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Current fftw source
author Chris Cannam
date Tue, 18 Oct 2016 13:40:26 +0100
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Chris@42 3 <!-- This manual is for FFTW
Chris@42 4 (version 3.3.5, 30 July 2016).
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Chris@42 6 Copyright (C) 2003 Matteo Frigo.
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Chris@42 24 <head>
Chris@42 25 <title>FFTW 3.3.5: Allocating aligned memory in Fortran</title>
Chris@42 26
Chris@42 27 <meta name="description" content="FFTW 3.3.5: Allocating aligned memory in Fortran">
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Chris@42 70
Chris@42 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
Chris@42 72 <a name="Allocating-aligned-memory-in-Fortran"></a>
Chris@42 73 <div class="header">
Chris@42 74 <p>
Chris@42 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>
Chris@42 76 </div>
Chris@42 77 <hr>
Chris@42 78 <a name="Allocating-aligned-memory-in-Fortran-1"></a>
Chris@42 79 <h3 class="section">7.5 Allocating aligned memory in Fortran</h3>
Chris@42 80
Chris@42 81 <a name="index-alignment-5"></a>
Chris@42 82 <a name="index-fftw_005falloc_005freal-5"></a>
Chris@42 83 <a name="index-fftw_005falloc_005fcomplex-5"></a>
Chris@42 84 <p>In order to obtain maximum performance in FFTW, you should store your
Chris@42 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
Chris@42 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
Chris@42 87 arrays. Unfortunately, standard Fortran arrays do <em>not</em> provide
Chris@42 88 any alignment guarantees. The <em>only</em> way to allocate aligned
Chris@42 89 memory in standard Fortran is to allocate it with an external C
Chris@42 90 function, like the <code>fftw_alloc_real</code> and
Chris@42 91 <code>fftw_alloc_complex</code> functions. Fortunately, Fortran 2003 provides
Chris@42 92 a simple way to associate such allocated memory with a standard Fortran
Chris@42 93 array pointer that you can then use normally.
Chris@42 94 </p>
Chris@42 95 <p>We therefore recommend allocating all your input/output arrays using
Chris@42 96 the following technique:
Chris@42 97 </p>
Chris@42 98 <ol>
Chris@42 99 <li> Declare a <code>pointer</code>, <code>arr</code>, to your array of the desired type
Chris@42 100 and dimensions. For example, <code>real(C_DOUBLE), pointer :: a(:,:)</code>
Chris@42 101 for a 2d real array, or <code>complex(C_DOUBLE_COMPLEX), pointer ::
Chris@42 102 a(:,:,:)</code> for a 3d complex array.
Chris@42 103
Chris@42 104 </li><li> The number of elements to allocate must be an
Chris@42 105 <code>integer(C_SIZE_T)</code>. You can either declare a variable of this
Chris@42 106 type, e.g. <code>integer(C_SIZE_T) :: sz</code>, to store the number of
Chris@42 107 elements to allocate, or you can use the <code>int(..., C_SIZE_T)</code>
Chris@42 108 intrinsic function. e.g. set <code>sz = L * M * N</code> or use
Chris@42 109 <code>int(L * M * N, C_SIZE_T)</code> for an L&nbsp;&times;&nbsp;M&nbsp;&times;&nbsp;N array.
Chris@42 110
Chris@42 111 </li><li> Declare a <code>type(C_PTR) :: p</code> to hold the return value from
Chris@42 112 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.
Chris@42 113
Chris@42 114 </li><li> <a name="index-c_005ff_005fpointer-2"></a>
Chris@42 115 Associate your pointer <code>arr</code> with the allocated memory <code>p</code>
Chris@42 116 using the standard <code>c_f_pointer</code> subroutine: <code>call
Chris@42 117 c_f_pointer(p, arr, [...dimensions...])</code>, where
Chris@42 118 <code>[...dimensions...])</code> are an array of the dimensions of the array
Chris@42 119 (in the usual Fortran order). e.g. <code>call c_f_pointer(p, arr,
Chris@42 120 [L,M,N])</code> for an L&nbsp;&times;&nbsp;M&nbsp;&times;&nbsp;N array. (Alternatively, you can
Chris@42 121 omit the dimensions argument if you specified the shape explicitly
Chris@42 122 when declaring <code>arr</code>.) You can now use <code>arr</code> as a usual
Chris@42 123 multidimensional array.
Chris@42 124
Chris@42 125 </li><li> When you are done using the array, deallocate the memory by <code>call
Chris@42 126 fftw_free(p)</code> on <code>p</code>.
Chris@42 127
Chris@42 128 </li></ol>
Chris@42 129
Chris@42 130 <p>For example, here is how we would allocate an L&nbsp;&times;&nbsp;M 2d real array:
Chris@42 131 </p>
Chris@42 132 <div class="example">
Chris@42 133 <pre class="example"> real(C_DOUBLE), pointer :: arr(:,:)
Chris@42 134 type(C_PTR) :: p
Chris@42 135 p = fftw_alloc_real(int(L * M, C_SIZE_T))
Chris@42 136 call c_f_pointer(p, arr, [L,M])
Chris@42 137 <em>...use arr and arr(i,j) as usual...</em>
Chris@42 138 call fftw_free(p)
Chris@42 139 </pre></div>
Chris@42 140
Chris@42 141 <p>and here is an L&nbsp;&times;&nbsp;M&nbsp;&times;&nbsp;N 3d complex array:
Chris@42 142 </p>
Chris@42 143 <div class="example">
Chris@42 144 <pre class="example"> complex(C_DOUBLE_COMPLEX), pointer :: arr(:,:,:)
Chris@42 145 type(C_PTR) :: p
Chris@42 146 p = fftw_alloc_complex(int(L * M * N, C_SIZE_T))
Chris@42 147 call c_f_pointer(p, arr, [L,M,N])
Chris@42 148 <em>...use arr and arr(i,j,k) as usual...</em>
Chris@42 149 call fftw_free(p)
Chris@42 150 </pre></div>
Chris@42 151
Chris@42 152 <p>See <a href="Reversing-array-dimensions.html#Reversing-array-dimensions">Reversing array dimensions</a> for an example allocating a
Chris@42 153 single array and associating both real and complex array pointers with
Chris@42 154 it, for in-place real-to-complex transforms.
Chris@42 155 </p>
Chris@42 156 <hr>
Chris@42 157 <div class="header">
Chris@42 158 <p>
Chris@42 159 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>
Chris@42 160 </div>
Chris@42 161
Chris@42 162
Chris@42 163
Chris@42 164 </body>
Chris@42 165 </html>