Re: [Discuss-gnuradio] Request for comment: Adding fixed point to VOLK

[CEL]

Hi Albin,

I'd love this, especially if we end up with a kernel that we can use to
build decimating FIR filters that inherently convert to higher-bitwidth 
int or floating point.

I think this could be super useful in hardware flow graphs, where one
would get integer numbers from hardware anyway; in a low-SNR scenario
only processing gain (e.g. through filtering / correlation) would make
it sensible to use a larger integer or floating point's larger dynamic
range.

Not quite sure how that would preferably look like, but I'd imagine
Q1.15 going in, getting weighted internally with Q1.15 (or Q1.31 with
limited amplitude) coefficients to add Q1.31 numbers to a Q1.31
accumulator. (I'm assuming Q1.15 means 15 fractional binary digits and
one sign bit.)

(Oh, and yes, x86_64 / SIMD extensions have relatively nice conversion
instructions.)

Best regards,
Marcus

On Fri, 2019-05-31 at 09:50 +0200, Albin Stigö wrote:
> Hi,
> 
> Volk has functions for converting between int and float by casting and
> multiplying with a scalar. I'd like to purpose functions for the
> special case where scalar is 1/INT32_MAX (and 1/INT16_MAX etc) as this
> is a very common use case and might be available to an optimization.
> Certain CPUs (armv8 in particular [1], not sure about x86? [2]) has
> hardware instructions for converting fixed point to float and back.
> 
> In particular I'm interested in adding the common Q1.31 and Q1.15
> formats (full range signed int32_t and full range int16_t to
> (-1.0)-1.0 float).
> 
> Would be grateful for input regarding this and in particular ideas for
> naming the functions.
> 
> 
> --Albin
> 
> [1] 
> http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0801h/pge1427897637493.html
> [2] 
> https://software.intel.com/en-us/articles/fast-floating-point-to-integer-conversions
> 
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