On Tue, May 26, 2015 at 7:37 PM, John Malsbury <jmalsbury.perso...@gmail.com > wrote:
> I have a complex phase rotation function that uses a pre-generated > sin/cos LUT and some basic multiple/adds. > > As it turns out, the rotation calc, which uses "straight" C/C++ math is > still the bottleneck in a demod. > > I was wondering, is there some uber-efficient rotation block/class I > should be using? I notice there is a volk kernel for the job and > gr_rotator. But I also should mention that the phase rotation operation > must happen one sample at a time. This is due to the sequential nature of > the algorithm - ie. I can't align and call a kernel with hundreds of > nicely-aligned samples. > > Any advice? > > -John > To follow-up on Sylvain's questions: is the restriction really on doing single-sample rotation (because of some intermediate calculation to generate the phase advance for the next sample), or on the alignment? I'll note that *in general* intel doesn't take much (including, often, a non-measurable) hit on non-aligned SIMD operations. Also - even if you aren't operating in the hundreds of sample range, that the SIMD kernel can save you if you're operating on e.g. several sample as a time (for SSE it's only doing two at a time anyways, with a clean-up loop for an odd-number of samples, and AVX does four at a time, with a clean-up loop for non-multiples of four). Besides, who can resist looking at something called the rotatorpuppet for inspiration on how to call the main kernel? I'll also point out that the gr::blocks::rotator (which is not a block, i.e. separate from the rotator_cc, which is a block) has both a rotate() method that operates on a single sample, and a rotateN() method that operates on n samples. It is the later that calls down into the volk rotator kernel. -- Doug Geiger doug.gei...@bioradiation.net
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