Re: NaN trick

[Miles Bader <miles@...>]

Peter Cawley <lua@corsix.org> writes:
>> I think it should also work on current AMD 64bit platforms. This
>> would reduce a Lua value from 16 bytes down to 8 bytes on the Lua
>> stack and within tables!
>
> Though you then need to get slightly more creative about encoding the
> type. A double has 52 bits of mantissa; with 48 taken for a
> pointer-sized value, and 1 taken to ensure a quiet NaN, you only have
> 3 bits left to encode the type. If you borrow the sign bit as well,
> then you have 4 bits. For reference, the current implementation in 5.2
> beta uses 6 bits for encoding type.

The way other systems seem to do this, is to divide types into "true
immediates", and "pointer + language heap obj".

The former must have their tag uniquely encoded using only the bits
available in immediate values, but the latter can have extra tag
information in the header of the "language heap obj" pointed to by the
pointer (and of course gc flags etc can also be stored there).

For Lua, there seem to be 6 "true immediate" types:  number, nil,
boolean, lightuserdata, and lightcfunction (anything else?).  So,
adding a 7th immediate tag for the "lua_heap_object" case (which
handles string, table, function, userdata, and thread types), that
would seem to require only 3 tag bits in immediates...

[nil and boolean could be combined using other bits in the immediate
to distinguish them, meaning 6 immediate tags are used, leaving 2 for
future use...]

-Miles

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