> Where my_thread is a pointer to the current Fil-C thread, which Fil-C passes around as the first argument in all calls.
Does this just mean you reserve a register for the current thread? In which case you could explain it as a reserved register (like FS used for TLS). Describing it as "passes around as the first argument in all calls" makes it sound inefficient–but whether it actually is depends on how you implement it.
It is exactly as inefficient as “passing it around as the first argument” implies
There’s a speedup to be had by either reserving a GPR or using one of the segment registers
Lots of obvious stuff like this hasn’t been done yet! If you want to have the satisfaction of landing speedups then Fil-C is a fun thing you could contribute to :-)