Re: Some trivial questions about Lua 5.1.4 source

[Roberto Ierusalimschy <roberto@...>]

> 1. In lobject.h, both the CommonHeader fragment and the lua_TValue
> struct have a "tt" field. I take it this means "type tag". However, why
> is it a lu_byte-sized field in CommonHeader and a int-sized field in
> lua_TValue?
> I realize that that the type tag enumerations will always be less than
> lu_byte-sized.
>  And I don't think these two types are ever applied to a single block
>  of data; if I understand right, CommonHeader is always part of a
>  heap-allocated structure, and TValues are small objects (booleans,
>  numbers, pointers). But why do the declarations vary? Is that
>  deliberate? Is there a reason for it?

CommonHeader is always part of a heap-allocated structure. That is why
its tt is a byte (and why CommonHeader is not a structure itself);
sometimes the structure defines other bytes and we do not lose memory
due to allignment (e.g., 'isC' and 'nupvalues' in closures). TValues are
complete structures that represent all values in Lua (including pointers
to heap-allocated objects with a CommonHeader). They are not part of
other structures. Again due to allignment, tt will require sizeof(int)
(or even sizeof(double)) bytes there, so it is simpler to keep it an int
(the basic integer type in C).


> 2. A TValue is a type-tagged Lua object. Some fields point to Lua
> objects, themselves, directly, rather than to TValues. For instance, in
> lstate.h, struct global_State.mt is a array of pointers to to Tables,
> rather than an array of pointers to
> Tables-wrapped-as-TValues-with-a-LUA_TTABLE-tag. OK.
>     Now I'm a bit puzzled by why struct lua_State.l_gt and .env are
> TValues rather than simply Table*s. There isn't much space difference,
> but I was puzzled whether there was a reason, given that in some other
> cases simple Table*s were used. Perhaps the reason is that those two
> fields are exposed in the Lua environment? But there's also a third
> case: struct global_State.l_registry is also a TValue not a Table*.
> That's not normally exposed in Lua. Granted,  one can make it exposed.
> But then, the elements of struct global_State.mt can also be exposed
> through debug calls, and they're Tables not TValues. So I was wondering
> whether there was any principle governing when TValues were used and
> when the type-untagged Lua objects themselves (or pointers to them) were
> used.

When space is important (that is, almost any other structure out of
lua_State), we try to use pure pointers.

Otherwise, each of these cases is a TValue or a direct pointer depending
on its usage. For instance, l_registry being a TValue simplifies its
handling as a pseudo-index in the C API (see function 'index2addr').

-- Roberto