[libre-riscv-dev] [Bug 44] IEEE754 FPU inverse-sqrt

[bugzilla-daemon at libre-riscv.org]

http://bugs.libre-riscv.org/show_bug.cgi?id=44

--- Comment #11 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
(In reply to Jacob Lifshay from comment #10)
> (In reply to Luke Kenneth Casson Leighton from comment #9)
> > the Tanako paper is extremely obtuse, and assumes access to IEEE papers
> > is guaranteed.  i found an implementation of "On the Fly Conversion":
> > https://web.archive.org/web/20170808225020/http://etd.dtu.dk/thesis/211563/
> > ep08_21.pdf
> > 
> > this paper does not require access to paywalls.  page 36.
> 
> As far as I can tell, this paper doesn't actually have a rsqrt or sqrt
> algorithm.

it doesn't... what it *does* have is implementations of the primitives
which are *assumed* that the reader has access to or knows about (from
paywalled literature).

OTFC - on-the-fly-conversion - is available according to the Tanako paper -
in a paywalled journal.

searching "OTFC" is near-impossible to find, it is very obscure... the
only paper i could find that actually describes it has been *removed* from
the original location (etd.dtu.dk - the entire server is down).

likewise, it contains implementations of Carry-Save Adders (p122) which *again*
the Tanako paper *ASSUMES* that the reader is either familiar with or has
access to (paywalled) journals containing that information.

without these algorithms - OTFC and CSA - the Tanako paper is effectively
useless, unfortunately.

frustratingly, they're incredibly simple blocks.  the notes in the Tanako
paper state that OTFC may be implemented with a couple of MUXes and a shifter.

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