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<h3 class="section">6.3 FFTW Accuracy on Cell</h3>

<p>The SPEs are fully IEEE-754 compliant in double precision.  In single
precision, they only implement round-towards-zero as opposed to the
standard round-to-even mode.  (The PPE is fully IEEE-754 compliant
like all other PowerPC implementations.)  Because of the rounding
mode, FFTW is less accurate when running on the SPEs than on the PPE.
The accuracy loss is hard to quantify in general, but as a rough
guideline, the L2 norm of the relative roundoff error for random
inputs is 4 to 8 times larger than the corresponding calculation in
round-to-even arithmetic.  In other words, expect to lose 2 to 3 bits
of accuracy.

   <p>FFTW currently does not use any algorithm that degrades accuracy to
gain performance on the SPE.  One implication of this choice is that
large 1D transforms run slower than they would if we were willing to
sacrifice another bit or so of accuracy.

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author	Geogaddi\David <d.m.ronan@qmul.ac.uk>
date	Wed, 22 Jul 2015 15:28:00 +0100
parents	25bf17994ef1
children