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<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&nbsp;&times;&nbsp;M&nbsp;&times;&nbsp;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&nbsp;&times;&nbsp;M&nbsp;&times;&nbsp;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&nbsp;&times;&nbsp;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&nbsp;&times;&nbsp;M&nbsp;&times;&nbsp;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.

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author	Chris Cannam
date	Wed, 20 Mar 2013 15:35:50 +0000
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