Interesting! Making two separate calls at the call site would have some extra overhead compared to what objc_msgSend does. The caller needs to load the self and _cmd arguments twice, for example, and stash the IMP somewhere convenient in between the two calls. If objc_msg_lookup has a standard prologue and epilogue then you'll end up running two of those each time. You'll push and pop two return addresses on the stack rather than just one.
However, I'll happily accept that these are probably pretty small costs, especially since so much of it is just register gains which probably result in cost-free renamings in the hardware. It makes sense that the i-cache impact is more important.
However, I'll happily accept that these are probably pretty small costs, especially since so much of it is just register gains which probably result in cost-free renamings in the hardware. It makes sense that the i-cache impact is more important.