effect of goto on C++ compiler optimization

Question

What are the performance benefits or penalties of using goto with a modern C++ compiler?

I am writing a C++ code generator and use of

Answer 1

I was wondering, from a purely compiler optimzation prespective, the result that goto's have on the compiler's optimizer? Does it make code faster, slower, or generally no change in performance compared to using temporaries / flags.

Why do you care? Your primary concern should be getting your code generator to create the correct code. Efficiency is of much less importance than correctness. Your question should be "Will my use of gotos make my generated code more likely or less likely to be correct?"

Look at the code generated by lex/yacc or flex/bison. That code is chock full of gotos. There's a good reason for that. lex and yacc implement finite state machines. Since the machine goes to another state at state transitions, the goto is arguably the most natural tool for such transitions.

There is a simple way to eliminate those gotos in many cases by using a while loop around a switch statement. This is structured code. Per Douglas Jones (Jones D. W., How (not) to code a finite state machine, SIGPLAN Not. 23, 8 (Aug. 1988), 19-22.), this is the worst way to encode a FSM. He argues that a goto-based scheme is better.

He also argues that there is an even better approach, which is convert your FSM to a control flow diagram using graph theory techniques. That's not always easy. It is an NP hard problem. That's why you still see a lot of FSMs, particularly auto-generated FSMs, implemented as either a loop around a switch or with state transitions implemented via gotos.