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Current fftw source
author Chris Cannam
date Tue, 18 Oct 2016 13:40:26 +0100
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Chris@42 25 <title>FFTW 3.3.5: Upgrading from FFTW version 2</title>
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Chris@42 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
Chris@42 72 <a name="Upgrading-from-FFTW-version-2"></a>
Chris@42 73 <div class="header">
Chris@42 74 <p>
Chris@42 75 Next: <a href="Installation-and-Customization.html#Installation-and-Customization" accesskey="n" rel="next">Installation and Customization</a>, Previous: <a href="Calling-FFTW-from-Legacy-Fortran.html#Calling-FFTW-from-Legacy-Fortran" accesskey="p" rel="prev">Calling FFTW from Legacy Fortran</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> &nbsp; [<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>
Chris@42 76 </div>
Chris@42 77 <hr>
Chris@42 78 <a name="Upgrading-from-FFTW-version-2-1"></a>
Chris@42 79 <h2 class="chapter">9 Upgrading from FFTW version 2</h2>
Chris@42 80
Chris@42 81 <p>In this chapter, we outline the process for updating codes designed for
Chris@42 82 the older FFTW 2 interface to work with FFTW 3. The interface for FFTW
Chris@42 83 3 is not backwards-compatible with the interface for FFTW 2 and earlier
Chris@42 84 versions; codes written to use those versions will fail to link with
Chris@42 85 FFTW 3. Nor is it possible to write &ldquo;compatibility wrappers&rdquo; to
Chris@42 86 bridge the gap (at least not efficiently), because FFTW 3 has different
Chris@42 87 semantics from previous versions. However, upgrading should be a
Chris@42 88 straightforward process because the data formats are identical and the
Chris@42 89 overall style of planning/execution is essentially the same.
Chris@42 90 </p>
Chris@42 91 <p>Unlike FFTW 2, there are no separate header files for real and complex
Chris@42 92 transforms (or even for different precisions) in FFTW 3; all interfaces
Chris@42 93 are defined in the <code>&lt;fftw3.h&gt;</code> header file.
Chris@42 94 </p>
Chris@42 95 <a name="Numeric-Types"></a>
Chris@42 96 <h3 class="heading">Numeric Types</h3>
Chris@42 97
Chris@42 98 <p>The main difference in data types is that <code>fftw_complex</code> in FFTW 2
Chris@42 99 was defined as a <code>struct</code> with macros <code>c_re</code> and <code>c_im</code>
Chris@42 100 for accessing the real/imaginary parts. (This is binary-compatible with
Chris@42 101 FFTW 3 on any machine except perhaps for some older Crays in single
Chris@42 102 precision.) The equivalent macros for FFTW 3 are:
Chris@42 103 </p>
Chris@42 104 <div class="example">
Chris@42 105 <pre class="example">#define c_re(c) ((c)[0])
Chris@42 106 #define c_im(c) ((c)[1])
Chris@42 107 </pre></div>
Chris@42 108
Chris@42 109 <p>This does not work if you are using the C99 complex type, however,
Chris@42 110 unless you insert a <code>double*</code> typecast into the above macros
Chris@42 111 (see <a href="Complex-numbers.html#Complex-numbers">Complex numbers</a>).
Chris@42 112 </p>
Chris@42 113 <p>Also, FFTW 2 had an <code>fftw_real</code> typedef that was an alias for
Chris@42 114 <code>double</code> (in double precision). In FFTW 3 you should just use
Chris@42 115 <code>double</code> (or whatever precision you are employing).
Chris@42 116 </p>
Chris@42 117 <a name="Plans"></a>
Chris@42 118 <h3 class="heading">Plans</h3>
Chris@42 119
Chris@42 120 <p>The major difference between FFTW 2 and FFTW 3 is in the
Chris@42 121 planning/execution division of labor. In FFTW 2, plans were found for a
Chris@42 122 given transform size and type, and then could be applied to <em>any</em>
Chris@42 123 arrays and for <em>any</em> multiplicity/stride parameters. In FFTW 3,
Chris@42 124 you specify the particular arrays, stride parameters, etcetera when
Chris@42 125 creating the plan, and the plan is then executed for <em>those</em> arrays
Chris@42 126 (unless the guru interface is used) and <em>those</em> parameters
Chris@42 127 <em>only</em>. (FFTW 2 had &ldquo;specific planner&rdquo; routines that planned for
Chris@42 128 a particular array and stride, but the plan could still be used for
Chris@42 129 other arrays and strides.) That is, much of the information that was
Chris@42 130 formerly specified at execution time is now specified at planning time.
Chris@42 131 </p>
Chris@42 132 <p>Like FFTW 2&rsquo;s specific planner routines, the FFTW 3 planner overwrites
Chris@42 133 the input/output arrays unless you use <code>FFTW_ESTIMATE</code>.
Chris@42 134 </p>
Chris@42 135 <p>FFTW 2 had separate data types <code>fftw_plan</code>, <code>fftwnd_plan</code>,
Chris@42 136 <code>rfftw_plan</code>, and <code>rfftwnd_plan</code> for complex and real one- and
Chris@42 137 multi-dimensional transforms, and each type had its own &lsquo;<samp>destroy</samp>&rsquo;
Chris@42 138 function. In FFTW 3, all plans are of type <code>fftw_plan</code> and all are
Chris@42 139 destroyed by <code>fftw_destroy_plan(plan)</code>.
Chris@42 140 </p>
Chris@42 141 <p>Where you formerly used <code>fftw_create_plan</code> and <code>fftw_one</code> to
Chris@42 142 plan and compute a single 1d transform, you would now use
Chris@42 143 <code>fftw_plan_dft_1d</code> to plan the transform. If you used the generic
Chris@42 144 <code>fftw</code> function to execute the transform with multiplicity
Chris@42 145 (<code>howmany</code>) and stride parameters, you would now use the advanced
Chris@42 146 interface <code>fftw_plan_many_dft</code> to specify those parameters. The
Chris@42 147 plans are now executed with <code>fftw_execute(plan)</code>, which takes all
Chris@42 148 of its parameters (including the input/output arrays) from the plan.
Chris@42 149 </p>
Chris@42 150 <p>In-place transforms no longer interpret their output argument as scratch
Chris@42 151 space, nor is there an <code>FFTW_IN_PLACE</code> flag. You simply pass the
Chris@42 152 same pointer for both the input and output arguments. (Previously, the
Chris@42 153 output <code>ostride</code> and <code>odist</code> parameters were ignored for
Chris@42 154 in-place transforms; now, if they are specified via the advanced
Chris@42 155 interface, they are significant even in the in-place case, although they
Chris@42 156 should normally equal the corresponding input parameters.)
Chris@42 157 </p>
Chris@42 158 <p>The <code>FFTW_ESTIMATE</code> and <code>FFTW_MEASURE</code> flags have the same
Chris@42 159 meaning as before, although the planning time will differ. You may also
Chris@42 160 consider using <code>FFTW_PATIENT</code>, which is like <code>FFTW_MEASURE</code>
Chris@42 161 except that it takes more time in order to consider a wider variety of
Chris@42 162 algorithms.
Chris@42 163 </p>
Chris@42 164 <p>For multi-dimensional complex DFTs, instead of <code>fftwnd_create_plan</code>
Chris@42 165 (or <code>fftw2d_create_plan</code> or <code>fftw3d_create_plan</code>), followed by
Chris@42 166 <code>fftwnd_one</code>, you would use <code>fftw_plan_dft</code> (or
Chris@42 167 <code>fftw_plan_dft_2d</code> or <code>fftw_plan_dft_3d</code>). followed by
Chris@42 168 <code>fftw_execute</code>. If you used <code>fftwnd</code> to to specify strides
Chris@42 169 etcetera, you would instead specify these via <code>fftw_plan_many_dft</code>.
Chris@42 170 </p>
Chris@42 171 <p>The analogues to <code>rfftw_create_plan</code> and <code>rfftw_one</code> with
Chris@42 172 <code>FFTW_REAL_TO_COMPLEX</code> or <code>FFTW_COMPLEX_TO_REAL</code> directions
Chris@42 173 are <code>fftw_plan_r2r_1d</code> with kind <code>FFTW_R2HC</code> or
Chris@42 174 <code>FFTW_HC2R</code>, followed by <code>fftw_execute</code>. The stride etcetera
Chris@42 175 arguments of <code>rfftw</code> are now in <code>fftw_plan_many_r2r</code>.
Chris@42 176 </p>
Chris@42 177 <p>Instead of <code>rfftwnd_create_plan</code> (or <code>rfftw2d_create_plan</code> or
Chris@42 178 <code>rfftw3d_create_plan</code>) followed by
Chris@42 179 <code>rfftwnd_one_real_to_complex</code> or
Chris@42 180 <code>rfftwnd_one_complex_to_real</code>, you now use <code>fftw_plan_dft_r2c</code>
Chris@42 181 (or <code>fftw_plan_dft_r2c_2d</code> or <code>fftw_plan_dft_r2c_3d</code>) or
Chris@42 182 <code>fftw_plan_dft_c2r</code> (or <code>fftw_plan_dft_c2r_2d</code> or
Chris@42 183 <code>fftw_plan_dft_c2r_3d</code>), respectively, followed by
Chris@42 184 <code>fftw_execute</code>. As usual, the strides etcetera of
Chris@42 185 <code>rfftwnd_real_to_complex</code> or <code>rfftwnd_complex_to_real</code> are no
Chris@42 186 specified in the advanced planner routines,
Chris@42 187 <code>fftw_plan_many_dft_r2c</code> or <code>fftw_plan_many_dft_c2r</code>.
Chris@42 188 </p>
Chris@42 189 <a name="Wisdom-2"></a>
Chris@42 190 <h3 class="heading">Wisdom</h3>
Chris@42 191
Chris@42 192 <p>In FFTW 2, you had to supply the <code>FFTW_USE_WISDOM</code> flag in order to
Chris@42 193 use wisdom; in FFTW 3, wisdom is always used. (You could simulate the
Chris@42 194 FFTW 2 wisdom-less behavior by calling <code>fftw_forget_wisdom</code> after
Chris@42 195 every planner call.)
Chris@42 196 </p>
Chris@42 197 <p>The FFTW 3 wisdom import/export routines are almost the same as before
Chris@42 198 (although the storage format is entirely different). There is one
Chris@42 199 significant difference, however. In FFTW 2, the import routines would
Chris@42 200 never read past the end of the wisdom, so you could store extra data
Chris@42 201 beyond the wisdom in the same file, for example. In FFTW 3, the
Chris@42 202 file-import routine may read up to a few hundred bytes past the end of
Chris@42 203 the wisdom, so you cannot store other data just beyond it.<a name="DOCF11" href="#FOOT11"><sup>11</sup></a>
Chris@42 204 </p>
Chris@42 205 <p>Wisdom has been enhanced by additional humility in FFTW 3: whereas FFTW
Chris@42 206 2 would re-use wisdom for a given transform size regardless of the
Chris@42 207 stride etc., in FFTW 3 wisdom is only used with the strides etc. for
Chris@42 208 which it was created. Unfortunately, this means FFTW 3 has to create
Chris@42 209 new plans from scratch more often than FFTW 2 (in FFTW 2, planning
Chris@42 210 e.g. one transform of size 1024 also created wisdom for all smaller
Chris@42 211 powers of 2, but this no longer occurs).
Chris@42 212 </p>
Chris@42 213 <p>FFTW 3 also has the new routine <code>fftw_import_system_wisdom</code> to
Chris@42 214 import wisdom from a standard system-wide location.
Chris@42 215 </p>
Chris@42 216 <a name="Memory-allocation"></a>
Chris@42 217 <h3 class="heading">Memory allocation</h3>
Chris@42 218
Chris@42 219 <p>In FFTW 3, we recommend allocating your arrays with <code>fftw_malloc</code>
Chris@42 220 and deallocating them with <code>fftw_free</code>; this is not required, but
Chris@42 221 allows optimal performance when SIMD acceleration is used. (Those two
Chris@42 222 functions actually existed in FFTW 2, and worked the same way, but were
Chris@42 223 not documented.)
Chris@42 224 </p>
Chris@42 225 <p>In FFTW 2, there were <code>fftw_malloc_hook</code> and <code>fftw_free_hook</code>
Chris@42 226 functions that allowed the user to replace FFTW&rsquo;s memory-allocation
Chris@42 227 routines (e.g. to implement different error-handling, since by default
Chris@42 228 FFTW prints an error message and calls <code>exit</code> to abort the program
Chris@42 229 if <code>malloc</code> returns <code>NULL</code>). These hooks are not supported in
Chris@42 230 FFTW 3; those few users who require this functionality can just
Chris@42 231 directly modify the memory-allocation routines in FFTW (they are defined
Chris@42 232 in <code>kernel/alloc.c</code>).
Chris@42 233 </p>
Chris@42 234 <a name="Fortran-interface"></a>
Chris@42 235 <h3 class="heading">Fortran interface</h3>
Chris@42 236
Chris@42 237 <p>In FFTW 2, the subroutine names were obtained by replacing &lsquo;<samp>fftw_</samp>&rsquo;
Chris@42 238 with &lsquo;<samp>fftw_f77</samp>&rsquo;; in FFTW 3, you replace &lsquo;<samp>fftw_</samp>&rsquo; with
Chris@42 239 &lsquo;<samp>dfftw_</samp>&rsquo; (or &lsquo;<samp>sfftw_</samp>&rsquo; or &lsquo;<samp>lfftw_</samp>&rsquo;, depending upon the
Chris@42 240 precision).
Chris@42 241 </p>
Chris@42 242 <p>In FFTW 3, we have begun recommending that you always declare the type
Chris@42 243 used to store plans as <code>integer*8</code>. (Too many people didn&rsquo;t notice
Chris@42 244 our instruction to switch from <code>integer</code> to <code>integer*8</code> for
Chris@42 245 64-bit machines.)
Chris@42 246 </p>
Chris@42 247 <p>In FFTW 3, we provide a <code>fftw3.f</code> &ldquo;header file&rdquo; to include in
Chris@42 248 your code (and which is officially installed on Unix systems). (In FFTW
Chris@42 249 2, we supplied a <code>fftw_f77.i</code> file, but it was not installed.)
Chris@42 250 </p>
Chris@42 251 <p>Otherwise, the C-Fortran interface relationship is much the same as it
Chris@42 252 was before (e.g. return values become initial parameters, and
Chris@42 253 multi-dimensional arrays are in column-major order). Unlike FFTW 2, we
Chris@42 254 do provide some support for wisdom import/export in Fortran
Chris@42 255 (see <a href="Wisdom-of-Fortran_003f.html#Wisdom-of-Fortran_003f">Wisdom of Fortran?</a>).
Chris@42 256 </p>
Chris@42 257 <a name="Threads"></a>
Chris@42 258 <h3 class="heading">Threads</h3>
Chris@42 259
Chris@42 260 <p>Like FFTW 2, only the execution routines are thread-safe. All planner
Chris@42 261 routines, etcetera, should be called by only a single thread at a time
Chris@42 262 (see <a href="Thread-safety.html#Thread-safety">Thread safety</a>). <em>Unlike</em> FFTW 2, there is no special
Chris@42 263 <code>FFTW_THREADSAFE</code> flag for the planner to allow a given plan to be
Chris@42 264 usable by multiple threads in parallel; this is now the case by default.
Chris@42 265 </p>
Chris@42 266 <p>The multi-threaded version of FFTW 2 required you to pass the number of
Chris@42 267 threads each time you execute the transform. The number of threads is
Chris@42 268 now stored in the plan, and is specified before the planner is called by
Chris@42 269 <code>fftw_plan_with_nthreads</code>. The threads initialization routine used
Chris@42 270 to be called <code>fftw_threads_init</code> and would return zero on success;
Chris@42 271 the new routine is called <code>fftw_init_threads</code> and returns zero on
Chris@42 272 failure. See <a href="Multi_002dthreaded-FFTW.html#Multi_002dthreaded-FFTW">Multi-threaded FFTW</a>.
Chris@42 273 </p>
Chris@42 274 <p>There is no separate threads header file in FFTW 3; all the function
Chris@42 275 prototypes are in <code>&lt;fftw3.h&gt;</code>. However, you still have to link to
Chris@42 276 a separate library (<code>-lfftw3_threads -lfftw3 -lm</code> on Unix), as well as
Chris@42 277 to the threading library (e.g. POSIX threads on Unix).
Chris@42 278 </p>
Chris@42 279 <div class="footnote">
Chris@42 280 <hr>
Chris@42 281 <h4 class="footnotes-heading">Footnotes</h4>
Chris@42 282
Chris@42 283 <h3><a name="FOOT11" href="#DOCF11">(11)</a></h3>
Chris@42 284 <p>We
Chris@42 285 do our own buffering because GNU libc I/O routines are horribly slow for
Chris@42 286 single-character I/O, apparently for thread-safety reasons (whether you
Chris@42 287 are using threads or not).</p>
Chris@42 288 </div>
Chris@42 289 <hr>
Chris@42 290 <div class="header">
Chris@42 291 <p>
Chris@42 292 Next: <a href="Installation-and-Customization.html#Installation-and-Customization" accesskey="n" rel="next">Installation and Customization</a>, Previous: <a href="Calling-FFTW-from-Legacy-Fortran.html#Calling-FFTW-from-Legacy-Fortran" accesskey="p" rel="prev">Calling FFTW from Legacy Fortran</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> &nbsp; [<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>
Chris@42 293 </div>
Chris@42 294
Chris@42 295
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