linux-kernel - Re: [RFC PATCH] dynamic_debug: Remove _

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Date:	Wed, 23 May 2012 11:10:43 -0700
From:	Joe Perches <joe@...ches.com>
To:	Jim Cromie <jim.cromie@...il.com>
Cc:	Jason Baron <jbaron@...hat.com>, linux-kernel@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...e.de>
Subject: Re: [RFC PATCH] dynamic_debug: Remove __used attribute from metadata

On Wed, 2012-05-23 at 12:00 -0600, Jim Cromie wrote:
> On Thu, May 17, 2012 at 2:02 PM, Joe Perches <joe@...ches.com> wrote:
> > The __used attribute in the declaration of the
> > dynamic_debug metadata stops the compiler from
> > optimizing and eliminating constant tests and
> > the metadata declaration used in things like:
> >
> >   #define DEBUG_LEVEL 0
> >   if (DEBUG_LEVEL > 1)
> >        pr_debug("foo...");
> >
> > This is a common construct for debugging macros
> > with a constant "level" test.
> >
> > When dynamic_debug is not configured, this is
> > pr_debug and format string is eliminated unless
> > DEBUG_LEVEL is greater than 1.
> >
> > Remove the unnecessary __used attribute so the
> > even the dynamic_debug use of pr_debug can be
> > appropriately optimized away completely.
> >
> 
> 
> Im a bit puzzled - the __used attr is in the
> #if defined(CONFIG_DYNAMIC_DEBUG) branch.

Correct, the optimization is for CONFIG_DYNAMIC_DEBUG
enabled.

> If its not config'd, the METADATA is not compiled,
> and this should have no effect.
> Did you mean enabled instead of configured ?

Nope.

	if (0)
		dynamic_pr_debug("foo %u\n", bar);

will not be optimized away completely because
dynamic_pr_debug uses
	DEFINE_DYNAMIC_DEBUG_METADATA(descriptor, fmt);
and that DEFINE includes __used

so even though the compiler knows that the branch
can not be taken, it still declares the known
unnecessary metadata.

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