linux-kernel - Re: [PATCH 08/16] tracing: have event_trace

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Message-Id: <20090313122235.43EB.A69D9226@jp.fujitsu.com>
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?


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