calc: faster sums ...

[Michael Meeks]

Hi Kohei,

	I'd love some input (if you have a minute) on the attached. The
punch-line is, that if we want to do really fast arithmetic, we start to
need to do some odd things; while I suspect that this piece of unrolling
can be done with the iterator - the next step I'm poking at (SSE3
assembler ;-) is not going to like that.

	The win from this on a sum heavy sheet calculation is from 14.4s
seconds to recalculate to 9s (actually testing using the
'puncture_mdds_encap' approach ;-) - a 35%+ speedup.

	I guess, what I'd most like is a:

	'const double *getDoubles(size_t nAtOffset);

	Or somesuch =) or, failing that - a chunked double iterator that would
give a pointer and size (reflecting the underlying chunks but giving
some freedom there).

	Thoughts much appreciated =)

	ATB,

		Michael.

-- 
 michael.meeks at collabora.com  <><, Pseudo Engineer, itinerant idiot
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