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diff src/fftw-3.3.3/doc/html/Plan-execution-in-Fortran.html @ 95:89f5e221ed7b
Add FFTW3
| author | Chris Cannam <cannam@all-day-breakfast.com> |
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| date | Wed, 20 Mar 2013 15:35:50 +0000 |
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--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/src/fftw-3.3.3/doc/html/Plan-execution-in-Fortran.html Wed Mar 20 15:35:50 2013 +0000 @@ -0,0 +1,113 @@ +<html lang="en"> +<head> +<title>Plan execution in Fortran - FFTW 3.3.3</title> +<meta http-equiv="Content-Type" content="text/html"> +<meta name="description" content="FFTW 3.3.3"> +<meta name="generator" content="makeinfo 4.13"> +<link title="Top" rel="start" href="index.html#Top"> +<link rel="up" href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran" title="Calling FFTW from Modern Fortran"> +<link rel="prev" href="FFTW-Fortran-type-reference.html#FFTW-Fortran-type-reference" title="FFTW Fortran type reference"> +<link rel="next" href="Allocating-aligned-memory-in-Fortran.html#Allocating-aligned-memory-in-Fortran" title="Allocating aligned memory in Fortran"> +<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> +<!-- +This manual is for FFTW +(version 3.3.3, 25 November 2012). + +Copyright (C) 2003 Matteo Frigo. + +Copyright (C) 2003 Massachusetts Institute of Technology. + + Permission is granted to make and distribute verbatim copies of + this manual provided the copyright notice and this permission + notice are preserved on all copies. + + Permission is granted to copy and distribute modified versions of + this manual under the conditions for verbatim copying, provided + that the entire resulting derived work is distributed under the + terms of a permission notice identical to this one. + + Permission is granted to copy and distribute translations of this + manual into another language, under the above conditions for + modified versions, except that this permission notice may be + stated in a translation approved by the Free Software Foundation. + --> +<meta http-equiv="Content-Style-Type" content="text/css"> +<style type="text/css"><!-- + pre.display { font-family:inherit } + pre.format { font-family:inherit } + pre.smalldisplay { font-family:inherit; font-size:smaller } + pre.smallformat { font-family:inherit; font-size:smaller } + pre.smallexample { font-size:smaller } + pre.smalllisp { font-size:smaller } + span.sc { font-variant:small-caps } + span.roman { font-family:serif; font-weight:normal; } + span.sansserif { font-family:sans-serif; font-weight:normal; } +--></style> +</head> +<body> +<div class="node"> +<a name="Plan-execution-in-Fortran"></a> +<p> +Next: <a rel="next" accesskey="n" href="Allocating-aligned-memory-in-Fortran.html#Allocating-aligned-memory-in-Fortran">Allocating aligned memory in Fortran</a>, +Previous: <a rel="previous" accesskey="p" href="FFTW-Fortran-type-reference.html#FFTW-Fortran-type-reference">FFTW Fortran type reference</a>, +Up: <a rel="up" accesskey="u" href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran">Calling FFTW from Modern Fortran</a> +<hr> +</div> + +<h3 class="section">7.4 Plan 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 modern Fortran is: +<pre class="example"> call fftw_execute(plan) +</pre> + <p><a name="index-fftw_005fexecute-554"></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>fftw_execute</code>, the semantics of Fortran (unlike C) allow the +compiler to assume that the input/output arrays are not changed by +<code>fftw_execute</code>. As a consequence, certain compilers end up +repositioning the call to <code>fftw_execute</code>, assuming incorrectly +that it does nothing to the arrays. + + <p>There are various workarounds to this, but the safest and simplest +thing is to not use <code>fftw_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 fftw_execute_dft(plan, in, out) +</pre> + <p><a name="index-fftw_005fexecute_005fdft-555"></a> +There are a few things to be careful of, however: + + <ul> +<li><a name="index-fftw_005fexecute_005fdft_005fr2c-556"></a><a name="index-fftw_005fexecute_005fdft_005fc2r-557"></a><a name="index-fftw_005fexecute_005fr2r-558"></a>You must use the correct type of execute function, matching the way +the plan was created. Complex DFT plans should use +<code>fftw_execute_dft</code>, Real-input (r2c) DFT plans should use use +<code>fftw_execute_dft_r2c</code>, and real-output (c2r) DFT plans should +use <code>fftw_execute_dft_c2r</code>. The various r2r plans should use +<code>fftw_execute_r2r</code>. Fortunately, if you use the wrong one you +will get a compile-time type-mismatch error (unlike legacy Fortran). + + <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 tricky of these is the +requirement that the new arrays have the same alignment as the +original arrays; the best (and possibly only) way to guarantee this +is to use the ‘<samp><span class="samp">fftw_alloc</span></samp>’ functions to allocate your arrays (see <a href="Allocating-aligned-memory-in-Fortran.html#Allocating-aligned-memory-in-Fortran">Allocating aligned memory in Fortran</a>). Alternatively, you can +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-559"></a> +</ul> + +<!-- --> + </body></html> +