linux-kernel - Re: [PATCH 1/3] typecheck: introduce assign_if() and assign_if

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Message-ID: <20140227234850.GC7308@joshc.qualcomm.com>
Date:	Thu, 27 Feb 2014 17:48:50 -0600
From:	Josh Cartwright <joshc@...eaurora.org>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/3] typecheck: introduce assign_if() and
 assign_if_enabled()

On Thu, Feb 27, 2014 at 11:00:32AM -0800, Greg Kroah-Hartman wrote:
> On Mon, Feb 24, 2014 at 11:08:25AM -0600, Josh Cartwright wrote:
> > +/*
> > + * Intended for use in static object initializers,
> > + * assign_if(const_expr, function) evaluates to 'function' if 'const_expr',
> > + * otherwise NULL.
> > + *
> > + * The type of the assign_if() expression is typeof(function), and therefore
> > + * can provide typechecking regardless of 'const_expr'.
> > + *
> > + * gcc considers 'function' to be used and will not generate a 'defined but not
> > + * used' warning when not 'const_expr', however, gcc is smart enough to
> > + * eliminate 'function' if assign_if() is the only reference.
> > + */
> 
> What version of gcc started doing this?  Does llvm also do this?

I'll need to dig up some old gcc's to give this a more thorough
testing; testing with clang 3.4, and it appears to have the same
behavior, at least when I throw a trivial usecase at it.

	$ clang --version
	clang version 3.4 (tags/RELEASE_34/final)
	Target: x86_64-unknown-linux-gnu
	Thread model: posix

	$ cat test.c
	static void foo(void)
	{
		extern void BROKEN(void);
		BROKEN();
	}
	void (*callback)(void) = 0 ? foo : 0;

	$ clang -Wall -Werror -c test.c

	$ size test.o
	   text	   data	    bss	    dec	    hex	filename
	      0	      0	      8	      8	      8	/tmp/test.o

	$ nm test.o
	0000000000000000 B callback

Although, given the feedback on the other patches, assign_if() may
become just be a solution in search of a problem :).

Thanks,
  Josh

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