Calling C functions using the old stack API would cause the JIT to abort. In Garrysmod's case, that includes such simple operations as constructing and performing arithmetic on vectors.

So when I was building a high-performance voxel library for Garrysmod, I ended up splitting my hot code into chunks that the JIT compiler would usually be happy with. One fast loop to generate a mesh, then a slow loop to convert that into a mesh structure the engine wants. Very carefully designed methods for bit-packing trees of 12-bit integer voxel data into flat tables of floats.

We ran Lua for the entire core of a game in a 400kb preallocated on the PSP about 20 years ago. I know of many places that used it long before us.