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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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cannam@167 1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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cannam@167 3 <!-- This manual is for FFTW
cannam@167 4 (version 3.3.8, 24 May 2018).
cannam@167 5
cannam@167 6 Copyright (C) 2003 Matteo Frigo.
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cannam@167 8 Copyright (C) 2003 Massachusetts Institute of Technology.
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cannam@167 24 <head>
cannam@167 25 <title>FFTW 3.3.8: Allocating aligned memory in Fortran</title>
cannam@167 26
cannam@167 27 <meta name="description" content="FFTW 3.3.8: Allocating aligned memory in Fortran">
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cannam@167 36 <link href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran" rel="up" title="Calling FFTW from Modern Fortran">
cannam@167 37 <link href="Accessing-the-wisdom-API-from-Fortran.html#Accessing-the-wisdom-API-from-Fortran" rel="next" title="Accessing the wisdom API from Fortran">
cannam@167 38 <link href="Plan-execution-in-Fortran.html#Plan-execution-in-Fortran" rel="prev" title="Plan execution in Fortran">
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cannam@167 68 </head>
cannam@167 69
cannam@167 70 <body lang="en">
cannam@167 71 <a name="Allocating-aligned-memory-in-Fortran"></a>
cannam@167 72 <div class="header">
cannam@167 73 <p>
cannam@167 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>
cannam@167 75 </div>
cannam@167 76 <hr>
cannam@167 77 <a name="Allocating-aligned-memory-in-Fortran-1"></a>
cannam@167 78 <h3 class="section">7.5 Allocating aligned memory in Fortran</h3>
cannam@167 79
cannam@167 80 <a name="index-alignment-5"></a>
cannam@167 81 <a name="index-fftw_005falloc_005freal-5"></a>
cannam@167 82 <a name="index-fftw_005falloc_005fcomplex-5"></a>
cannam@167 83 <p>In order to obtain maximum performance in FFTW, you should store your
cannam@167 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
cannam@167 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
cannam@167 86 arrays. Unfortunately, standard Fortran arrays do <em>not</em> provide
cannam@167 87 any alignment guarantees. The <em>only</em> way to allocate aligned
cannam@167 88 memory in standard Fortran is to allocate it with an external C
cannam@167 89 function, like the <code>fftw_alloc_real</code> and
cannam@167 90 <code>fftw_alloc_complex</code> functions. Fortunately, Fortran 2003 provides
cannam@167 91 a simple way to associate such allocated memory with a standard Fortran
cannam@167 92 array pointer that you can then use normally.
cannam@167 93 </p>
cannam@167 94 <p>We therefore recommend allocating all your input/output arrays using
cannam@167 95 the following technique:
cannam@167 96 </p>
cannam@167 97 <ol>
cannam@167 98 <li> Declare a <code>pointer</code>, <code>arr</code>, to your array of the desired type
cannam@167 99 and dimensions. For example, <code>real(C_DOUBLE), pointer :: a(:,:)</code>
cannam@167 100 for a 2d real array, or <code>complex(C_DOUBLE_COMPLEX), pointer ::
cannam@167 101 a(:,:,:)</code> for a 3d complex array.
cannam@167 102
cannam@167 103 </li><li> The number of elements to allocate must be an
cannam@167 104 <code>integer(C_SIZE_T)</code>. You can either declare a variable of this
cannam@167 105 type, e.g. <code>integer(C_SIZE_T) :: sz</code>, to store the number of
cannam@167 106 elements to allocate, or you can use the <code>int(..., C_SIZE_T)</code>
cannam@167 107 intrinsic function. e.g. set <code>sz = L * M * N</code> or use
cannam@167 108 <code>int(L * M * N, C_SIZE_T)</code> for an L&nbsp;&times;&nbsp;M&nbsp;&times;&nbsp;N
cannam@167 109 array.
cannam@167 110
cannam@167 111 </li><li> Declare a <code>type(C_PTR) :: p</code> to hold the return value from
cannam@167 112 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.
cannam@167 113
cannam@167 114 </li><li> <a name="index-c_005ff_005fpointer-2"></a>
cannam@167 115 Associate your pointer <code>arr</code> with the allocated memory <code>p</code>
cannam@167 116 using the standard <code>c_f_pointer</code> subroutine: <code>call
cannam@167 117 c_f_pointer(p, arr, [...dimensions...])</code>, where
cannam@167 118 <code>[...dimensions...])</code> are an array of the dimensions of the array
cannam@167 119 (in the usual Fortran order). e.g. <code>call c_f_pointer(p, arr,
cannam@167 120 [L,M,N])</code> for an L&nbsp;&times;&nbsp;M&nbsp;&times;&nbsp;N
cannam@167 121 array. (Alternatively, you can
cannam@167 122 omit the dimensions argument if you specified the shape explicitly
cannam@167 123 when declaring <code>arr</code>.) You can now use <code>arr</code> as a usual
cannam@167 124 multidimensional array.
cannam@167 125
cannam@167 126 </li><li> When you are done using the array, deallocate the memory by <code>call
cannam@167 127 fftw_free(p)</code> on <code>p</code>.
cannam@167 128
cannam@167 129 </li></ol>
cannam@167 130
cannam@167 131 <p>For example, here is how we would allocate an L&nbsp;&times;&nbsp;M
cannam@167 132 2d real array:
cannam@167 133 </p>
cannam@167 134 <div class="example">
cannam@167 135 <pre class="example"> real(C_DOUBLE), pointer :: arr(:,:)
cannam@167 136 type(C_PTR) :: p
cannam@167 137 p = fftw_alloc_real(int(L * M, C_SIZE_T))
cannam@167 138 call c_f_pointer(p, arr, [L,M])
cannam@167 139 <em>...use arr and arr(i,j) as usual...</em>
cannam@167 140 call fftw_free(p)
cannam@167 141 </pre></div>
cannam@167 142
cannam@167 143 <p>and here is an L&nbsp;&times;&nbsp;M&nbsp;&times;&nbsp;N
cannam@167 144 3d complex array:
cannam@167 145 </p>
cannam@167 146 <div class="example">
cannam@167 147 <pre class="example"> complex(C_DOUBLE_COMPLEX), pointer :: arr(:,:,:)
cannam@167 148 type(C_PTR) :: p
cannam@167 149 p = fftw_alloc_complex(int(L * M * N, C_SIZE_T))
cannam@167 150 call c_f_pointer(p, arr, [L,M,N])
cannam@167 151 <em>...use arr and arr(i,j,k) as usual...</em>
cannam@167 152 call fftw_free(p)
cannam@167 153 </pre></div>
cannam@167 154
cannam@167 155 <p>See <a href="Reversing-array-dimensions.html#Reversing-array-dimensions">Reversing array dimensions</a> for an example allocating a
cannam@167 156 single array and associating both real and complex array pointers with
cannam@167 157 it, for in-place real-to-complex transforms.
cannam@167 158 </p>
cannam@167 159 <hr>
cannam@167 160 <div class="header">
cannam@167 161 <p>
cannam@167 162 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>
cannam@167 163 </div>
cannam@167 164
cannam@167 165
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