Re: NaN trick

[Miles Bader <miles@...>]

On Fri, Jul 8, 2011 at 4:54 PM, Axel Kittenberger <axkibe@gmail.com> wrote:
> 6+1 (for NIL; BOOLEAN and NONE) = 7, 3 bit suffice ;-)

Lua 5.2 also has "light C function" (LUA_TLCF), which I think is an
immediate-only pointer type, so that needs its own immediate tag.
Also, as Patrick pointed out, there are 3 internal object types that
might need their own tags with pointers (LUA_TPROTO, LUA_TUPVAL,
LUA_TDEADKEY), but I'm a bit fuzzy on exactly what constraints they
have.

Luckily, though, all the "Lua heap object" types (string, table, function,
userdata, thread) can use one immediate tag value, with a separate tag
field in the Lua object header to distinguish them.

So the immediate tags might be:

   LUA_IMM_TNUMBER		0
   LUA_IMM_TMISCVAL		1   /* NIL, BOOL, NONE, using lower imm bits */
   LUA_IMM_THEAPVAL		2   /* TABLE, STRING, FUNCTION, USERDATA, etc */
   LUA_IMM_TLIGHTUSERDATA	3
   LUA_IMM_TLIGHTCFUNC		4
   ...

And the tags in heap object headers:

   LUA_HEAP_TTABLE		0
   LUA_HEAP_TSTRING		1
   LUA_HEAP_TFUNCTION		2
   LUA_HEAP_TUSERDATA		3
   LUA_HEAP_TTHREAD		4
   ...

I dunno offhand where TPROTO, UPVAL, and DEADKEY should go, but
there seems to be room either way...

-Miles

-- 
Cat is power.  Cat is peace.