Re: Re: [ANN] LuaJIT x64 port sponsorship program

[Mike Pall <mikelu-0912@...>]

Peter Cawley wrote:
> On Wed, Dec 9, 2009 at 11:06 AM, Mike Pall <mikelu-0912@mike.de> wrote:
> > http://lua-users.org/lists/lua-l/2009-08/msg00202.html
> 
> What does this mean for light userdata? Can you only give LuaJit2
> light userdata which live in the lower 4GB, or does it transparently
> box them in a special type of full userdata?

You can pass it any pointer value that is legal on x64 in
user-mode under the currently supported 48 bit memory model. In
practice this means it's accepted by lua_pushlightuserdata() if
the highest 16 bits are zero (user-mode pointers can only point to
addresses 0 .. 2^47-1).

That's why I said 'GC object' pointers are 32 bit. Lightuserdata
is not a GC object and not boxed either. See lj_obj.h:

Internal object tags:

Internal tags overlap the MSW of a number object (must be a double).
Interpreted as a double these are special NaNs. The FPU only generates
one type of NaN (0xfff8_0000_0000_0000). So MSWs > 0xfff80000 are available
for use as internal tags. Small negative numbers are used to shorten the
encoding of type comparisons (reg/mem against sign-ext. 8 bit immediate).

                 ---MSW---.---LSW---
primitive types |  itype  |         |
lightuserdata   |  itype  |  void * |  (32 bit platforms)
lightuserdata   |fffc|    void *    |  (64 bit platforms, 48 bit pointers)
GC objects      |  itype  |  GCRef  |
number           -------double------

--Mike