Posits - a new approach could sink floating point computation

I do think that’s a really interesting and potentially valuable trade off to explore. However, that is not how posits (and unums before them) have been pitched. Rather, they have been sold as “you don’t have to worry about numerical precision errors anymore”. Whereas the reality is that you have to be even more careful and aware of numerical precision. In exchange for that, you can save significant time, memory and energy. Fair enough. Sometimes that’s a worthwhile trade off. It’s the repeated claims that posits/unums give you something for free with no downside that are problematic. Explicitly giving up scale invariance in exchange for dynamic precision is a potentially very useful trade off, but don’t pretend it’s not a trade off. A format which has a) dynamic precision and b) some way of addressing the representation of operation error precisely and c) a mechanism for tracking what the minimum precision throughout a computation would be very interesting indeed. Perhaps that’s what posits are converging to, but they’re not there yet.

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Do you think that time and effort invested in this has better payoff than devoting the same amount of resources to error analysis and algorithm design for numerical stability, sticking to IEEE 754 floating point?

Eg for the calculation above, simply working with logs would be a fairly standard thing to do. Details of course depend on the context.

Scale invariance in Float16 is a bad joke anyway. So for specifically 16 bit numbers, I think the answer is “yes”.

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Adding hardware “quire” accumulators for IEEE floats would be a killer hardware feature. There’s an ongoing conversation about Python’s fsum and superaccumulators, which are surprisingly fast on modern hardware, with various people trying to optimize them better. It seems like something where a hardware mechanims for getting guaranteed exact sums would be a game changer.

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I rarely use Float16 for computation (only for storage, very occasionally), but I am under the impression that most nontrivial computations would need to be expertly designed to get acceptable accuracy in IEEE 754 and also for posits (at least I don’t see any specific feature of posits that allows one to skip this step).

Also, I think that Float16 is kind of a red herring here, as most of the focus seems to be on 32-bit.

A quire is a data type, and when used as an accumulator I think everyone will see the need to make it as register-like as possible. But yes, you can store the quire register in memory and load it from memory. You can also add the contents of a quire value stored in memory to the quire register (and I imagine we need subtraction support, also). I feel good about the practicality of this up to 32-bit posits, for which the quire is 512 bits. (If you attempt to build a quire for 32-bit floats, you’ll find it needs to be 640 bits or thereabouts, an ugly number from an architecture perspective.) If we find that 64-bit posits are needed for some applications, the quire is 2048 bits and that starts to look rather unwieldy.

Although I’m intrigued by the ACRITH and XSC approach, my strong preference is that all use of the quire be explicit in the source code, not automatic. As soon as things become automatic (covert), differences arise in the result. We have a real shot at achieving perfect bitwise reproducibility with posits, if the Draft Standard is followed and we can keep language designers from performing any covert optimizations that can affect the rounding in any way. I’ve hunted down every source of irreproducibility in the way IEEE 754 floats work and corrected those in the Posit Draft Standard. Posits should be as reproducible as integer calculations where the precision (number of bits) in the integer is explicit at all times. If I may be so bold as to offer a meme:

Kahan-Gustafsonmeme

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Completely missing from the discussion so far is Gustafson’s Valids, his complement to Posits. It’s similar to interval arithmetic, and should be helpful.

“In February 2017, Gustafson officially introduced unum type III, posits and valids.”

If you get away with fewer bits (half) for Posits compared to regular floats, then Valids are a good option.

See also: https://github.com/JuliaIntervals/IntervalArithmetic.jl that I assume you can use with Posits.

And off-topic (regarding FPGA discussion above), FPGA support seems at least on thee table (but not implemented, and I found Julia library related to):

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Valids are covered briefly in the “posit4” document on posithub.org. One reason for not going into more detail is because valids are just like Type II unums, expressed as a start-stop pair indicating the arc of the projective real circle that they traverse.

The big advantage of valids (or any other type of unum) over intervals that use IEEE floats as endpoints is that they distinguish between open and closed endpoints. When you want to work with intervals, you’re dealing with sets; you have to be able to intersect, union, and complement sets, but classic interval methods cannot do that because all the endpoints are closed. If you need to express underflow, for example, you use the open interval (0, minreal). Overflow is (maxreal, ∞). Valids give you back all the features like signed infinities and what IEEE 754 calls “negative zero” that posits eliminate to simplify hardware.

A valid is simply a pair of posits, where the last bit of each posit is the ubit (uncertainty bit). If the ubit = 0, the value is exact; if the ubit = 1, the value is the open interval between adjacent exact posits.

As an aside, Kahan’s “scathing” review of unums is riddled with errors and self-contradictions, like much of his unrefereed blogs. He got called on those errors in our debate, in which he also revealed he has only read snippets of The End of Error: Unum Computing, not the whole thing; for example, he claims that the Wrapping Problem is not mentioned in the book. Well, it was left out of the Index by mistake, but it is in the Table of Contents, the Glossary, and at least one Chapter is dedicated to it with full-page illustrations of what causes it and how to deal with it. A transcript of “The Great Debate” with Kahan (which predates the invention of posits) is at http://www.johngustafson.net/pdfs/DebateTranscription.pdf

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I’m sure you [all] know about the new IEEE 754-2019.

http://754r.ucbtest.org/background/

It has e.g. “The relaxed ordering of NaNs” probably of interest, to Julia gurus (one area where posits win).

It has “new tanPi, aSinPi, and aCosPi operations are recommended” (previously not thought needed), and I’m curious if posits have similar (does it have any trigonometry, or all in libraries?)? I only know of the dotproduct (and a few more?) extra operations posits have standardized vs. regular IEEE.

Also e.g. “5.3.1 {min,max}{Num,NumMag} operations, formerly required, are now deleted” and “9.6 new {min,max}imum{,Number,Magnitude,MagnitudeNumber} operations are recommended; NaN and signed zero handling are changed from 754-2008 5.3.1.” seems interesting vs. posits.

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Thanks for this update… I was not aware of this latest effort to rearrange the deck chairs on the Titanic. It may be one of the first indications of IEEE 754 attempting to keep up with posits and unums by adopting some of their features. In The End of Error: Unum Computing pp. 156–158 I wrote that switching to degrees as the unit solves the argument reduction problem. In the Draft Posit Standard I mandated tanPi, aSinPi, aCosPi, and other trig functions that treat all angles as an exactly-representable number times π. While traditional cos(x), sin(x) etc. may make calculus look more elegant, it turns math libraries for trig functions into very expensive random number generators for most of the dynamic range.

So many of the changes are now “Recommended.” Standards documents should not have recommendations or suggestions or options. They should have Requirements, and that’s it. IEEE 754 should be called a Guidelines document, not a Standard. It could have more than one level of “compliance” if they wanted to formally define that, but I don’t see them going that direction.

And yes, posits eliminate the kinds of issues that IEEE 754 faces with multiple NaN values, signed zero handling, and round-ties-to-even when both choices are odd. Someday we will look back on such difficulties and laugh.

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