Messages in this thread | | | Date | Thu, 12 Mar 2009 23:17:21 -0400 (EDT) | From | Steven Rostedt <> | Subject | Re: [PATCH 08/16] tracing: have event_trace_printk use static tracer |
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On Fri, 13 Mar 2009, KOSAKI Motohiro wrote:
> > +#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.
-- Steve
> > > > + \ > > + __trace_bprintk(ip, trace_printk_fmt, ##args); \ > > + } else \ > > + __trace_printk(ip, fmt, ##args); \ > > +} while (0) > > >
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