I'm not sure I understand your complaint about callee-saved registers. The point is that arguments are passed in registers, as opposed to on the stack, and most

I'm not sure I understand your complaint about callee-saved registers. The point is that arguments are passed in registers, as opposed to on the stack, and most calls are tail calls anyways (so `jmp`, not `call`/`ret`). It doesn't matter if the registers are caller-saved or callee-saved, as the context is destroyed anyways, the function never returns. In case of GHC, the functions could return, but the fast-path is that most functions access VM state, and many functions have very little need for registers; therefore, forcing registers be caller-saved would just cause most functions to push and pop the stack without any reason. If non-standard calling convention (e.g. C calling convention) functions are called only rarely, then using caller- or callee-saved registers doesn't really matter.