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Date: Fri, 13 Mar 2009 12:28:55 +0900 (JST) From: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> To: Steven Rostedt <rostedt@...dmis.org> Cc: kosaki.motohiro@...fujitsu.com, linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>, Andrew Morton <akpm@...ux-foundation.org>, Peter Zijlstra <peterz@...radead.org>, Frederic Weisbecker <fweisbec@...il.com>, Lai Jiangshan <laijs@...fujitsu.com>, Steven Rostedt <srostedt@...hat.com> Subject: Re: [PATCH 08/16] tracing: have event_trace_printk use static tracer > > > +#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? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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