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<h3 class="section">8.3 FFTW Execution in Fortran</h3>
<p>In C, in order to use a plan, one normally calls <code>fftw_execute</code>,
which executes the plan to perform the transform on the input/output
arrays passed when the plan was created (see <a href="Using-Plans.html#Using-Plans">Using Plans</a>). The
corresponding subroutine call in legacy Fortran is:
<pre class="example"> call dfftw_execute(plan)
</pre>
<p><a name="index-dfftw_005fexecute-584"></a>
However, we have had reports that this causes problems with some
recent optimizing Fortran compilers. The problem is, because the
input/output arrays are not passed as explicit arguments to
<code>dfftw_execute</code>, the semantics of Fortran (unlike C) allow the
compiler to assume that the input/output arrays are not changed by
<code>dfftw_execute</code>. As a consequence, certain compilers end up
optimizing out or repositioning the call to <code>dfftw_execute</code>,
assuming incorrectly that it does nothing.
<p>There are various workarounds to this, but the safest and simplest
thing is to not use <code>dfftw_execute</code> in Fortran. Instead, use the
functions described in <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>, which take
the input/output arrays as explicit arguments. For example, if the
plan is for a complex-data DFT and was created for the arrays
<code>in</code> and <code>out</code>, you would do:
<pre class="example"> call dfftw_execute_dft(plan, in, out)
</pre>
<p><a name="index-dfftw_005fexecute_005fdft-585"></a>
There are a few things to be careful of, however:
<ul>
<li>You must use the correct type of execute function, matching the way
the plan was created. Complex DFT plans should use
<code>dfftw_execute_dft</code>, Real-input (r2c) DFT plans should use use
<code>dfftw_execute_dft_r2c</code>, and real-output (c2r) DFT plans should
use <code>dfftw_execute_dft_c2r</code>. The various r2r plans should use
<code>dfftw_execute_r2r</code>.
<li>You should normally pass the same input/output arrays that were used when
creating the plan. This is always safe.
<li><em>If</em> you pass <em>different</em> input/output arrays compared to
those used when creating the plan, you must abide by all the
restrictions of the new-array execute functions (see <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>). The most difficult of these, in Fortran, is the
requirement that the new arrays have the same alignment as the
original arrays, because there seems to be no way in legacy Fortran to obtain
guaranteed-aligned arrays (analogous to <code>fftw_malloc</code> in C). You
can, of course, use the <code>FFTW_UNALIGNED</code> flag when creating the
plan, in which case the plan does not depend on the alignment, but
this may sacrifice substantial performance on architectures (like x86)
with SIMD instructions (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>).
<a name="index-FFTW_005fUNALIGNED-586"></a>
</ul>
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