[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Notes from Thursday's meeting...



	We began with Ivan, Dick, & I describing our objections to the
	online NaN proposal.  Jeff argued in favor of the proposal.  A
	general discussion of the issue ensued.  I cut off the
	discussion at 1:30 in favor of Mike & Eric's presentations.

	Mike showed Eric's slides first.  Eric's position is that
	millenial encoding is not easier than BCD in hardware.  He
	described a 10-bit adder with a carry at 1024-24.  (This is
	approximately correct for an add function but I can show him
	a much better way to do this for the adds within a multiply.)
	He compared this to the BCD adder which (with a 16-6 bit carry)
	looks quite similar.  He also showed 4 different multiplication
	methods but did not consider any even slightly redundant
	representation.  He made a point about the problems with Peter's
	exponent format & the need for divisions near the end of his
	presentation.

	Mike continued with the software part of their presentation.
	His slides described the variations possible with different
	"chunk" sizes.  (He mentioned that we agreed to the DPD format
	in 2/2003.  Sounds about right.)  He used the Telco benchmark
	as a guide to his results.  It seems that the performance is
	largely independent of chunk size.  His results suggest that
	the software cost of DPD lookup is never worse that 2% overall.
	When doing conversions alone (with no other work) it rose to
	about 4% (but of course nothing is getting done in this case).
	Mike also spoke of the combination field & the difficulties of
	using Peter's approach for the sake of lexical ordering.  He
	believes its about 30% added to the cost of conversions which
	he already showed was small.  He showed the logic equations for
	conversion of millenial to BCD which is, naturally, obscene.

	Peter began his case study of DPD or GBCD to BCD conversions in
	software on the Intel XScale architecture which is apparently
	some small embedded processor they sell.  First, several slides
	detail the DPD -> BCD assembly code.  It looks like it takes 14
	cycles on average (from 6.3 to 22.4 cycles).  Then GBCD -> BCD.
	The code was 11 instructions long (with 2 multiplies in it).
	He has two cycle multiplies but he hides the latency under the
	rearrangeent of the results &, therefore, takes 10.2 cycles in
	all cases.  (The code size in the DPD case was 55 instructions
	but only an average of 14 were executed in any given case.)
	Then Peter also discussed the combination field in the encoding
	of the exponent & the MSD.  He presented a new encoding that
	was similar to the existing encoding but included the whole
	exponent rather than just a couple of bits.  Interesting but
	not lexical.  Ah, but he CAN contrive a lexical ordering AFTER
	he decodes these bits.

	Finally, John wanted to talk about various notions of "format".
	DPD, GBCD, varients, internal formats, etc.  He raised the
	issue of external versus internal formats.  Or exchange versus
	computational formats.  He proposed a terminology of 4 formats:
	Computational, Internal, Memory, & Exchange.  He mentioned the
	possibility of just using strings as the exchange format.  His
	conclusion is that, for the average user, the representation
	just doesn't matter.  But for the fact that he presents his
	ideas BETTER than we have you might think that John had been
	listening in on the subcommittee meetings.

	At 3:40 or so we took our late break.

	After the break, Jeff presented his work from the subcommittee
	on our notions similar to John's.  Then Prof Kahan presented his
	variant of the same notions.  (He used the word coterie WRT
	decimal fixed point values differently than I understood it from
	our discussions.)  I touched on the relationship between decimal
	& binary on this issue when you bring expressions into the
	discussion.  Ivan also contributed in that he pointed out that
	care must be taken if predictable results are to be counted on
	from machine to machine.  A lot of time was spent on a compiler
	complexity issue that I really didn't understand in the end.

	We broke up for dinner around 5:30.
	-----

	Ivan, Dick, & I will talk on NaN issue 1/2 hour
	Eric (w guest rule) 1/2 hour
	Mike (w guest rule) 1/2 hour
	Peter Tang (w guest rule) 1/2 hour
	John Harrison (w guest rule) 1/2 hour


754 | revision | FAQ | references | list archive