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Current fftw source
author Chris Cannam <cannam@all-day-breakfast.com>
date Tue, 18 Oct 2016 13:40:26 +0100
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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 3.3.5: Allocating aligned memory in Fortran</title>
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72 <a name="Allocating-aligned-memory-in-Fortran"></a>
73 <div class="header">
74 <p>
75 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>
76 </div>
77 <hr>
78 <a name="Allocating-aligned-memory-in-Fortran-1"></a>
79 <h3 class="section">7.5 Allocating aligned memory in Fortran</h3>
80
81 <a name="index-alignment-5"></a>
82 <a name="index-fftw_005falloc_005freal-5"></a>
83 <a name="index-fftw_005falloc_005fcomplex-5"></a>
84 <p>In order to obtain maximum performance in FFTW, you should store your
85 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
86 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
87 arrays. Unfortunately, standard Fortran arrays do <em>not</em> provide
88 any alignment guarantees. The <em>only</em> way to allocate aligned
89 memory in standard Fortran is to allocate it with an external C
90 function, like the <code>fftw_alloc_real</code> and
91 <code>fftw_alloc_complex</code> functions. Fortunately, Fortran 2003 provides
92 a simple way to associate such allocated memory with a standard Fortran
93 array pointer that you can then use normally.
94 </p>
95 <p>We therefore recommend allocating all your input/output arrays using
96 the following technique:
97 </p>
98 <ol>
99 <li> Declare a <code>pointer</code>, <code>arr</code>, to your array of the desired type
100 and dimensions. For example, <code>real(C_DOUBLE), pointer :: a(:,:)</code>
101 for a 2d real array, or <code>complex(C_DOUBLE_COMPLEX), pointer ::
102 a(:,:,:)</code> for a 3d complex array.
103
104 </li><li> The number of elements to allocate must be an
105 <code>integer(C_SIZE_T)</code>. You can either declare a variable of this
106 type, e.g. <code>integer(C_SIZE_T) :: sz</code>, to store the number of
107 elements to allocate, or you can use the <code>int(..., C_SIZE_T)</code>
108 intrinsic function. e.g. set <code>sz = L * M * N</code> or use
109 <code>int(L * M * N, C_SIZE_T)</code> for an L&nbsp;&times;&nbsp;M&nbsp;&times;&nbsp;N 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 array. (Alternatively, you can
121 omit the dimensions argument if you specified the shape explicitly
122 when declaring <code>arr</code>.) You can now use <code>arr</code> as a usual
123 multidimensional array.
124
125 </li><li> When you are done using the array, deallocate the memory by <code>call
126 fftw_free(p)</code> on <code>p</code>.
127
128 </li></ol>
129
130 <p>For example, here is how we would allocate an L&nbsp;&times;&nbsp;M 2d real array:
131 </p>
132 <div class="example">
133 <pre class="example"> real(C_DOUBLE), pointer :: arr(:,:)
134 type(C_PTR) :: p
135 p = fftw_alloc_real(int(L * M, C_SIZE_T))
136 call c_f_pointer(p, arr, [L,M])
137 <em>...use arr and arr(i,j) as usual...</em>
138 call fftw_free(p)
139 </pre></div>
140
141 <p>and here is an L&nbsp;&times;&nbsp;M&nbsp;&times;&nbsp;N 3d complex array:
142 </p>
143 <div class="example">
144 <pre class="example"> complex(C_DOUBLE_COMPLEX), pointer :: arr(:,:,:)
145 type(C_PTR) :: p
146 p = fftw_alloc_complex(int(L * M * N, C_SIZE_T))
147 call c_f_pointer(p, arr, [L,M,N])
148 <em>...use arr and arr(i,j,k) as usual...</em>
149 call fftw_free(p)
150 </pre></div>
151
152 <p>See <a href="Reversing-array-dimensions.html#Reversing-array-dimensions">Reversing array dimensions</a> for an example allocating a
153 single array and associating both real and complex array pointers with
154 it, for in-place real-to-complex transforms.
155 </p>
156 <hr>
157 <div class="header">
158 <p>
159 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>
160 </div>
161
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163
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