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This manual is for FFTW
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(version 3.2.2, 12 July 2009).
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Copyright (C) 2003 Matteo Frigo.
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Copyright (C) 2003 Massachusetts Institute of Technology.
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<h3 class="section">6.3 FFTW Accuracy on Cell</h3>
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<p>The SPEs are fully IEEE-754 compliant in double precision. In single
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precision, they only implement round-towards-zero as opposed to the
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standard round-to-even mode. (The PPE is fully IEEE-754 compliant
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like all other PowerPC implementations.) Because of the rounding
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mode, FFTW is less accurate when running on the SPEs than on the PPE.
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The accuracy loss is hard to quantify in general, but as a rough
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guideline, the L2 norm of the relative roundoff error for random
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inputs is 4 to 8 times larger than the corresponding calculation in
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round-to-even arithmetic. In other words, expect to lose 2 to 3 bits
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<p>FFTW currently does not use any algorithm that degrades accuracy to
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gain performance on the SPE. One implication of this choice is that
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large 1D transforms run slower than they would if we were willing to
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sacrifice another bit or so of accuracy.
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