It certainly uses native machine code, but I don't think it generates any at runtime outside of the reverse-FFI closures (at least on linux)? PROT_EXEC at least

It certainly uses native machine code, but I don't think it generates any at runtime outside of the reverse-FFI closures (at least on linux)? PROT_EXEC at least isn't used outside of them, which'd be a minimum requirement for linux JITting. Running in a debugger an ffi_call to a "int add(int a, int b)" leads me to https://github.com/libffi/libffi/blob/1716f81e9a115d340429504563bc8e7fb2eeef2b/src/x86/unix64.S#L84-L104 https://github.com/libffi/libffi/blob/1716f81e9a115d34042950... as the assembly directly before the function is invoked, and, besides clearly not being JITted from me being able to link to it, it is clearly inefficient and unnecessarily general for the given call, loading 7 arguments instead of just the two necessary ones.