Extending Lua for multiple light userdata types

[Rena <hyperhacker@...>]

Recently I was thinking about how you might add a 64-bit integer type to Lua.

As I understand it, the existing primitive types such as number and
light userdata have a bit of a speed advantage over others, because
they're small enough to be stored directly on the stack or in a
register; that means one less level of indirection, plus no time
needed to allocate memory and collect them.

It'd be nice if we could have this advantage for our in64 types as
well, since they're only 8 bytes, and using full userdata for them
incurs some penalties in performance (memory has to be allocated and
freed, memory fragmentation can be an issue) and memory use (each
instance needs at least a pointer to its memory block and a pointer to
its metatable, so already the size triples, plus whatever overhead for
the allocator to keep track of those blocks).

A tempting idea is to just cast your int64 to void* and use light
userdata, and there's a module that does this:
https://github.com/cloudwu/lua-int64
but this has a few obvious issues:
-Requires that sizeof(void*) == sizeof(int64)
-int64 values share the same metatable as all other light userdata, so
accidentally invoking a method intended for some other object that
expects a pointer will lead to a segfault
-no real way to tell if a value is intended to be a pointer or not

It seems like these issues could be avoided, if there were a way to
create more "primitive" types, that function like light userdata. Each
type having its own metatable and data size (up to 8 bytes?). You
might have int64, uint64, pointers to different things?

-- 
Sent from my Game Boy.