All the good stuff was already suggested (esp. the possibility that gcc
isn't unrolling loops properly, that using the -S option would help
you determine this, etc..). I will only suggest something minor--almost
trivial. Try playing around with the 'register' keyword in C. I would
think that on a good compiler like gcc this type of thing can actually
hurt your program rather than help it (as gcc should be automatically
figuring out which variables deserve to stay in the registers, etc.). But
you never know! Anyway good luck.. :)
-Calin
On Thu, 24 Jan 2002, Arne Linder wrote:
> <color><param>0100,0100,0100</param>Hello everybody,
>
>
> my question is a bit off topic, but since I know from the conference in
> Milan, that there are a lot of programming specialists among us, I hope,
> you can help me.
>
>
> In my real-time application an ISR has to do a matrix multiplication in
> real time. This is done by the following code:
>
> ===
>
> </color> for (j=0; j<<Np; j++)
>
> {
>
> Element=0.0;
>
> k=(j+1)*Np;
>
> for (i=0; i<<Np; i++)
>
> Element+= *(P+k+i) * (*(W+i) - *(f+i));
>
> *(DU+j)=Element;
>
> }
>
> ===
>
>
> *DU, *W and *f are declared as double.
>
> Compiler options: -O2 -D__KERNEL__ -DMODULE -c -I/usr/src/rtai-
> 1.3/include -funroll-loops
>
>
> With Np=100 the calculation time is 51�s. This seems to be quite high
> for an AMD Duron processor with 900MHz. So I tried to find out, which
> part of the routine is consuming most of the time. I realized, that
> deleting the command "*(DU+j)=Element" reduces the processing time
> to 9�s.
>
> My first thought was, that due to cache misses the time to access *DU
> gives that huge difference, but changing "*(DU+j)=Element" into
> "Element=*(DU+j)" also resulted in 9�s processing time. (Of course the
> program didn't give the desired results in that case.)
>
>
> In my opinion the gcc compiler (version 2.95.2 19991024) refuses to
> unroll the loops correctly when inserting the line "*(DU+j)=Element;"
> into the outer for-loop. Does someone have an idea to solve this
> problem?
>
>
> Greetings from Wuppertal
>
>
> Arne Linder
>
>
>
> <nofill>
> Dipl.-Ing. Arne Linder
> Labor fuer elektrische Maschinen und Antriebe
> Fachbereich 13
> Bergische Universitaet - Gesamthochschule Wuppertal
> D-42097 Wuppertal
> e-mail: [EMAIL PROTECTED]
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