Re: Setting Float Precision in Lua.c

[KHMan <keinhong@...>]

On 6/6/2018 8:53 PM, Albert Chan wrote:
> > Why don't you compile your binaries for SSE2 only? Even easier, just compile to 64-bit binaries? Surprising you mentioned Windows uses extended precision by default when there is x64 on every 64-bit capable Intel/AMD/other chip... and has been so for many, many years already.
>
> I already picked 53-bits roundings.
> I use my own laptop behavior just as a example, same with fsum.lua
>
> David Gay's dtoa.c strtod maybe a better example.
> With 53-bits roundings, it optimized away common cases [1]
>
> strtod("123456789e-20", NULL)
> = 123456789 / 1e20   -- both numbers exactly represented in double
> = 1.23456789e-012    -- division guaranteed correct rounding

Say 123456789 is correct to 9 digits. Divide it by anything, it 
will be useful to less than 9 digits. Whatever perfect results to 
16 digits that is produced does not matter. The latter things only 
matter to mathematicians looking for some kind of perfection.


> > Avoid the 8087 datapath, it's not hard to avoid that legacy thing.
>
> seems we are in agreement here.
>
> But, if you believe in this, why not add it to Lua ?
> This will automatically let everyone avoid it too.
>
> > It's a banana skin, just walk around the banana skin. The rest of us, we don't need to be walking there.
>
> You already are walking there, just dont know it.
> It is hidden behind Lua default of showing 8 significant digits.
>
> Python3 default to show just enough digits to round-trip [2]
> So, even 1 ULP difference will shows up in the output.
>
> Should Lua put the banana skin in the trash ?
>
> --
>
> [1] https://www.exploringbinary.com/fast-path-decimal-to-floating-point-conversion/
> [2] https://www.exploringbinary.com/the-shortest-decimal-string-that-round-trips-examples/

Oh no, you've drunk too much of the kool-aid. Oh, I'm aware of the 
site above. Long discussion elsewhere last year. Here, read more 
to mess with your mind:

http://www.exploringbinary.com/17-digits-gets-you-there-once-youve-found-your-way/

https://www.exploringbinary.com/number-of-decimal-digits-in-a-binary-fraction/

https://www.exploringbinary.com/maximum-number-of-decimal-digits-in-binary-floating-point-numbers/

You've laser-focused onto some kind of path.

I don't think I will bother to change your mind any further. You 
work it out on your own.

Good luck in your request to Roberto & co.

--
Cheers,
Kein-Hong Man (esq.)
Selangor, Malaysia