Messages in this thread |  | | | From | KOSAKI Motohiro <> | | Subject | Re: [PATCH 08/16] tracing: have event_trace_printk use static tracer | | Date | Fri, 13 Mar 2009 12:28:55 +0900 (JST) |
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> > > +#define event_trace_printk(ip, fmt, args...) \ > > > +do { \ > > > + __trace_printk_check_format(fmt, ##args); \ > > > + tracing_record_cmdline(current); \ > > > + if (__builtin_constant_p(fmt)) { \ > > > + static const char *trace_printk_fmt \ > > > + __attribute__((section("__trace_printk_fmt"))) = \ > > > + __builtin_constant_p(fmt) ? fmt : NULL; \ > > > > Why __builtin_constant_p(fmt) evaluate twice? > > It's explained in another patch. But this was a real PITA. We first tried > this with just the "if (__builtin_return_p(fmt))" but the way gcc works, > it handles the global data assignments before optimizing out condition > logic. Thus we ended up getting errors about can not initialize static > variable with a non constant. > > But the ? : operation of the assignment is optimized before the assignment > is made. Thus, if fmt is not constant, then we avoid this warning. Then > during the conditional optimization, gcc will remove that part of the code > altogether. > > Thus the double __builtin_constant_p(fmt) is needed twice. Try taking out > one of them and see what happens with: > > myfunc(const char *fmt) { > > event_trace_printk(fmt); > > } > > Of course the way this is made, we may not call it that way, but I wanted > to be safe.
Thanks for kindful explain. So, I guess many developer feel it's strange. adding comment is better?
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