Re: Re: NaN packing for TValue

[Yura Sokolov <funny.falcon@...>]

-10.01.-28163 22:59, Henk Boom пишет:
> I spent a while trying to figure out how it would work on 64-bit
> before I reread your email and saw that it was x86 only ;)
>
> This makes me wonder, how does luajit do it on 64-bit? I can
> understand that in terms of supply, much less than 64 bits is
> sufficient, but how do you store pointers that happen not to lie in
> the available range? mmap'd storage in particular gives back pointers
> that are clearly uncontiguous with the rest of your allocated memory.
>
>      henk
Well, x64 at the moment uses only 48 bits for addresses.

1bit for a sign + 11bits for mantis + 1bit for quiet NaN + 48 bits for 
an address = 61bit
So that there is 3bits for tagging.

Currently Lua uses 13 different tag values, so that it would be tricky 
to pack them into three bits :)
But possible in general. May be, one could use lower bits from aligned 
address pointer.
Manipulating with them would not be so simple.
And I have no machine running in 64bit mode at the moment :(

For x86 I used another 16 free bits for speed and simplicity.


Sokolov Yura aka funny_falcon