Re: Lua 5.2 #... proposal and patch.

[Tony Finch <dot@...>]

Benoit Germain <bnt.germain@gmail.com> wrote:
>
> So, if I understand correctly, there is one single monolithic stack of
> 'slots' somewhere in the VM where call frames are 'allocated', these
> being made of a given number of slots, the first pointing to the
> callinfo, and the remainder containing the parameters and locals.

The callinfo stack is actually a second parallel stack, which holds
information like saved stack pointers and instruction pointers. The value
stack that I talked about in my previous message just contains the values
that you directly manipulate in your Lua program: local variables are
slots in this stack, as are temporary values created when evaluating
expressions. (FORTH systems have a similar two-stack setup. I think the
LuaJIT interpreter uses a single unified stack more like typical compiled
languages.)

> I suppose then that the the default 20 slots we get for a C function
> simply belong the top call frame on this global stack, with the
> additional advantage of being re-sizable by the C function?

Right. The whole stack is resizable. In pure Lua code, Lua knows how many
stack slots each function needs for its locals and temporaries, so it
checks once at the start of the function that enough space is available.
For C functions it just checks that LUA_MINSTACK (i.e. 20) slots are
available. Lua may need to resize the stack during a function if you use
... (because of the unpredictable number of arguments), and similarly C
functions can call lua_checkstack.

Tony.
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