Re: A faster sum

[Victor Eijkhout]

On May 21, 2005, at 4:19 PM, Keith Goodman wrote:

> So does it make sense to modify the built-in function sum to use
> matrix multiplication?

I wouldn't do it.

I'm still a bit surprised. The multiplication approach takes twice as 
many operations, but it's probably faster because it uses Atlas, which 
tries to optimize use of bandwidth, cache lines, number of registers 
&c. Since the memory behaviour is more important than the actual 
efficiency of operations (to a degree), I guess the Atlas matrix-vector 
multiply will be more efficient.

However, this depends on all sorts of assumptions about architecture. 
I'd leave the code the way it is, and preserve this as a bit of 'octave 
folklore'.

V.



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