| Re: Accelerating a one-line program doing matrix multiplication [message #72742 is a reply to message #72696] |
Thu, 30 September 2010 09:38   |
Axel Martínez
Messages: 22
Registered: June 2010
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Junior Member |
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On Sep 30, 5:03 pm, Paolo <pgri...@gmail.com> wrote:
> On Sep 30, 10:59 am, Paolo <pgri...@gmail.com> wrote:
>
>
>
>> [skip]
>
>>> FUNCTION vc2rc_accel, v0,v1,v2,v3,vc
>>> npoints = (SIZE(vc, /DIMENSIONS))[1]
>>> for i=0L, npoints-1 DO BEGIN
>>> vc[*,i] = vc[0,i] * v1 + vc[1,i] * v2 + vc[2,i] * v3 + v0
>>> endfor
>>> RETURN, vc
>>> END
>
>> No, that's using a for loop - that's why it is slow.
>> You want something like this (no loops):
>
>> vc0=vc[0,*]
>> vc1=vc[1,*]
>> vc2=vc[2,*]
>> vc[0,*]=vc0*v1[0]+vc1*v2[0]+vc3*v3[0]
>> vc[1,*]=vc0*v1[1]+vc1*v2[1]+...
>> vc[2,*]=vc0*v1[2]+...
>
>> Ciao,
>> Paolo
>
> well, there's a bit of an index mismatch
> in there...
>
> vc[0,*]=vc0*v1[0]+vc1*v2[0]+vc2*v3[0]
> vc[1,*]=vc0*v1[1]+vc1*v2[1]+vc2*v3[1]
> vc[2,*]=vc0*v1[2]+vc1*v2[2]+vc2*v3[2]
Got it, thanks. I changed it in my test, it turned out to be slower
than the original version (about 30% slower).
By now, seeing no way to improve the original version with an
arbitrary vc, I stick to the version of "adding REBINS" for the case
where all indexes from an image are given as input. This is the
critical case for my application anyways. When a smaller region is
wanted, the original version can be used.
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