|
cannam@95
|
1 <html lang="en">
|
|
cannam@95
|
2 <head>
|
|
cannam@95
|
3 <title>Allocating aligned memory in Fortran - FFTW 3.3.3</title>
|
|
cannam@95
|
4 <meta http-equiv="Content-Type" content="text/html">
|
|
cannam@95
|
5 <meta name="description" content="FFTW 3.3.3">
|
|
cannam@95
|
6 <meta name="generator" content="makeinfo 4.13">
|
|
cannam@95
|
7 <link title="Top" rel="start" href="index.html#Top">
|
|
cannam@95
|
8 <link rel="up" href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran" title="Calling FFTW from Modern Fortran">
|
|
cannam@95
|
9 <link rel="prev" href="Plan-execution-in-Fortran.html#Plan-execution-in-Fortran" title="Plan execution in Fortran">
|
|
cannam@95
|
10 <link rel="next" href="Accessing-the-wisdom-API-from-Fortran.html#Accessing-the-wisdom-API-from-Fortran" title="Accessing the wisdom API from Fortran">
|
|
cannam@95
|
11 <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
|
|
cannam@95
|
12 <!--
|
|
cannam@95
|
13 This manual is for FFTW
|
|
cannam@95
|
14 (version 3.3.3, 25 November 2012).
|
|
cannam@95
|
15
|
|
cannam@95
|
16 Copyright (C) 2003 Matteo Frigo.
|
|
cannam@95
|
17
|
|
cannam@95
|
18 Copyright (C) 2003 Massachusetts Institute of Technology.
|
|
cannam@95
|
19
|
|
cannam@95
|
20 Permission is granted to make and distribute verbatim copies of
|
|
cannam@95
|
21 this manual provided the copyright notice and this permission
|
|
cannam@95
|
22 notice are preserved on all copies.
|
|
cannam@95
|
23
|
|
cannam@95
|
24 Permission is granted to copy and distribute modified versions of
|
|
cannam@95
|
25 this manual under the conditions for verbatim copying, provided
|
|
cannam@95
|
26 that the entire resulting derived work is distributed under the
|
|
cannam@95
|
27 terms of a permission notice identical to this one.
|
|
cannam@95
|
28
|
|
cannam@95
|
29 Permission is granted to copy and distribute translations of this
|
|
cannam@95
|
30 manual into another language, under the above conditions for
|
|
cannam@95
|
31 modified versions, except that this permission notice may be
|
|
cannam@95
|
32 stated in a translation approved by the Free Software Foundation.
|
|
cannam@95
|
33 -->
|
|
cannam@95
|
34 <meta http-equiv="Content-Style-Type" content="text/css">
|
|
cannam@95
|
35 <style type="text/css"><!--
|
|
cannam@95
|
36 pre.display { font-family:inherit }
|
|
cannam@95
|
37 pre.format { font-family:inherit }
|
|
cannam@95
|
38 pre.smalldisplay { font-family:inherit; font-size:smaller }
|
|
cannam@95
|
39 pre.smallformat { font-family:inherit; font-size:smaller }
|
|
cannam@95
|
40 pre.smallexample { font-size:smaller }
|
|
cannam@95
|
41 pre.smalllisp { font-size:smaller }
|
|
cannam@95
|
42 span.sc { font-variant:small-caps }
|
|
cannam@95
|
43 span.roman { font-family:serif; font-weight:normal; }
|
|
cannam@95
|
44 span.sansserif { font-family:sans-serif; font-weight:normal; }
|
|
cannam@95
|
45 --></style>
|
|
cannam@95
|
46 </head>
|
|
cannam@95
|
47 <body>
|
|
cannam@95
|
48 <div class="node">
|
|
cannam@95
|
49 <a name="Allocating-aligned-memory-in-Fortran"></a>
|
|
cannam@95
|
50 <p>
|
|
cannam@95
|
51 Next: <a rel="next" accesskey="n" href="Accessing-the-wisdom-API-from-Fortran.html#Accessing-the-wisdom-API-from-Fortran">Accessing the wisdom API from Fortran</a>,
|
|
cannam@95
|
52 Previous: <a rel="previous" accesskey="p" href="Plan-execution-in-Fortran.html#Plan-execution-in-Fortran">Plan execution in Fortran</a>,
|
|
cannam@95
|
53 Up: <a rel="up" accesskey="u" href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran">Calling FFTW from Modern Fortran</a>
|
|
cannam@95
|
54 <hr>
|
|
cannam@95
|
55 </div>
|
|
cannam@95
|
56
|
|
cannam@95
|
57 <h3 class="section">7.5 Allocating aligned memory in Fortran</h3>
|
|
cannam@95
|
58
|
|
cannam@95
|
59 <p><a name="index-alignment-560"></a><a name="index-fftw_005falloc_005freal-561"></a><a name="index-fftw_005falloc_005fcomplex-562"></a>In order to obtain maximum performance in FFTW, you should store your
|
|
cannam@95
|
60 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
|
|
cannam@95
|
61 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
|
|
cannam@95
|
62 arrays. Unfortunately, standard Fortran arrays do <em>not</em> provide
|
|
cannam@95
|
63 any alignment guarantees. The <em>only</em> way to allocate aligned
|
|
cannam@95
|
64 memory in standard Fortran is to allocate it with an external C
|
|
cannam@95
|
65 function, like the <code>fftw_alloc_real</code> and
|
|
cannam@95
|
66 <code>fftw_alloc_complex</code> functions. Fortunately, Fortran 2003 provides
|
|
cannam@95
|
67 a simple way to associate such allocated memory with a standard Fortran
|
|
cannam@95
|
68 array pointer that you can then use normally.
|
|
cannam@95
|
69
|
|
cannam@95
|
70 <p>We therefore recommend allocating all your input/output arrays using
|
|
cannam@95
|
71 the following technique:
|
|
cannam@95
|
72
|
|
cannam@95
|
73 <ol type=1 start=1>
|
|
cannam@95
|
74
|
|
cannam@95
|
75 <li>Declare a <code>pointer</code>, <code>arr</code>, to your array of the desired type
|
|
cannam@95
|
76 and dimensions. For example, <code>real(C_DOUBLE), pointer :: a(:,:)</code>
|
|
cannam@95
|
77 for a 2d real array, or <code>complex(C_DOUBLE_COMPLEX), pointer ::
|
|
cannam@95
|
78 a(:,:,:)</code> for a 3d complex array.
|
|
cannam@95
|
79
|
|
cannam@95
|
80 <li>The number of elements to allocate must be an
|
|
cannam@95
|
81 <code>integer(C_SIZE_T)</code>. You can either declare a variable of this
|
|
cannam@95
|
82 type, e.g. <code>integer(C_SIZE_T) :: sz</code>, to store the number of
|
|
cannam@95
|
83 elements to allocate, or you can use the <code>int(..., C_SIZE_T)</code>
|
|
cannam@95
|
84 intrinsic function. e.g. set <code>sz = L * M * N</code> or use
|
|
cannam@95
|
85 <code>int(L * M * N, C_SIZE_T)</code> for an L × M × N array.
|
|
cannam@95
|
86
|
|
cannam@95
|
87 <li>Declare a <code>type(C_PTR) :: p</code> to hold the return value from
|
|
cannam@95
|
88 FFTW'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.
|
|
cannam@95
|
89
|
|
cannam@95
|
90 <li><a name="index-c_005ff_005fpointer-563"></a>Associate your pointer <code>arr</code> with the allocated memory <code>p</code>
|
|
cannam@95
|
91 using the standard <code>c_f_pointer</code> subroutine: <code>call
|
|
cannam@95
|
92 c_f_pointer(p, arr, [...dimensions...])</code>, where
|
|
cannam@95
|
93 <code>[...dimensions...])</code> are an array of the dimensions of the array
|
|
cannam@95
|
94 (in the usual Fortran order). e.g. <code>call c_f_pointer(p, arr,
|
|
cannam@95
|
95 [L,M,N])</code> for an L × M × N array. (Alternatively, you can
|
|
cannam@95
|
96 omit the dimensions argument if you specified the shape explicitly
|
|
cannam@95
|
97 when declaring <code>arr</code>.) You can now use <code>arr</code> as a usual
|
|
cannam@95
|
98 multidimensional array.
|
|
cannam@95
|
99
|
|
cannam@95
|
100 <li>When you are done using the array, deallocate the memory by <code>call
|
|
cannam@95
|
101 fftw_free(p)</code> on <code>p</code>.
|
|
cannam@95
|
102
|
|
cannam@95
|
103 </ol>
|
|
cannam@95
|
104
|
|
cannam@95
|
105 <p>For example, here is how we would allocate an L × M 2d real array:
|
|
cannam@95
|
106
|
|
cannam@95
|
107 <pre class="example"> real(C_DOUBLE), pointer :: arr(:,:)
|
|
cannam@95
|
108 type(C_PTR) :: p
|
|
cannam@95
|
109 p = fftw_alloc_real(int(L * M, C_SIZE_T))
|
|
cannam@95
|
110 call c_f_pointer(p, arr, [L,M])
|
|
cannam@95
|
111 <em>...use arr and arr(i,j) as usual...</em>
|
|
cannam@95
|
112 call fftw_free(p)
|
|
cannam@95
|
113 </pre>
|
|
cannam@95
|
114 <p>and here is an L × M × N 3d complex array:
|
|
cannam@95
|
115
|
|
cannam@95
|
116 <pre class="example"> complex(C_DOUBLE_COMPLEX), pointer :: arr(:,:,:)
|
|
cannam@95
|
117 type(C_PTR) :: p
|
|
cannam@95
|
118 p = fftw_alloc_complex(int(L * M * N, C_SIZE_T))
|
|
cannam@95
|
119 call c_f_pointer(p, arr, [L,M,N])
|
|
cannam@95
|
120 <em>...use arr and arr(i,j,k) as usual...</em>
|
|
cannam@95
|
121 call fftw_free(p)
|
|
cannam@95
|
122 </pre>
|
|
cannam@95
|
123 <p>See <a href="Reversing-array-dimensions.html#Reversing-array-dimensions">Reversing array dimensions</a> for an example allocating a
|
|
cannam@95
|
124 single array and associating both real and complex array pointers with
|
|
cannam@95
|
125 it, for in-place real-to-complex transforms.
|
|
cannam@95
|
126
|
|
cannam@95
|
127 <!-- -->
|
|
cannam@95
|
128 </body></html>
|
|
cannam@95
|
129
|
