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

[Tom Rondeau]

On Thu, Feb 6, 2014 at 5:31 PM, Tom Tsou <address@hidden> wrote:
> On Thu, Feb 6, 2014 at 11:58 AM, Douglas Geiger
> <address@hidden> wrote:
>> I'll suggest that the generic kernel should be trusted to do the correct
>> thing, or at least have the correct set of intentions, and that if you want
>> to change the behavior of the generic kernel, that you are introducing a
>> change that will break applications depending on this behavior.
>>
>> I believe the use of casting the subtraction before the comparison (where:
>> ((int16_t) (src0[i] - src0[i + 1]) > 0) ? src0[i] : src0[i+1]; is the full
>> comparison) was intentional to ensure overflow subtraction vs. saturation
>> subtraction.
>
> Assuming this kernel is used for some type of trellis decoding, the
> overflow makes sense as it avoids the need for normalization of
> accumulated metrics. It does, however, require that the spread of
> input values be bounded for it to work. Otherwise, you have
> comparisons between overflow values and non-overflow values, which
> makes the comparison ambiguous and the output not-sane.
>
>   -TT

Yeah, Nathan emailed me back off-list and explained that to me. Makes sense now.

Thanks,

Tom