114 how much space to allocate, and this may be <em>larger</em> than the
115 local portion of the array (see <a href="MPI-Data-Distribution.html#MPI-Data-Distribution">MPI Data Distribution</a>), you should
116 <em>always</em> allocate your arrays dynamically using FFTW&rsquo;s allocation
117 routines as described in <a href="Allocating-aligned-memory-in-Fortran.html#Allocating-aligned-memory-in-Fortran">Allocating aligned memory in Fortran</a>.
118 (Coincidentally, this also provides the best performance by
119 guaranteeding proper data alignment.)
120
121 </li><li> Because all sizes in the MPI FFTW interface are declared as
122 <code>ptrdiff_t</code> in C, you should use <code>integer(C_INTPTR_T)</code> in
123 Fortran (see <a href="FFTW-Fortran-type-reference.html#FFTW-Fortran-type-reference">FFTW Fortran type reference</a>).
124
125 </li><li> <a name="index-fftw_005fexecute_005fdft-1"></a>
126 <a name="index-fftw_005fmpi_005fexecute_005fdft-1"></a>
127 <a name="index-new_002darray-execution-3"></a>
128 In Fortran, because of the language semantics, we generally recommend
129 using the new-array execute functions for all plans, even in the
130 common case where you are executing the plan on the same arrays for
131 which the plan was created (see <a href="Plan-execution-in-Fortran.html#Plan-execution-in-Fortran">Plan execution in Fortran</a>).
132 However, note that in the MPI interface these functions are changed:
133 <code>fftw_execute_dft</code> becomes <code>fftw_mpi_execute_dft</code>,
134 etcetera. See <a href="Using-MPI-Plans.html#Using-MPI-Plans">Using MPI Plans</a>.
135
136 </li></ul>
137
138 <p>For example, here is a Fortran code snippet to perform a distributed
139 L&nbsp;&times;&nbsp;M
140 complex DFT in-place. (This assumes you have already
141 initialized MPI with <code>MPI_init</code> and have also performed
142 <code>call fftw_mpi_init</code>.)
143 </p>
144 <div class="example">
145 <pre class="example"> use, intrinsic :: iso_c_binding
146 include 'fftw3-mpi.f03'
147 integer(C_INTPTR_T), parameter :: L = ...
148 integer(C_INTPTR_T), parameter :: M = ...
149 type(C_PTR) :: plan, cdata
150 complex(C_DOUBLE_COMPLEX), pointer :: data(:,:)
151 integer(C_INTPTR_T) :: i, j, alloc_local, local_M, local_j_offset
152
153 ! <span class="roman">get local data size and allocate (note dimension reversal)</span>
154 alloc_local = fftw_mpi_local_size_2d(M, L, MPI_COMM_WORLD, &amp;
155 local_M, local_j_offset)
156 cdata = fftw_alloc_complex(alloc_local)
157 call c_f_pointer(cdata, data, [L,local_M])
158
159 ! <span class="roman">create MPI plan for in-place forward DFT (note dimension reversal)</span>
160 plan = fftw_mpi_plan_dft_2d(M, L, data, data, MPI_COMM_WORLD, &amp;
161 FFTW_FORWARD, FFTW_MEASURE)
162
163 ! <span class="roman">initialize data to some function</span> my_function(i,j)
164 do j = 1, local_M
165 do i = 1, L
166 data(i, j) = my_function(i, j + local_j_offset)
167 end do
168 end do
169
170 ! <span class="roman">compute transform (as many times as desired)</span>
171 call fftw_mpi_execute_dft(plan, data, data)
172
173 call fftw_destroy_plan(plan)
174 call fftw_free(cdata)
175 </pre></div>
176
177 <p>Note that when we called <code>fftw_mpi_local_size_2d</code> and
178 <code>fftw_mpi_plan_dft_2d</code> with the dimensions in reversed order,
179 since a L&nbsp;&times;&nbsp;M
180 Fortran array is viewed by FFTW in C as a
181 M&nbsp;&times;&nbsp;L
182 array. This means that the array was distributed over
183 the <code>M</code> dimension, the local portion of which is a
184 L&nbsp;&times;&nbsp;local_M
185 array in Fortran. (You must <em>not</em> use an
186 <code>allocate</code> statement to allocate an L&nbsp;&times;&nbsp;local_M
187 array,
188 however; you must allocate <code>alloc_local</code> complex numbers, which
189 may be greater than <code>L * local_M</code>, in order to reserve space for
190 intermediate steps of the transform.) Finally, we mention that
191 because C&rsquo;s array indices are zero-based, the <code>local_j_offset</code>
192 argument can conveniently be interpreted as an offset in the 1-based
193 <code>j</code> index (rather than as a starting index as in C).
194 </p>
195 <p>If instead you had used the <code>ior(FFTW_MEASURE,
196 FFTW_MPI_TRANSPOSED_OUT)</code> flag, the output of the transform would be a
197 transposed M&nbsp;&times;&nbsp;local_L
198 array, associated with the <em>same</em>
199 <code>cdata</code> allocation (since the transform is in-place), and which
200 you could declare with:
201 </p>
202 <div class="example">
203 <pre class="example"> complex(C_DOUBLE_COMPLEX), pointer :: tdata(:,:)
204 ...
205 call c_f_pointer(cdata, tdata, [M,local_L])
206 </pre></div>
207
208 <p>where <code>local_L</code> would have been obtained by changing the
209 <code>fftw_mpi_local_size_2d</code> call to:
210 </p>
211 <div class="example">
212 <pre class="example"> alloc_local = fftw_mpi_local_size_2d_transposed(M, L, MPI_COMM_WORLD, &amp;
213 local_M, local_j_offset, local_L, local_i_offset)
214 </pre></div>
215 <div class="footnote">
216 <hr>
217 <h4 class="footnotes-heading">Footnotes</h4>
218
219 <h3><a name="FOOT8" href="#DOCF8">(8)</a></h3>
220 <p>Technically, this is because you aren&rsquo;t
221 actually calling the C functions directly. You are calling wrapper
222 functions that translate the communicator with <code>MPI_Comm_f2c</code>
223 before calling the ordinary C interface. This is all done
224 transparently, however, since the <code>fftw3-mpi.f03</code> interface file
225 renames the wrappers so that they are called in Fortran with the same
226 names as the C interface functions.</p>
227 </div>
228 <hr>
229 <div class="header">
230 <p>
231 Previous: <a href="FFTW-MPI-Reference.html#FFTW-MPI-Reference" accesskey="p" rel="prev">FFTW MPI Reference</a>, Up: <a href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" accesskey="u" rel="up">Distributed-memory FFTW with MPI</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>
232 </div>
233
234
235
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237 </html>