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Subject: Re: optimizing out inline functions
From: Joe Perches <joe@perches.com>
To: Johannes Weiner <hannes@saeurebad.de>
Cc: James Kosin <jkosin@beta.intcomgrp.com>, linux-kernel@vger.kernel.org
In-Reply-To: <87lk1trdr8.fsf@saeurebad.de>
References: <ayzYV-7mv-5@gated-at.bofh.it> <ayA8E-89e-29@gated-at.bofh.it>
	 <483DC28B.4060001@support.intcomgrp.com>  <87lk1trdr8.fsf@saeurebad.de>
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Date: Wed, 28 May 2008 20:04:22 -0700
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On Thu, 2008-05-29 at 05:27 +0200, Johannes Weiner wrote:
> James Kosin <jkosin@beta.intcomgrp.com> writes:
> >> But we do not have KCONFIG_DEBUG_SOMETHING available
> >> so the second best is to use an empty function
> >> to keep the typechecking in place.
> >> IIRC gcc optimize both away.
> > Another way would be to have:
> > static inline void some_debug_function(var1)
> > {
> >    #ifdef KCONFIG_DEBUG_SOMETHING
> >       something = var1;
> >       printk(some debug text);
> >    #endif
> > }

A potential issue is a possible unnecessary call of any
function used as an argument to some_debug_function.

int unnecessary_debug_test(void)
{
	int foo;
	[]
	return foo;
}

static inline void some_debug_function(int bar)
{
}

some_debug_function(unnecessary_debug_test())

unnecessary_debug_test will still get called

Macro wrappers/statement expressions can be used to avoid
the function call.  see kernel.h/pr_debug for an example.

#ifdef DEBUG
/* If you are writing a driver, please use dev_dbg instead */
#define pr_debug(fmt, arg...) \
	printk(KERN_DEBUG fmt, ##arg)
#else
#define pr_debug(fmt, arg...) \
	({ if (0) printk(KERN_DEBUG fmt, ##arg); 0; })
#endif