Mercurial > hg > sv-dependency-builds

annotate src/fftw-3.3.8/doc/html/Allocating-aligned-memory-in-Fortran.html @ 83:ae30d91d2ffe

Find changesets by keywords (author, files, the commit message), revision number or hash, or revset expression.
Replace these with versions built using an older toolset (so as to avoid ABI compatibilities when linking on Ubuntu 14.04 for packaging purposes)
author Chris Cannam
date Fri, 07 Feb 2020 11:51:13 +0000
parents d0c2a83c1364
children
rev   line source
Chris@82 1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
Chris@82 2 <html>
Chris@82 3 <!-- This manual is for FFTW
Chris@82 4 (version 3.3.8, 24 May 2018).
Chris@82 5
Chris@82 6 Copyright (C) 2003 Matteo Frigo.
Chris@82 7
Chris@82 8 Copyright (C) 2003 Massachusetts Institute of Technology.
Chris@82 9
Chris@82 10 Permission is granted to make and distribute verbatim copies of this
Chris@82 11 manual provided the copyright notice and this permission notice are
Chris@82 12 preserved on all copies.
Chris@82 13
Chris@82 14 Permission is granted to copy and distribute modified versions of this
Chris@82 15 manual under the conditions for verbatim copying, provided that the
Chris@82 16 entire resulting derived work is distributed under the terms of a
Chris@82 17 permission notice identical to this one.
Chris@82 18
Chris@82 19 Permission is granted to copy and distribute translations of this manual
Chris@82 20 into another language, under the above conditions for modified versions,
Chris@82 21 except that this permission notice may be stated in a translation
Chris@82 22 approved by the Free Software Foundation. -->
Chris@82 23 <!-- Created by GNU Texinfo 6.3, http://www.gnu.org/software/texinfo/ -->
Chris@82 24 <head>
Chris@82 25 <title>FFTW 3.3.8: Allocating aligned memory in Fortran</title>
Chris@82 26
Chris@82 27 <meta name="description" content="FFTW 3.3.8: Allocating aligned memory in Fortran">
Chris@82 28 <meta name="keywords" content="FFTW 3.3.8: Allocating aligned memory in Fortran">
Chris@82 29 <meta name="resource-type" content="document">
Chris@82 30 <meta name="distribution" content="global">
Chris@82 31 <meta name="Generator" content="makeinfo">
Chris@82 32 <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
Chris@82 33 <link href="index.html#Top" rel="start" title="Top">
Chris@82 34 <link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
Chris@82 35 <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
Chris@82 36 <link href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran" rel="up" title="Calling FFTW from Modern Fortran">
Chris@82 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">
Chris@82 38 <link href="Plan-execution-in-Fortran.html#Plan-execution-in-Fortran" rel="prev" title="Plan execution in Fortran">
Chris@82 39 <style type="text/css">
Chris@82 40 <!--
Chris@82 41 a.summary-letter {text-decoration: none}
Chris@82 42 blockquote.indentedblock {margin-right: 0em}
Chris@82 43 blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
Chris@82 44 blockquote.smallquotation {font-size: smaller}
Chris@82 45 div.display {margin-left: 3.2em}
Chris@82 46 div.example {margin-left: 3.2em}
Chris@82 47 div.lisp {margin-left: 3.2em}
Chris@82 48 div.smalldisplay {margin-left: 3.2em}
Chris@82 49 div.smallexample {margin-left: 3.2em}
Chris@82 50 div.smalllisp {margin-left: 3.2em}
Chris@82 51 kbd {font-style: oblique}
Chris@82 52 pre.display {font-family: inherit}
Chris@82 53 pre.format {font-family: inherit}
Chris@82 54 pre.menu-comment {font-family: serif}
Chris@82 55 pre.menu-preformatted {font-family: serif}
Chris@82 56 pre.smalldisplay {font-family: inherit; font-size: smaller}
Chris@82 57 pre.smallexample {font-size: smaller}
Chris@82 58 pre.smallformat {font-family: inherit; font-size: smaller}
Chris@82 59 pre.smalllisp {font-size: smaller}
Chris@82 60 span.nolinebreak {white-space: nowrap}
Chris@82 61 span.roman {font-family: initial; font-weight: normal}
Chris@82 62 span.sansserif {font-family: sans-serif; font-weight: normal}
Chris@82 63 ul.no-bullet {list-style: none}
Chris@82 64 -->
Chris@82 65 </style>
Chris@82 66
Chris@82 67
Chris@82 68 </head>
Chris@82 69
Chris@82 70 <body lang="en">
Chris@82 71 <a name="Allocating-aligned-memory-in-Fortran"></a>
Chris@82 72 <div class="header">
Chris@82 73 <p>
Chris@82 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>
Chris@82 75 </div>
Chris@82 76 <hr>
Chris@82 77 <a name="Allocating-aligned-memory-in-Fortran-1"></a>
Chris@82 78 <h3 class="section">7.5 Allocating aligned memory in Fortran</h3>
Chris@82 79
Chris@82 80 <a name="index-alignment-5"></a>
Chris@82 81 <a name="index-fftw_005falloc_005freal-5"></a>
Chris@82 82 <a name="index-fftw_005falloc_005fcomplex-5"></a>
Chris@82 83 <p>In order to obtain maximum performance in FFTW, you should store your
Chris@82 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
Chris@82 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
Chris@82 86 arrays. Unfortunately, standard Fortran arrays do <em>not</em> provide
Chris@82 87 any alignment guarantees. The <em>only</em> way to allocate aligned
Chris@82 88 memory in standard Fortran is to allocate it with an external C
Chris@82 89 function, like the <code>fftw_alloc_real</code> and
Chris@82 90 <code>fftw_alloc_complex</code> functions. Fortunately, Fortran 2003 provides
Chris@82 91 a simple way to associate such allocated memory with a standard Fortran
Chris@82 92 array pointer that you can then use normally.
Chris@82 93 </p>
Chris@82 94 <p>We therefore recommend allocating all your input/output arrays using
Chris@82 95 the following technique:
Chris@82 96 </p>
Chris@82 97 <ol>
Chris@82 98 <li> Declare a <code>pointer</code>, <code>arr</code>, to your array of the desired type
Chris@82 99 and dimensions. For example, <code>real(C_DOUBLE), pointer :: a(:,:)</code>
Chris@82 100 for a 2d real array, or <code>complex(C_DOUBLE_COMPLEX), pointer ::
Chris@82 101 a(:,:,:)</code> for a 3d complex array.
Chris@82 102
Chris@82 103 </li><li> The number of elements to allocate must be an
Chris@82 104 <code>integer(C_SIZE_T)</code>. You can either declare a variable of this
Chris@82 105 type, e.g. <code>integer(C_SIZE_T) :: sz</code>, to store the number of
Chris@82 106 elements to allocate, or you can use the <code>int(..., C_SIZE_T)</code>
Chris@82 107 intrinsic function. e.g. set <code>sz = L * M * N</code> or use
Chris@82 108 <code>int(L * M * N, C_SIZE_T)</code> for an L&nbsp;&times;&nbsp;M&nbsp;&times;&nbsp;N
Chris@82 109 array.
Chris@82 110
Chris@82 111 </li><li> Declare a <code>type(C_PTR) :: p</code> to hold the return value from
Chris@82 112 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@82 113
Chris@82 114 </li><li> <a name="index-c_005ff_005fpointer-2"></a>
Chris@82 115 Associate your pointer <code>arr</code> with the allocated memory <code>p</code>
Chris@82 116 using the standard <code>c_f_pointer</code> subroutine: <code>call
Chris@82 117 c_f_pointer(p, arr, [...dimensions...])</code>, where
Chris@82 118 <code>[...dimensions...])</code> are an array of the dimensions of the array
Chris@82 119 (in the usual Fortran order). e.g. <code>call c_f_pointer(p, arr,
Chris@82 120 [L,M,N])</code> for an L&nbsp;&times;&nbsp;M&nbsp;&times;&nbsp;N
Chris@82 121 array. (Alternatively, you can
Chris@82 122 omit the dimensions argument if you specified the shape explicitly
Chris@82 123 when declaring <code>arr</code>.) You can now use <code>arr</code> as a usual
Chris@82 124 multidimensional array.
Chris@82 125
Chris@82 126 </li><li> When you are done using the array, deallocate the memory by <code>call
Chris@82 127 fftw_free(p)</code> on <code>p</code>.
Chris@82 128
Chris@82 129 </li></ol>
Chris@82 130
Chris@82 131 <p>For example, here is how we would allocate an L&nbsp;&times;&nbsp;M
Chris@82 132 2d real array:
Chris@82 133 </p>
Chris@82 134 <div class="example">
Chris@82 135 <pre class="example"> real(C_DOUBLE), pointer :: arr(:,:)
Chris@82 136 type(C_PTR) :: p
Chris@82 137 p = fftw_alloc_real(int(L * M, C_SIZE_T))
Chris@82 138 call c_f_pointer(p, arr, [L,M])
Chris@82 139 <em>...use arr and arr(i,j) as usual...</em>
Chris@82 140 call fftw_free(p)
Chris@82 141 </pre></div>
Chris@82 142
Chris@82 143 <p>and here is an L&nbsp;&times;&nbsp;M&nbsp;&times;&nbsp;N
Chris@82 144 3d complex array:
Chris@82 145 </p>
Chris@82 146 <div class="example">
Chris@82 147 <pre class="example"> complex(C_DOUBLE_COMPLEX), pointer :: arr(:,:,:)
Chris@82 148 type(C_PTR) :: p
Chris@82 149 p = fftw_alloc_complex(int(L * M * N, C_SIZE_T))
Chris@82 150 call c_f_pointer(p, arr, [L,M,N])
Chris@82 151 <em>...use arr and arr(i,j,k) as usual...</em>
Chris@82 152 call fftw_free(p)
Chris@82 153 </pre></div>
Chris@82 154
Chris@82 155 <p>See <a href="Reversing-array-dimensions.html#Reversing-array-dimensions">Reversing array dimensions</a> for an example allocating a
Chris@82 156 single array and associating both real and complex array pointers with
Chris@82 157 it, for in-place real-to-complex transforms.
Chris@82 158 </p>
Chris@82 159 <hr>
Chris@82 160 <div class="header">
Chris@82 161 <p>
Chris@82 162 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>
Chris@82 163 </div>
Chris@82 164
Chris@82 165
Chris@82 166
Chris@82 167 </body>
Chris@82 168 </html>