158 lines
7.5 KiB
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158 lines
7.5 KiB
HTML
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<!-- This manual is for FFTW
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(version 3.3.10, 10 December 2020).
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Copyright (C) 2003 Matteo Frigo.
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Copyright (C) 2003 Massachusetts Institute of Technology.
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Permission is granted to make and distribute verbatim copies of this
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manual provided the copyright notice and this permission notice are
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<title>Allocating aligned memory in Fortran (FFTW 3.3.10)</title>
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Calling-FFTW-from-Modern-Fortran.html" rel="up" title="Calling FFTW from Modern Fortran">
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<link href="Accessing-the-wisdom-API-from-Fortran.html" rel="next" title="Accessing the wisdom API from Fortran">
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<span id="Allocating-aligned-memory-in-Fortran"></span><div class="header">
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<p>
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Next: <a href="Accessing-the-wisdom-API-from-Fortran.html" accesskey="n" rel="next">Accessing the wisdom API from Fortran</a>, Previous: <a href="Plan-execution-in-Fortran.html" accesskey="p" rel="prev">Plan execution in Fortran</a>, Up: <a href="Calling-FFTW-from-Modern-Fortran.html" accesskey="u" rel="up">Calling FFTW from Modern Fortran</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<span id="Allocating-aligned-memory-in-Fortran-1"></span><h3 class="section">7.5 Allocating aligned memory in Fortran</h3>
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<span id="index-alignment-5"></span>
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<span id="index-fftw_005falloc_005freal-5"></span>
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<span id="index-fftw_005falloc_005fcomplex-5"></span>
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<p>In order to obtain maximum performance in FFTW, you should store your
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data in arrays that have been specially aligned in memory (see <a href="SIMD-alignment-and-fftw_005fmalloc.html">SIMD alignment and fftw_malloc</a>). Enforcing alignment also permits you to
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safely use the new-array execute functions (see <a href="New_002darray-Execute-Functions.html">New-array Execute Functions</a>) to apply a given plan to more than one pair of in/out
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arrays. Unfortunately, standard Fortran arrays do <em>not</em> provide
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any alignment guarantees. The <em>only</em> way to allocate aligned
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memory in standard Fortran is to allocate it with an external C
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function, like the <code>fftw_alloc_real</code> and
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<code>fftw_alloc_complex</code> functions. Fortunately, Fortran 2003 provides
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a simple way to associate such allocated memory with a standard Fortran
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array pointer that you can then use normally.
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</p>
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<p>We therefore recommend allocating all your input/output arrays using
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the following technique:
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</p>
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<ol>
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<li> Declare a <code>pointer</code>, <code>arr</code>, to your array of the desired type
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and dimensions. For example, <code>real(C_DOUBLE), pointer :: a(:,:)</code>
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for a 2d real array, or <code>complex(C_DOUBLE_COMPLEX), pointer ::
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a(:,:,:)</code> for a 3d complex array.
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</li><li> The number of elements to allocate must be an
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<code>integer(C_SIZE_T)</code>. You can either declare a variable of this
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type, e.g. <code>integer(C_SIZE_T) :: sz</code>, to store the number of
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elements to allocate, or you can use the <code>int(..., C_SIZE_T)</code>
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intrinsic function. e.g. set <code>sz = L * M * N</code> or use
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<code>int(L * M * N, C_SIZE_T)</code> for an L × M × N
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array.
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</li><li> Declare a <code>type(C_PTR) :: p</code> to hold the return value from
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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.
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</li><li> <span id="index-c_005ff_005fpointer-2"></span>
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Associate your pointer <code>arr</code> with the allocated memory <code>p</code>
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using the standard <code>c_f_pointer</code> subroutine: <code>call
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c_f_pointer(p, arr, [...dimensions...])</code>, where
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<code>[...dimensions...])</code> are an array of the dimensions of the array
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(in the usual Fortran order). e.g. <code>call c_f_pointer(p, arr,
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[L,M,N])</code> for an L × M × N
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array. (Alternatively, you can
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omit the dimensions argument if you specified the shape explicitly
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when declaring <code>arr</code>.) You can now use <code>arr</code> as a usual
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multidimensional array.
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</li><li> When you are done using the array, deallocate the memory by <code>call
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fftw_free(p)</code> on <code>p</code>.
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</li></ol>
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<p>For example, here is how we would allocate an L × M
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2d real array:
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</p>
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<div class="example">
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<pre class="example"> real(C_DOUBLE), pointer :: arr(:,:)
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type(C_PTR) :: p
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p = fftw_alloc_real(int(L * M, C_SIZE_T))
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call c_f_pointer(p, arr, [L,M])
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<em>...use arr and arr(i,j) as usual...</em>
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call fftw_free(p)
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</pre></div>
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<p>and here is an L × M × N
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3d complex array:
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</p>
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<div class="example">
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<pre class="example"> complex(C_DOUBLE_COMPLEX), pointer :: arr(:,:,:)
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type(C_PTR) :: p
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p = fftw_alloc_complex(int(L * M * N, C_SIZE_T))
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call c_f_pointer(p, arr, [L,M,N])
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<em>...use arr and arr(i,j,k) as usual...</em>
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call fftw_free(p)
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</pre></div>
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<p>See <a href="Reversing-array-dimensions.html">Reversing array dimensions</a> for an example allocating a
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single array and associating both real and complex array pointers with
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it, for in-place real-to-complex transforms.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Accessing-the-wisdom-API-from-Fortran.html" accesskey="n" rel="next">Accessing the wisdom API from Fortran</a>, Previous: <a href="Plan-execution-in-Fortran.html" accesskey="p" rel="prev">Plan execution in Fortran</a>, Up: <a href="Calling-FFTW-from-Modern-Fortran.html" accesskey="u" rel="up">Calling FFTW from Modern Fortran</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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