[Discuss-gnuradio] gr_math.h addition, SDR Forum

[Robert McGwier]

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