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2 <head>
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3 <title>FFTW Accuracy on Cell - FFTW 3.2.1</title>
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4 <meta http-equiv="Content-Type" content="text/html">
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5 <meta name="description" content="FFTW 3.2.1">
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6 <meta name="generator" content="makeinfo 4.8">
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7 <link title="Top" rel="start" href="index.html#Top">
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8 <link rel="up" href="FFTW-on-the-Cell-Processor.html#FFTW-on-the-Cell-Processor" title="FFTW on the Cell Processor">
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9 <link rel="prev" href="Cell-Caveats.html#Cell-Caveats" title="Cell Caveats">
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10 <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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11 <!--
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12 This manual is for FFTW
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13 (version 3.2.1, 5 February 2009).
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14
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15 Copyright (C) 2003 Matteo Frigo.
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16
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17 Copyright (C) 2003 Massachusetts Institute of Technology.
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18
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19 Permission is granted to make and distribute verbatim copies of
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20 this manual provided the copyright notice and this permission
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21 notice are preserved on all copies.
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22
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23 Permission is granted to copy and distribute modified versions of
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24 this manual under the conditions for verbatim copying, provided
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25 that the entire resulting derived work is distributed under the
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26 terms of a permission notice identical to this one.
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27
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28 Permission is granted to copy and distribute translations of this
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29 manual into another language, under the above conditions for
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30 modified versions, except that this permission notice may be
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31 stated in a translation approved by the Free Software Foundation.
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32 -->
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44 --></style>
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45 </head>
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46 <body>
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47 <div class="node">
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48 <p>
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49 <a name="FFTW-Accuracy-on-Cell"></a>
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50 Previous: <a rel="previous" accesskey="p" href="Cell-Caveats.html#Cell-Caveats">Cell Caveats</a>,
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51 Up: <a rel="up" accesskey="u" href="FFTW-on-the-Cell-Processor.html#FFTW-on-the-Cell-Processor">FFTW on the Cell Processor</a>
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52 <hr>
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53 </div>
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54
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55 <h3 class="section">6.3 FFTW Accuracy on Cell</h3>
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56
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57 <p>The SPEs are fully IEEE-754 compliant in double precision. In single
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58 precision, they only implement round-towards-zero as opposed to the
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59 standard round-to-even mode. (The PPE is fully IEEE-754 compliant
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60 like all other PowerPC implementations.) Because of the rounding
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61 mode, FFTW is less accurate when running on the SPEs than on the PPE.
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62 The accuracy loss is hard to quantify in general, but as a rough
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63 guideline, the L2 norm of the relative roundoff error for random
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64 inputs is 4 to 8 times larger than the corresponding calculation in
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65 round-to-even arithmetic. In other words, expect to lose 2 to 3 bits
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66 of accuracy.
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67
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68 <p>FFTW currently does not use any algorithm that degrades accuracy to
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69 gain performance on the SPE. One implication of this choice is that
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70 large 1D transforms run slower than they would if we were willing to
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71 sacrifice another bit or so of accuracy.
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72
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73 <!-- ************************************************************ -->
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74 </body></html>
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75
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