Mercurial > hg > sv-dependency-builds
diff src/fftw-3.3.3/doc/html/Allocating-aligned-memory-in-Fortran.html @ 95:89f5e221ed7b
Add FFTW3
| author | Chris Cannam <cannam@all-day-breakfast.com> |
|---|---|
| date | Wed, 20 Mar 2013 15:35:50 +0000 |
| parents | |
| children |
line wrap: on
line diff
--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/src/fftw-3.3.3/doc/html/Allocating-aligned-memory-in-Fortran.html Wed Mar 20 15:35:50 2013 +0000 @@ -0,0 +1,129 @@ +<html lang="en"> +<head> +<title>Allocating aligned memory in Fortran - FFTW 3.3.3</title> +<meta http-equiv="Content-Type" content="text/html"> +<meta name="description" content="FFTW 3.3.3"> +<meta name="generator" content="makeinfo 4.13"> +<link title="Top" rel="start" href="index.html#Top"> +<link rel="up" href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran" title="Calling FFTW from Modern Fortran"> +<link rel="prev" href="Plan-execution-in-Fortran.html#Plan-execution-in-Fortran" title="Plan execution in Fortran"> +<link rel="next" href="Accessing-the-wisdom-API-from-Fortran.html#Accessing-the-wisdom-API-from-Fortran" title="Accessing the wisdom API from Fortran"> +<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> +<!-- +This manual is for FFTW +(version 3.3.3, 25 November 2012). + +Copyright (C) 2003 Matteo Frigo. + +Copyright (C) 2003 Massachusetts Institute of Technology. + + Permission is granted to make and distribute verbatim copies of + this manual provided the copyright notice and this permission + notice are preserved on all copies. + + Permission is granted to copy and distribute modified versions of + this manual under the conditions for verbatim copying, provided + that the entire resulting derived work is distributed under the + terms of a permission notice identical to this one. + + Permission is granted to copy and distribute translations of this + manual into another language, under the above conditions for + modified versions, except that this permission notice may be + stated in a translation approved by the Free Software Foundation. + --> +<meta http-equiv="Content-Style-Type" content="text/css"> +<style type="text/css"><!-- + pre.display { font-family:inherit } + pre.format { font-family:inherit } + pre.smalldisplay { font-family:inherit; font-size:smaller } + pre.smallformat { font-family:inherit; font-size:smaller } + pre.smallexample { font-size:smaller } + pre.smalllisp { font-size:smaller } + span.sc { font-variant:small-caps } + span.roman { font-family:serif; font-weight:normal; } + span.sansserif { font-family:sans-serif; font-weight:normal; } +--></style> +</head> +<body> +<div class="node"> +<a name="Allocating-aligned-memory-in-Fortran"></a> +<p> +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>, +Previous: <a rel="previous" accesskey="p" href="Plan-execution-in-Fortran.html#Plan-execution-in-Fortran">Plan execution in Fortran</a>, +Up: <a rel="up" accesskey="u" href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran">Calling FFTW from Modern Fortran</a> +<hr> +</div> + +<h3 class="section">7.5 Allocating aligned memory in Fortran</h3> + +<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 +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 +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 +arrays. Unfortunately, standard Fortran arrays do <em>not</em> provide +any alignment guarantees. The <em>only</em> way to allocate aligned +memory in standard Fortran is to allocate it with an external C +function, like the <code>fftw_alloc_real</code> and +<code>fftw_alloc_complex</code> functions. Fortunately, Fortran 2003 provides +a simple way to associate such allocated memory with a standard Fortran +array pointer that you can then use normally. + + <p>We therefore recommend allocating all your input/output arrays using +the following technique: + + <ol type=1 start=1> + + <li>Declare a <code>pointer</code>, <code>arr</code>, to your array of the desired type +and dimensions. For example, <code>real(C_DOUBLE), pointer :: a(:,:)</code> +for a 2d real array, or <code>complex(C_DOUBLE_COMPLEX), pointer :: +a(:,:,:)</code> for a 3d complex array. + + <li>The number of elements to allocate must be an +<code>integer(C_SIZE_T)</code>. You can either declare a variable of this +type, e.g. <code>integer(C_SIZE_T) :: sz</code>, to store the number of +elements to allocate, or you can use the <code>int(..., C_SIZE_T)</code> +intrinsic function. e.g. set <code>sz = L * M * N</code> or use +<code>int(L * M * N, C_SIZE_T)</code> for an L × M × N array. + + <li>Declare a <code>type(C_PTR) :: p</code> to hold the return value from +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. + + <li><a name="index-c_005ff_005fpointer-563"></a>Associate your pointer <code>arr</code> with the allocated memory <code>p</code> +using the standard <code>c_f_pointer</code> subroutine: <code>call +c_f_pointer(p, arr, [...dimensions...])</code>, where +<code>[...dimensions...])</code> are an array of the dimensions of the array +(in the usual Fortran order). e.g. <code>call c_f_pointer(p, arr, +[L,M,N])</code> for an L × M × N array. (Alternatively, you can +omit the dimensions argument if you specified the shape explicitly +when declaring <code>arr</code>.) You can now use <code>arr</code> as a usual +multidimensional array. + + <li>When you are done using the array, deallocate the memory by <code>call +fftw_free(p)</code> on <code>p</code>. + + </ol> + + <p>For example, here is how we would allocate an L × M 2d real array: + +<pre class="example"> real(C_DOUBLE), pointer :: arr(:,:) + type(C_PTR) :: p + p = fftw_alloc_real(int(L * M, C_SIZE_T)) + call c_f_pointer(p, arr, [L,M]) + <em>...use arr and arr(i,j) as usual...</em> + call fftw_free(p) +</pre> + <p>and here is an L × M × N 3d complex array: + +<pre class="example"> complex(C_DOUBLE_COMPLEX), pointer :: arr(:,:,:) + type(C_PTR) :: p + p = fftw_alloc_complex(int(L * M * N, C_SIZE_T)) + call c_f_pointer(p, arr, [L,M,N]) + <em>...use arr and arr(i,j,k) as usual...</em> + call fftw_free(p) +</pre> + <p>See <a href="Reversing-array-dimensions.html#Reversing-array-dimensions">Reversing array dimensions</a> for an example allocating a +single array and associating both real and complex array pointers with +it, for in-place real-to-complex transforms. + +<!-- --> + </body></html> +