Gr_math.h addition, SDR Forum

I made an addition to gr_math.h today in an attempt to get some stalls
for branching out of code where we have an if statement run on every
bleeding sample.

gr_branchless_clipping is the new addition to gr_math as a static inline
float. It may surprise that calls to fabs and multiple additions and a
multiplication are cheaper than the “if branching”, but on average they
can be significantly cheaper. fabs can be intrinsic and I hope we are
doing that. To get full performance we need to be.

If there are places in your code where you are doing

if (a > clip), clip a
else if (a<-clip) clip a

this can be significantly faster

There are more things like this Tom and I are experimenting with in
Tom’s receiver branch. We moved a bunch of per sample junk out to the
“once per function call” position and saw significant savings.

We are at the end of SDR Forum and Matt and fred harris have left so the
fun is wearing off and there is only so much beer you can drink before
you really want to go back to work.

In a really funny occurrence, the sdr challenge to university entrants
was a bit upsetting to the powers that be. They had some major sponsors
and part of the challenge was to use their gifts and give them cred and
publicity. All of the winners blew off the hardware when it was a pain
in the rear, and used USRP and Gnu Radio. One of the teams did manage
to get the sponsor hardware to finally work by hacking the
microprocessor firmware and getting it to NFS boot ARM linux!. Virginia
Tech is a really fantastic place for software radio education. They are
big users of gnu radio, took most of the major prizes, including the
big one I mentioned. I might be granted persona non grata status as I
raiding the students cache for my summer projects! Tom had his name on
several papers. My partner at work, another big gnu radio user, won a
best paper award for two of his papers on cognitive radio. Matt came
and the big shots were twisting his arm big time to be a sponsor and
member. We all had a really good time. ;-).

Bob


AMSAT Director and VP Engineering. Member: ARRL, AMSAT-DL,
TAPR, Packrats, NJQRP, QRP ARCI, QCWA, FRC. ARRL SDR WG Chair
“An optimist may see a light where there is none, but why
must the pessimist always run to blow it out?” Descartes