OT : Approximate / fast math libraries ?

[Chris Jones]

Hi,

On Friday 31 August 2007 8:48:49 pm Globe Trotter wrote:
> 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.

I don't have overall control, so cannot just switch to -03 myself. I  did try 
it out though and it didn't make a lot of difference. Maybe I should 
investigate that a little deeper though to make sure something wasn't wrong.

Also, I agree with you in general on the precision, just in one particular 
case I know I need much less than I am currently getting, and using atan2 
seems to be taking significant cpu (w.r.t. the rest of the method, i.e. 50% 
or so).

Chris

>
> Trotter
>
> ----- Original Message ----
> From: Chris Jones <jonesc at hep.phy.cam.ac.uk>
> To: For users of Fedora <fedora-list at redhat.com>
> 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.cp
>p
>
> ( 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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