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Add FFTW 3.3.8 source, and a Linux build
author Chris Cannam
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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25 <title>FFTW 3.3.8: Allocating aligned memory in Fortran</title>
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71 <a name="Allocating-aligned-memory-in-Fortran"></a>
72 <div class="header">
73 <p>
74 Next: <a href="Accessing-the-wisdom-API-from-Fortran.html#Accessing-the-wisdom-API-from-Fortran" accesskey="n" rel="next">Accessing the wisdom API from Fortran</a>, Previous: <a href="Plan-execution-in-Fortran.html#Plan-execution-in-Fortran" accesskey="p" rel="prev">Plan execution in Fortran</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>
75 </div>
76 <hr>
77 <a name="Allocating-aligned-memory-in-Fortran-1"></a>
78 <h3 class="section">7.5 Allocating aligned memory in Fortran</h3>
79
80 <a name="index-alignment-5"></a>
81 <a name="index-fftw_005falloc_005freal-5"></a>
82 <a name="index-fftw_005falloc_005fcomplex-5"></a>
83 <p>In order to obtain maximum performance in FFTW, you should store your
84 data in arrays that have been specially aligned in memory (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>). Enforcing alignment also permits you to
85 safely use the new-array execute functions (see <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>) to apply a given plan to more than one pair of in/out
86 arrays. Unfortunately, standard Fortran arrays do <em>not</em> provide
87 any alignment guarantees. The <em>only</em> way to allocate aligned
88 memory in standard Fortran is to allocate it with an external C
89 function, like the <code>fftw_alloc_real</code> and
90 <code>fftw_alloc_complex</code> functions. Fortunately, Fortran 2003 provides
91 a simple way to associate such allocated memory with a standard Fortran
92 array pointer that you can then use normally.
93 </p>
94 <p>We therefore recommend allocating all your input/output arrays using
95 the following technique:
96 </p>
97 <ol>
98 <li> Declare a <code>pointer</code>, <code>arr</code>, to your array of the desired type
99 and dimensions. For example, <code>real(C_DOUBLE), pointer :: a(:,:)</code>
100 for a 2d real array, or <code>complex(C_DOUBLE_COMPLEX), pointer ::
101 a(:,:,:)</code> for a 3d complex array.
102
103 </li><li> The number of elements to allocate must be an
104 <code>integer(C_SIZE_T)</code>. You can either declare a variable of this
105 type, e.g. <code>integer(C_SIZE_T) :: sz</code>, to store the number of
106 elements to allocate, or you can use the <code>int(..., C_SIZE_T)</code>
107 intrinsic function. e.g. set <code>sz = L * M * N</code> or use
108 <code>int(L * M * N, C_SIZE_T)</code> for an L&nbsp;&times;&nbsp;M&nbsp;&times;&nbsp;N
109 array.
110
111 </li><li> Declare a <code>type(C_PTR) :: p</code> to hold the return value from
112 FFTW&rsquo;s allocation routine. Set <code>p = fftw_alloc_real(sz)</code> for a real array, or <code>p = fftw_alloc_complex(sz)</code> for a complex array.
113
114 </li><li> <a name="index-c_005ff_005fpointer-2"></a>
115 Associate your pointer <code>arr</code> with the allocated memory <code>p</code>
116 using the standard <code>c_f_pointer</code> subroutine: <code>call
117 c_f_pointer(p, arr, [...dimensions...])</code>, where
118 <code>[...dimensions...])</code> are an array of the dimensions of the array
119 (in the usual Fortran order). e.g. <code>call c_f_pointer(p, arr,
120 [L,M,N])</code> for an L&nbsp;&times;&nbsp;M&nbsp;&times;&nbsp;N
121 array. (Alternatively, you can
122 omit the dimensions argument if you specified the shape explicitly
123 when declaring <code>arr</code>.) You can now use <code>arr</code> as a usual
124 multidimensional array.
125
126 </li><li> When you are done using the array, deallocate the memory by <code>call
127 fftw_free(p)</code> on <code>p</code>.
128
129 </li></ol>
130
131 <p>For example, here is how we would allocate an L&nbsp;&times;&nbsp;M
132 2d real array:
133 </p>
134 <div class="example">
135 <pre class="example"> real(C_DOUBLE), pointer :: arr(:,:)
136 type(C_PTR) :: p
137 p = fftw_alloc_real(int(L * M, C_SIZE_T))
138 call c_f_pointer(p, arr, [L,M])
139 <em>...use arr and arr(i,j) as usual...</em>
140 call fftw_free(p)
141 </pre></div>
142
143 <p>and here is an L&nbsp;&times;&nbsp;M&nbsp;&times;&nbsp;N
144 3d complex array:
145 </p>
146 <div class="example">
147 <pre class="example"> complex(C_DOUBLE_COMPLEX), pointer :: arr(:,:,:)
148 type(C_PTR) :: p
149 p = fftw_alloc_complex(int(L * M * N, C_SIZE_T))
150 call c_f_pointer(p, arr, [L,M,N])
151 <em>...use arr and arr(i,j,k) as usual...</em>
152 call fftw_free(p)
153 </pre></div>
154
155 <p>See <a href="Reversing-array-dimensions.html#Reversing-array-dimensions">Reversing array dimensions</a> for an example allocating a
156 single array and associating both real and complex array pointers with
157 it, for in-place real-to-complex transforms.
158 </p>
159 <hr>
160 <div class="header">
161 <p>
162 Next: <a href="Accessing-the-wisdom-API-from-Fortran.html#Accessing-the-wisdom-API-from-Fortran" accesskey="n" rel="next">Accessing the wisdom API from Fortran</a>, Previous: <a href="Plan-execution-in-Fortran.html#Plan-execution-in-Fortran" accesskey="p" rel="prev">Plan execution in Fortran</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>
163 </div>
164
165
166
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