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Date: Sat, 10 Mar 2007 12:36:41 -0800 (PST)
From: Trent Piepho <xyzzy@speakeasy.org>
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To: Linus Torvalds <torvalds@linux-foundation.org>
cc: Christoph Hellwig <hch@infradead.org>,
       Rusty Russell <rusty@rustcorp.com.au>,
       lkml - Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] Use more gcc extensions in the Linux headers
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On Fri, 9 Mar 2007, Linus Torvalds wrote:
> On Fri, 9 Mar 2007, Christoph Hellwig wrote:
> Well, since Rusty's macro was hoddible *anyway*, I don't think I'd apply
> it as-is. Breaking icc for something that ugly and not-very-important
> simply makes no sense.
>
> There are better ways to do this.
>
> For one, you could (and should!) abstract these kinds of things out,
> rather than put them in another macro that really does something totally
> different. Then, the macro could have become
>
> 	#define ARRAY_SIZE (sizeof_expression + 0*error_if_not_array)

/* Error if X is a pointer, 0 otherwise */
#define ERROR_IF_POINTER(x) \
	sizeof(int[-__builtin_types_compatible_p(typeof(x), typeof(&x[0]))])

/* Warning (div by zero) if x is a pointer, 0 otherwise */
#define WARN_IF_POINTER(x) \
	(0/!__builtin_types_compatible_p(typeof(x), typeof(&x[0])))

The gcc docs say __builtin_types_compatible_p returns 1 or 0, so the !!
isn't necessary.  And my gcc at least returns 0 for sizeof(int[0]).