Movement against float.init being nan

[mw]

On Sunday, 21 August 2022 at 05:03:43 UTC, Ali Çehreli wrote:
> On 8/20/22 20:44, Walter Bright wrote:
>
> > I.e. a "payload" can be put in the mantissa bits of a NaN,
> which can be
> > used to provide more information about the NaN.
>
> I think it is called "nan boxing."
>
> > I've never seen anyone
> > use this, though, but someone might
>
> I heard about it either at a meetup or at a DConf. The speaker 
> was explaining that exact technique.

Just learnt from here:

https://github.com/Robert-van-Engelen/tinylisp/issues/12#issuecomment-1752142467

> > Another question I want ask is: the NaN boxing trick is OK, 
> > but why it's needed here (for education purpose Lisp 
> > implementation, this trick I think actually distracted 
> > learners from learning Lisp)? Why cannot we just use bit 
> > fields or a small struct as tagged union? Also by using NaN 
> > boxing, are we actually wasting 13 bits for each double?
> 
> Lots of modern PL implementations use NaN boxing instead of 
> tagged structs/unions. Nothing gets wasted, because a cell has 
> to be a fixed size anyway, which is the larger size of all 
> possible values it can hold. Adding a tag therefore makes the 
> cell structure larger by a byte, at least, to hold a tag. That 
> also makes addressing less efficient, since cells are no longer 
> guaranteed to be 32 bit aligned (or the tag has to be 32 bits, 
> which wasts more bits).


I think I understand it now, the benefits of using NaN boxing is 
that: in a 64-bit double

all the doubles are still fully represented
use 48 bit as pointer can still address 262,144 GB memory, that 
is good enough for any computer today
only integer's range gets shrunk: from 64-bit to 48-bit (49 with 
sign bit depends on if we choose to use it), that's good for most 
purpose, and if one really need > 48-bit integer type, then use 
the fat BigInt (class / struct) then (which will be treated as 
object pointer type).
(1) and (3) means all the scalar types are stored as scalar, not 
pointers to some other representations.

That's very compact design indeed.