Re: [Qemu-devel] [PATCH v2] trace: Provide a per-event status define for conditional compilation

[Stefan Hajnoczi]

2011/12/6 Lluís Vilanova <address@hidden>:
> Adds a 'TRACE_${NAME}_ENABLED' preprocessor define for each tracing event in
> "trace.h".

I think we should take it a step further: support
TRACE_${NAME}_ENABLED at run-time.  This means skipping the trace
event code if the event is currently disabled.  If the user enables it
at runtime then the if (TRACE_${NAME}_ENABLED) will evaluate to true
and we execute the code.

SystemTap has support for this and it would be easy to do runtime
support for stderr and simple (where we can test
trace_list[event_id].state).

The SystemTap mechanism is a macro, e.g. QEMU_${NAME}_ENABLED(), which
fits perfectly with what you have posted.

What do you think?

(The difference is that your patch plus compiler dead-code elimination
would completely remove the code when built without a trace event.
What I'm suggesting becomes a test and branch to skip over the code
which always gets compiled in.)

Stefan