You're forgetting the overhead of popping / pushing registers. Both in terms of direct instruction overhead, and indirectly through code size and working set bl

You're forgetting the overhead of popping / pushing registers. Both in terms of direct instruction overhead, and indirectly through code size and working set bloat. Which, especially for smaller functions, can be significant. It's one of the problems with a register-oriented architecture. Sometimes this can be optimized away, but not always.