Re: OT : Approximate / fast math libraries ?

Btw, I usually use -O3 rather than -O2. I agree with the other poster: I am not sure getting rid of precision is a great idea.

Trotter

----- Original Message ----
From: Chris Jones <jonesc@xxxxxxxxxxxxxxxxx>
To: For users of Fedora <fedora-list@xxxxxxxxxx>
Sent: Friday, August 31, 2007 2:41:56 PM
Subject: Re: OT : Approximate / fast math libraries ?

Hi,

Thanks for your feedback.

What exactly is your need? Contact me off-list and maybe I can
help. Have you profiled your code? I have found that people
often do not actually know where their code is spending its
time. I once sped up an app which was universally acknowledged
to be slow "because it uses floating point." I sped it up 3x.

Yes, I have profiled the code, quite extensively, using the valgrind/calltree
application. From this I know this that I'm know I've tidying up this to the
point where its hard to find big improvements, the cpu time is fairly well
spread around, not isolated in a few places. So am now looking a a few places
where math calls are taking more time than I would hope. I'm not going to get
factors in speed in the overall application, but I hope in a few places
things can be improvemed a lot locally.

Also, the project is not small, massive in fact, and I'm only writting one
small part. If you are interested you can find it here

http://lhcb-release-area.web.cern.ch/LHCb-release-area/DOC/brunel/releases/latest/doxygen/index.html

It also has to be supported on a *lot* of hardware. Basically gcc 3.2.3 based
Scientific Linux 3 machines, gcc 3.4.6 SL4 machines (32 and 64 bit) and (not
my decision), windows VC 7.1. I cannot rely on for instance SSE math calls
etc.

Taking an example from another thread, one place I'm trying to understand is
where I use atan2 see

http://www.hep.phy.cam.ac.uk/~jonesc/atan2.png

for the profiler output. atan2 is taking 50% of the time of this method. Not
here I don't need that much precision on the result - say +- O(2*pi/100).
Anything you can suggest here - The code is here

http://www.hep.phy.cam.ac.uk/~jonesc/RichPhotonRecoUsingCKEstiFromRadius.cpp

( note though its full of internal classes etc...)

cheers Chris




I modified the parsing routines it used, not the floating point.

Mike
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