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Tidy up for ROLI
author Geogaddi\David <d.m.ronan@qmul.ac.uk>
date Tue, 17 May 2016 18:50:19 +0100
parents 636c989477e7
children
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--- a/Lib/fftw-3.2.1/doc/html/.svn/text-base/FFTW-Execution-in-Fortran.html.svn-base	Wed May 04 11:02:59 2016 +0100
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-<title>FFTW Execution in Fortran - FFTW 3.2.1</title>
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-<a name="FFTW-Execution-in-Fortran"></a>
-Next:&nbsp;<a rel="next" accesskey="n" href="Fortran-Examples.html#Fortran-Examples">Fortran Examples</a>,
-Previous:&nbsp;<a rel="previous" accesskey="p" href="FFTW-Constants-in-Fortran.html#FFTW-Constants-in-Fortran">FFTW Constants in Fortran</a>,
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-<hr>
-</div>
-
-<h3 class="section">7.3 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 Fortran is:
-<pre class="example">             call dfftw_execute(plan)
-</pre>
-   <p><a name="index-dfftw_005fexecute-338"></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-339"></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 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-340"></a>
-</ul>
-
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