Optimizer trace: what's on the execution path when trace=off?

Need to check again what is on the execution path (and so adds CPU overhead), when the tracing is not enabled.

The original tracing proposal had everything wrapped inside if-statements, something like

#define OPT_TRACE(thd_arg, X)  if (unlikely(thd_arg->trace_enabled))  { X; }

With tracing code using RAII objects, we can't have everything inside if-s.
MySQL doesn't have everything inside if-statements either.
What is the added overhead?