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cannam@167: <title>FFTW 3.3.8: FFTW Execution in Fortran</title>
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cannam@167: Next: <a href="Fortran-Examples.html#Fortran-Examples" accesskey="n" rel="next">Fortran Examples</a>, Previous: <a href="FFTW-Constants-in-Fortran.html#FFTW-Constants-in-Fortran" accesskey="p" rel="prev">FFTW Constants in Fortran</a>, Up: <a href="Calling-FFTW-from-Legacy-Fortran.html#Calling-FFTW-from-Legacy-Fortran" accesskey="u" rel="up">Calling FFTW from Legacy Fortran</a> [<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>
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cannam@167: <hr>
cannam@167: <a name="FFTW-Execution-in-Fortran-1"></a>
cannam@167: <h3 class="section">8.3 FFTW Execution in Fortran</h3>
cannam@167:
cannam@167: <p>In C, in order to use a plan, one normally calls <code>fftw_execute</code>,
cannam@167: which executes the plan to perform the transform on the input/output
cannam@167: arrays passed when the plan was created (see <a href="Using-Plans.html#Using-Plans">Using Plans</a>). The
cannam@167: corresponding subroutine call in legacy Fortran is:
cannam@167: </p><div class="example">
cannam@167: <pre class="example"> call dfftw_execute(plan)
cannam@167: </pre></div>
cannam@167: <a name="index-dfftw_005fexecute"></a>
cannam@167:
cannam@167: <p>However, we have had reports that this causes problems with some
cannam@167: recent optimizing Fortran compilers. The problem is, because the
cannam@167: input/output arrays are not passed as explicit arguments to
cannam@167: <code>dfftw_execute</code>, the semantics of Fortran (unlike C) allow the
cannam@167: compiler to assume that the input/output arrays are not changed by
cannam@167: <code>dfftw_execute</code>. As a consequence, certain compilers end up
cannam@167: optimizing out or repositioning the call to <code>dfftw_execute</code>,
cannam@167: assuming incorrectly that it does nothing.
cannam@167: </p>
cannam@167: <p>There are various workarounds to this, but the safest and simplest
cannam@167: thing is to not use <code>dfftw_execute</code> in Fortran. Instead, use the
cannam@167: functions described in <a href="New_002darray-Execute-Functions.html#New_002darray-Execute-Functions">New-array Execute Functions</a>, which take
cannam@167: the input/output arrays as explicit arguments. For example, if the
cannam@167: plan is for a complex-data DFT and was created for the arrays
cannam@167: <code>in</code> and <code>out</code>, you would do:
cannam@167: </p><div class="example">
cannam@167: <pre class="example"> call dfftw_execute_dft(plan, in, out)
cannam@167: </pre></div>
cannam@167: <a name="index-dfftw_005fexecute_005fdft"></a>
cannam@167:
cannam@167: <p>There are a few things to be careful of, however:
cannam@167: </p>
cannam@167: <ul>
cannam@167: <li> You must use the correct type of execute function, matching the way
cannam@167: the plan was created. Complex DFT plans should use
cannam@167: <code>dfftw_execute_dft</code>, Real-input (r2c) DFT plans should use use
cannam@167: <code>dfftw_execute_dft_r2c</code>, and real-output (c2r) DFT plans should
cannam@167: use <code>dfftw_execute_dft_c2r</code>. The various r2r plans should use
cannam@167: <code>dfftw_execute_r2r</code>.
cannam@167:
cannam@167: </li><li> You should normally pass the same input/output arrays that were used when
cannam@167: creating the plan. This is always safe.
cannam@167:
cannam@167: </li><li> <em>If</em> you pass <em>different</em> input/output arrays compared to
cannam@167: those used when creating the plan, you must abide by all the
cannam@167: 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
cannam@167: requirement that the new arrays have the same alignment as the
cannam@167: original arrays, because there seems to be no way in legacy Fortran to obtain
cannam@167: guaranteed-aligned arrays (analogous to <code>fftw_malloc</code> in C). You
cannam@167: can, of course, use the <code>FFTW_UNALIGNED</code> flag when creating the
cannam@167: plan, in which case the plan does not depend on the alignment, but
cannam@167: this may sacrifice substantial performance on architectures (like x86)
cannam@167: with SIMD instructions (see <a href="SIMD-alignment-and-fftw_005fmalloc.html#SIMD-alignment-and-fftw_005fmalloc">SIMD alignment and fftw_malloc</a>).
cannam@167: <a name="index-FFTW_005fUNALIGNED-3"></a>
cannam@167:
cannam@167: </li></ul>
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cannam@167: <p>
cannam@167: Next: <a href="Fortran-Examples.html#Fortran-Examples" accesskey="n" rel="next">Fortran Examples</a>, Previous: <a href="FFTW-Constants-in-Fortran.html#FFTW-Constants-in-Fortran" accesskey="p" rel="prev">FFTW Constants in Fortran</a>, Up: <a href="Calling-FFTW-from-Legacy-Fortran.html#Calling-FFTW-from-Legacy-Fortran" accesskey="u" rel="up">Calling FFTW from Legacy Fortran</a> [<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>
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