Re: [Discuss-gnuradio] [VOLK] 16i_max_star_horizontal_16i non-saturating subtraction

[Tom Rondeau]

On Wed, Feb 5, 2014 at 4:04 PM, West, Nathan
<address@hidden> wrote:
> I was doing some work with this kernel and came across an odd result
> that I think is caused by a non-saturating add in the generic
> proto-kernel, that should also be relevant to the 16i_max_star_16i.
>
> I haven't looked too much in to the SSE versions, but the generic
> versions are doing a comparison by subtracting two values and
> comparing the result to 0. At least in the QA on armhf this is causing
> wrap-around so that the smaller of the two numbers is returned as the
> max.
>
> Since I'm pretty sure this is part of the max* operator defined by
> Viterbi in his "justification and implementation of a MAP decoder" I
> think this result is incorrect unless I'm misunderstanding part of the
> operator.
>
> Can somebody with a little more insight in to this kernel ping back if
> this is intended behavior? If not, is there some benefit to doing
> (x-y)>0 vs x>y as the comparison?
>
> Thanks,
> Nathan

Just from what you've said here, I would agree that (x-y)>0 is a bit
dangerous, and I don't see why we can't use x>y. Does it work for you
if you make that change?

Tom